Sustainable Water Management for Irrigated Asparagus

Final Report for SW02-013

Project Type: Research and Education
Funds awarded in 2002: $23,013.50
Projected End Date: 12/31/2003
Matching Non-Federal Funds: $10,000.00
Region: Western
State: Utah
Principal Investigator:
Dr. Daniel Drost
Utah State University
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Project Information

Abstract:

Improvements in asparagus productivity (spear yield and stand longevity) require a better understanding of present farming practices and how best to manage irrigation. Detailed farm evaluations were conducted on working asparagus farms in California and Washington, two major asparagus producing regions in the western U.S. Current production practices and on-farm plant evaluations were conducted over several years in asparagus fields planted in 2002. Growers employed different harvest strategies, irrigation methods, and amounts which had a significant impact on asparagus fern development, root growth, and crop yield. Growers employing more harvest pressure in the year after planting tended to have less root growth and higher plant stand reductions than those taking smaller harvests during the first harvest season. Irrigation method also appears to influence plant stand reduction with higher plant losses in commercial sprinkler- and drip-irrigated fields when compared to furrow-irrigated asparagus.
In a replicated experiment, we assessed the role harvest strategy, irrigation method and amount had on asparagus plant growth and productivity during the first 3 years of growth. Plants were harvested in the first or second year after planting and were watered at 0, 75, or 150% of evapo-transpiration with drip or sprinkler irrigation systems. While early harvest had little significant effect on yield, plants harvested one year after planting had fewer stems and stem dry weight than those harvested in the second year. Irrigation method had a greater impact on root growth than on fern growth or yield. Plants grown with drip irrigation had greater root mass than those grown with sprinkler irrigation. Although yields were not different between drip- and sprinkler-irrigated asparagus, drip-irrigated plants had a higher percentage of large and very large spears. Fern growth (number and weight of stems) increased when more frequent and higher applications of water were applied during the summer. However, the yield difference between the 150 and 75% ET treatments was not significant while a further reduction in applied water to 0% of ET reduced spear yield by more than 50% when compared to the 75% ET treatment. Growers need to pay more careful attention to harvest pressure and use carbohydrate monitoring to assess the changes in root carbohydrate content as a means of assessing when to stop harvest. These approaches will ensure that plants have sufficient energy reserves to grow enough fern and have adequate time to recharge the root system after harvest. Our farm and research findings suggest that asparagus growers should seriously consider their choice of irrigation system, should monitor soil water to avoid over irrigating, and not over-harvest young plantings. With these improvements, stand longevity should be improved, water resources will be conserved, and long-term yields should be maintained.

Project Objectives:

Objective 1: Assessing growth and productivity in commercial asparagus fields

Objective 2: To develop irrigation application guidelines that optimizes productivity

Objective 3: To provide asparagus industry water recommendations based on observations gathered in commercial settings and research conducted under controlled conditions and to use this information to ensure optimal production

Introduction:

Asparagus producers have reported a decrease in plant longevity and plant productivity in asparagus fields. Historically, asparagus fields remained in production for 15 to 20 years (Dean et al., 1993). Currently asparagus field longevity has decreased to 10 years or less according to the asparagus growers interviewed for this study. Growers are interested in what causes this decrease in field longevity. Different harvest durations (Shelton and Lacy, 1980; Takatori et al., 1970), irrigation methods (Drost, 1999), tillage practices (Putnam, 1972), or other cultural practices may contribute to stand loss or low levels of productivity.
It has been reported that when comparing different harvest durations, longer harvests period lengths reduce total yield harvested (Haber, 1932), lowers the total yields over the lifetime of a crop (Shelton and Lacy 1980), decreases spear quality (Paschold et al., 2002), and decreases plant populations (Takatori et al., 1970). As little as 10 to 20 days shorter period for carbohydrate assimilation during the summer reduces marketable and total yields (Knaflewski and Krzesinski, 2002). The optimal length of harvest (number of cuts or days) varies with crop age and location. Young asparagus crops typically have less root carbohydrate (CHO) reserves, due to smaller root mass, to support the same harvest duration as established crops. Asparagus crops located in areas with longer growing seasons can have an advantage of more time to establish root mass and CHO reserves and can sustain longer harvests at younger ages without as much detrimental impact to the crop. However, the optimal harvest period is unknown and may be shorter than currently practiced.
Varying amounts of applied water can affect plant growth, development, and yields. Soil moisture stress has been shown to have long-term negative effects on asparagus. Soil moisture stress reduces fern number and weight (Cannell and Takatori, 1970), limits yields by reducing spear number (Takatori et at., 1970), and reduces spear diameter (Hartmann, 1981) or average spear size (Roth and Gardner, 1990). As irrigation rates decreased from 80% to 0% ET during the fern growth period, the number of ferns, roots, and buds, and fern and root fresh weight decreased (Drost, 1996). Others have shown when precipitation levels increased during July and August, asparagus yields increased in the following year (Hartmann, 1981; Hartmann et al., 1990). Yields decreased when precipitation levels were high during September because precipitation promoted bud break and fern growth later in the year than desirable (Hartmann et al., 1990).
In the western U.S., asparagus production is dependent on irrigation. Hanna and Doneen (1958) reported that 92 cm of precipitation and irrigation were needed in coastal southern California. Cannell and Takatori (1970) reported water requirements of 51 to 77 cm in northern California. The annual water requirements in the Imperial Valley of southern California were in excess of 330 cm (Robinson et al., 1984) while Roth and Gardner (1989) reported that 270 to 310 cm of water were necessary for asparagus in Arizona. In Washington, asparagus growers should apply 50 to 75 cm during the fern growth period (Dean et al., 1993) while in Utah, recommendations range from 55 to 80 cm (Drost, 1996).
In Objective 1, we monitored and evaluated young asparagus plantings on commercial farms to better understand growth, root development, and stand longevity as it is influenced by standard farming practices used in the asparagus industries of the western U.S. In addition, we also began a long-term irrigation study (Objective 2) to address the questions of how harvest pressure, irrigation method and amount impacts plant growth and productivity. Information gathered from these two studies (Objective 3) were presented to asparagus growers throughout the U.S. and Canada in a variety of different formats.

Cooperators

Click linked name(s) to expand
  • Randy Seth Peterson

Research

Materials and methods:
Objective 1

Eleven commercial asparagus fields of 40 acres or greater were monitored starting in the early spring of 2003. Five of the fields were located near Pasco and Yakima, Washington and six were near Stockton, California. The primary focus of this study was to evaluate how long-term growth is affected by cultural practices like harvest duration and amount, irrigation methods and amounts, and tillage. Sites were chosen from growers who had planted their asparagus crop in the spring of 2002.
Specifics on farm practices needed to be determined so a questionnaire was sent to the growers. The questionnaire asked about what cultivars they planted, whether they planted crowns or seedlings, plant and row spacing, harvest length and amount, nutrient applications, irrigation methods and amounts, and cultivation practices. This information was used to compare and contrast the cultural practices of the various growers.
Measurements of plant population, fern number per length of row, fern weight, carbohydrate content in the roots, root mass, and root distribution were monitored during the 2003-2004 growing seasons. To avoid variations due to location of sampling, sites were chosen in each field where repeated measurements would be taken from year to year. These sample sites were chosen based on their representation of the whole field and uniformity within sampling locations. Six sampling sites enabled the root and plant performance data to be taken at comparable locations on each sample date and allowed information to be related to earlier measured results.
Beginning plant populations were estimated from the plant and row spacing provided by the growers. Each year plant populations were again estimated by subtracting plant loss from the original planting populations. Plant loss was estimated by measuring the distance between plants in a 6.1 m length of row. Any distance greater than 30 cm between plants was determined to be sufficiently greater than original plant spacing and was then assumed to indicate the death of a plant. Fern number was sampled by counting all stems in the 6.1 m length of row. Fern mass was calculated by weighing 10 random stems harvested in the row opposite the sample row. Because asparagus takes four-five years to establish, additional data collection from this study will show the long-term affects of cultural practices on asparagus growth and development. Washington fields were only studied during 2003, after additional support from the Washington Asparagus Commission was withdrawn because of the industry's desire to reduce expenditures due to difficult economic circumstances within the industry. Monitoring continued in California through 2004.
Carbohydrate levels in the roots were sampled by collecting fleshy storage roots at specific times throughout each season. A shovel was used to make vertical cuts, the section of soil was removed, and the storage roots collected, and placed in plastic bags, and stored on ice. The roots were transported to Utah State University, washed, surface dried, and frozen. After 24 hours, frozen samples where thawed, cut to 1-2 cm pieces, and then pressed to extract the juice. A refractometer (model REF103; reading range 0-32%) was used to obtain the percentage of sugar in the juice (BRIX). The resulting numbers were then assessed with AspireUS (www.aspireus.com) to convert the refractometer reading to mg fructose/g root dry weight (CHO). Carbohydrate sampling began in March and ended in November during 2003 and 2004. Key sampling times include pre-harvest, near the ends of the harvest and fern establishment periods, during mid-summer and late summer fern growth periods, and after dormancy begins at the end of the season.
Root mass and distribution was sampled in the spring of 2003 to estimate the root growth during the 2002 season and again at the end of the 2003 and 2004 growing seasons. Soil cores (7 x 30 cm) to 60 cm deep, taken at two distances from the row, were used to collect the fleshy storage roots. Sampling was repeated in six locations correlating with fern data collection sites on each farm. Fleshy roots were sorted from the soil, washed, and fresh and dry weight determined. Root distribution graphs were generated from the data using a contouring program in Surfer 7 (Golden Software, Inc. Golden, Colorado; www.goldensoftware.com).
To estimate the total energy available for growth, CHO was converted to kg CHO/ha by multiplying the estimated root mass by the CHO values. Since the majority of root growth occurs from about four weeks after harvest period (around July) until the end of the growing season (Drost and Wilcox-Lee, 2000), root mass was estimated in the fall and used to obtain kg CHO/ha for July to November of that same year and March through June of the next year. This estimation of kg CHO/ha may be useful in predicting future yields.
Statistics were not calculated for the data collected from this portion of the study. Mean values were calculated from the six replications within each farm and were used as broad comparisons between farms and states.

Objective 2

In April 2002, one-year-old “Jersey Giant” asparagus crowns were planted in Millville silt loam soil (coarse silty, carbonatic, mesic typic haploxeroll) at the Utah State University Greenville Farm in North Logan, Utah. Plots were 6.1 m long (20 plants) by two rows. Rows were spaced 1.5 m apart with guard rows between each treatment and there were five replications. Prior to planting, phosphorous (113 kg/ha P2O5) was banded in the bottom of 23 cm deep trenches. Crowns were spaced 30 cm apart in trenches and were covered with 5-6 cm of soil and additional soil was added during the year. All rows were fertilized with 113 kg of nitrogen per ha in mid-June. Weeds were suppressed during the summer with shallow tillage and hand weeding. In November 2002, an 8-12 cm of soil was mounded over the rows for winter protection and to accommodate for soil settling.
In the spring of 2003, fern was mowed and the soil leveled in preparation for spear emergence. A pre-emergent herbicides (metribuzin; 2.3 kg ai/ha and napropamide; 4.5 kg ai/ha) was applied for residual weed control, and glyphosate; 2.3 kg ai/ha was tank mixed for control of existing weeds. Glyphosate (2.3 kg ai/ha) was applied between the rows after fern was mature and for spot treatments of perennial weeds periodically during the year. Disulfoton (1.2 liters per hectare) was applied in early and mid summer for aphid control. Nitrogen was applied at 113 kg/ha in June to promote healthy fern growth.
In the spring of 2004, fern was mowed and herbicides applied as in previous years. Nitrogen (113 kg/ha), for fern growth, and Disulfoton (1.2 liter per hectare), for aphid control, were injected into the irrigation systems and spread on the non-irrigated treatments during fern establishment period of early June.

Irrigation Management 2002-2004
All rows were irrigated the same during the establishment year of 2002. Irrigation events occurred every two weeks through drip tape (T-Tape; 30 cm emitter spacing; 1.7 liters per minute). Five irrigation treatments were imposed in June 2003. Treatments included a non-irrigated control, sprinkler irrigated at 75% evapo-transpiration (ET), sprinkler at 150% ET, drip at 75% ET, and drip at 150% ET. Irrigation was applied from June to mid September. Drip tape (T-Tape) was placed 20 cm from the center of the row and 10 to 15 cm deep along the length of the row. The drip tape was placed on the side of the row opposite the guard row. Sprinkler treatments were set up to direct the spray from the outer parameter of the plots toward the centers. Catch can tests were preformed and the sprinkler system averaged an application rate of 3.3 cm of water per hour per plot with a distribution uniformity of 70%.
The irrigation needs were calculated using the FAO crop coefficient (Kc) values (Allen et al., 1998), available soil water content, crop rooting depth, ETo (grass reference Penman-Monteith FAO 56 equation) and precipitation values from a local weather station. The FAO Kc value used was 0.957 (mid-season), which is the value for the fern growth period, and 0.5 (initial) which is the value during the harvest period. Percent canopy cover was calculated at the initiation of irrigation treatments each year. The percentage of canopy cover was multiplied by the difference in Kc initial and Kc mid-season and this number was added to the Kc initial providing an adjusted value.
Reference ETo was adjusted to either 75 or 150 percent of ET. In 2003, 100% grass reference ETo from June 10 to September 14, 2003 was 55 cm with 5.5 cm of rain and from June 8 to September 20, 2004 was 59 cm with 4.8 cm of rain. Average rooting depth (from coring samples) was 60 cm in 2003 and 75 cm in 2004. The soil water depletion threshold was set at 50 percent and the length of irrigations were determined by the drip tape emitter outputs and by catch can tests conducted for the sprinkler treatments. For this study, 9.2 cm in 2003 and 11.5 cm in 2004, of available water was stored in the root zone.
A neutron probe was used to monitor soil moisture content in 2003 and 2004. Readings were taken periodically from early July to mid-September in 2003 and from mid-June to early October in 2004. Access tubes were positioned 38 cm from the row between the treatment rows in two replications. Readings were taken at 23, 38, 53, 69, 84, and 99 cm below the soil surface.

Harvest Strategies 2002-2004
Two different harvest regimes were imposed in 2003. One row of each treatment was harvested in the year after planting (common grower practice) and the other was not harvested until the second year after planting. Plots were harvested for three weeks starting on April 15, 2003 with 17 cuts during the period. All spears taller than 23 cm were harvested, weighed, trimmed to 23 cm, and graded into size classes based on the diameter of the spear. The size categories were very small (less than 8 mm), small (8.1 mm to 13 mm), medium (13.1 mm to 17.5 mm), large (17.6 mm to 22 mm), very large (greater than 22.1 mm), and cull (damaged, diseased, or bent and broke when straightened) as specified by US standards for grades of fresh asparagus (Anon, 1997). In 2004, all plots were harvested for three weeks starting on April 7 and finishing on April 28 and those rows harvested first in 2003 were cut for an additional three weeks. Spears were counted and graded on Monday, Wednesday, Thursday, and Saturday and counted, weighed, and graded on Tuesdays and Fridays.

Plant Data Collection 2002-2004
In August and November, 2002 fern number was counted, while fern fresh and dry weight, and fern height data was collected in November. All the fern from two randomly selected plants in each row were removed, weighed, and then dried and weighed again. In 2003 and 2004, fern number and fern heights were recorded in July and October and fresh and dry fern weights were determined in November.
Root distribution, depth, and root mass were estimated in early June 2003, prior to imposing the irrigation treatments. Soil cores were taken at 0, 30, and 60 cm from the row center between two plants in five replications. Soil cores were 8.9 cm in diameter and were divided into 15 cm segments to a depth of 75 cm. In April of 2004, soil cores in three replications were taken between two plants in each row of each treatment. Soil cores were taken at 0, 30, and 60 cm from the row center with a core diameter of 5.3 cm. The core was divided into 15 cm segments down to 90 cm deep. Fleshy storage roots were separated from the soil, washed, weighed fresh, and dried prior to CHO being determined. Root distribution was mapped using Surfer 7 (www.goldensoftware.com).
Root CHO (mg fructose/g root dry weight) was analytically evaluated using a modified Anthrone method. Dried asparagus roots were ground, 50 mg was added to a test tube, five ml of distilled H2O was added, and the samples heated at 90 C for 30 minutes. Samples were then centrifuged for 15 minutes at 700 g and 0.5 ml of the sample was reacted with 5 ml of Anthrone solution (0.25 g anthrone, 5 g thiourea, and 1 liter of 66 percent sulfuric acid) and incubated in the dark for 25 minutes. Samples were then scanned with a Diode Array Spectrophotometer at 620 nm and compared to known standards. Standards were dilutions made from 10 g fructose/1 liters distilled H2O, equivalent to values expected in asparagus roots.
All data were analyzed by standard analysis of variance to determine main effects and interactions of harvest treatments, ET treatments, and irrigation methods. Data was analyzed again with non-irrigated and 0% ET treatments eliminated from data, and means were separated using Tukey t-Test (LSD). (SAS Institute, Cary, North Carolina)

Research results and discussion:
Objective 1

Washington
Asparagus crop production practices vary by a growers own experiences and farming goals. Growers were surveyed about their practices including cultivar planted, use of crowns or seedlings, irrigation methods and amounts and frequency of application, row and plant spacing, crop yield and length of harvest, fertilizer applications, and cultivation practices for the years of 2002 and 2003.
Washington growers planted the Green Giant cultivar as one-year-old crowns from the same crown production farm and planted them in March or April of 2002. Three of the asparagus farms used sprinkler irrigation systems including one pivot, one wheel line, and one stationary hand line (Table 1). Two farms used furrow irrigation on mounded or flat beds. Irrigation frequency varied from every 3 days to 14 days for sprinkler-irrigated fields and every 14 to 21 days in the furrow-irrigated sites. The length of each irrigation event was 12 to 24 hours for both sprinkler and furrow systems.
From the planting information provided by the growers, the planted population in 2002 was 43,000 to 64,600 asparagus crowns/ha (Table 1). Plant stands at the end of 2003 were 37,000 to 52,500 asparagus plants/ha for a stand reduction of 6 to 19%. Furrow-irrigated fields had a population reduction of 8.5% while sprinkle- irrigated fields had an average reduction of 15%.
Harvest amount and duration varied by farm. Spear harvest in Washington was 577 to 952 kg/ha in 2003, the first harvest year after planting (Table 1). Harvest lengths ranged from 14 to 21 days with 7 to 21 cuts over that harvest period. The ending date of the harvest period differed by 18 days between the growers. In 2004, spears harvested ranged from 3,000 to 4,300 kg/ha, harvest lengths ranged from 41 to 56 days, and the ending date of the harvest period differed by 18 days between the growers. Fern growth was measured at the end of the 2003 season. The stem weight ranged from 1.66 to 4.06 kg fresh weight/10 stems with fields averaging 24 to 32 stems per meter (Table 2).
Carbohydrate levels were used to evaluate the health of an asparagus field. In the spring of 2003, CHO values ranged from 440 to 520 mg fructose/g root dry weight and reflect the previous years CHO accumulation (Table 2). Depletion occurred until early summer (July), due to CHO utilization during the harvest and fern establishment periods, when values ranged from 180 to 330 mg fructose/g root dry weight. Root CHO levels accumulated to the pre-harvest values on all farms by October 2003.
Root mass and root distribution were estimated in the spring and fall of 2003 using a root coring procedure. The spring samples represent the amount of root growth from the previous year and the fall samples represented the root mass accumulation during the 2003 growing season (Table 2). Root mass varied greatly from farm to farm with calculated fresh weights of 7,000 to 42,000 kg/ha in the spring of 2003. By the fall of 2003, root mass ranged from 28,000 to 47,000 kg/ha with a percent increase of 13 to 404%. On average, sprinkler irrigated farms had a 50% increase in root biomass while furrow irrigated farms had a 140% increase in root growth.
Figure 1 (Farms A-E) illustrates the distribution of root mass in the soil profile for the different Washington farms surveyed in 2003. These root maps are similar in appearance to topographic maps where iso-lines represent a change in root mass with changes in depth and distance. More lines closer together represent a rapid change in root mass while fewer lines, further apart, suggest a more evenly distributed root system. The highest root mass concentrations are located at approximately 17 cm below the soil surface and indicate the depth where the crown is located. Washington furrow-irrigated farms (B&E) averaged 72% of the root mass in the upper 30 cm of the sampled profile while more than 77% of the storage roots were located above 30 cm depth in sprinkler-irrigated farms (A,C,& D).

California
In California, growers planted the cultivar “UC 157” in January or February of 2002 as one-year-old crowns. Four of the asparagus farms in California used furrow irrigation with mounded beds and two farms used drip irrigation (Table 3). The frequency of irrigation applications varied from every two to three days in drip fields to one time during the summer for furrow-irrigated sites. The length of each irrigation ranged from 5 to 12 hours.
In 2002, the planted population was 28,700 to 32,300 asparagus plants/ha (Table 3). By the end of 2003, stand reductions varied from 3 to 18% with a calculated population of 26,000 to 28,100 plants/ha. At the end of 2004, stand reductions ranged from 4 to 17 % and were estimated at 27,000 to 28,000 plants/ha. Drip-irrigated fields had the highest stand reduction with a 16% decrease in 2003 with no additional losses in 2004. Furrow irrigated fields averaged stand reductions of 8% in 2003 with little change from 2003-2004.
In California, growers harvested from 0 to 1,300 kg/ha of spears in 2003 and 2,500 to 3,200 kg/ha in 2004 (Table 4). Harvest lengths varied from 0 to 32 days (0 to 13 cuts) over that harvest period in 2003 and from 47 to 58 days (28 to 62 cuts) in 2004. The ending date of the harvest period differed by approximately 30 days in 2003 and 6 days in 2004. Not all growers were willing to provide us with detailed information regarding their harvest practices. Fern growth was measured at the end of the 2003 and 2004 seasons. The fern weighed from 0.90 to 1.32 kg/10 stems in 2003 and from 0.77 to 1.58 kg/10 stems in 2004. The number of stems varied from 21 to 39 stems per meter in 2003 and from 25 to 42 stems per meter in 2004. Generally drip-irrigated fields had few stems (27 fern/1.46 kg) of greater weight than the furrow irrigated fields (32 fern/0.93 kg).
Root data were collected using the soil coring method in the spring and fall of 2003 and the fall of 2004 (Table 5). Calculated root fresh weights ranged from 7,900 to 36,100 kg/ha in the spring of 2003. By the fall of 2003, root mass varied from 13,000 to 43,200 kg/ha (-5% to 192% change). In the fall of 2004, root mass ranged from 17,000 to 59,100 kg/ha (-2% to 73% change).
Carbohydrate values ranged from 346 to 417 mg fructose/g root dry weight in March of 2003 (Table 5). During the harvest and fern establishment period (April and July), CHO values decreased to between 216 to 321 mg fructose/g root dry weight. As CHO’s were replenished during the summer, final CHO levels reached 572 to 712 mg fructose/g root dry weight by November. In March of 2004, levels had decreased to 496 to 614 mg fructose/g root dry weight as a result of respiration and early spear growth. Carbohydrate levels were decreased until May or June due to harvest and fern growth with seasonal low values varying from 221 to 320 mg fructose/g root dry weight. By November of 2004, CHO levels were replenished to values from 422 to 696 mg fructose/ g root dry weight.
Figure 2 (Farms A-F) illustrates the distribution of root mass in the soil profile for the different farms surveyed in California in the fall of 2003 and 2004. The greatest root mass concentration was located approximately 17 cm deep in the soil profile, indicating the location of the crown. Line patterns indicate root concentrations and rooting pattern. California furrow irrigated farms (B,C,& F) averaged 43% of the total root mass in the upper 30 cm of the sampled profile in 2003 and 56% in 2004. Drip-irrigated farms (D&E) averaged 70% of the total root mass in the top 30 cm in 2003 but only 30% within this soil zone in 2004.

Objective 2

Irrigation Treatments
From 12 June (start of irrigation treatments) to 17 September 2003 the 0%, 75%, and 150% ET treatments received a total of 0, 23 (5 applications), and 41 (9 applications) cm of applied irrigation water respectively. During this time period 5.5 cm of precipitation fell on all treatments and the total ETo in 2003 was 54.3 cm and the adjusted Kc value was 0.6. In 2004, zero, 29 (5 applications), and 57 (10 applications) cm of irrigation was applied between 3 June and 17 September for the 0, 75 and 150% ET treatments. Plots received 7.3 cm of precipitation, ETo was 52.9 cm, and the adjusted Kc was 0.86.

Effects of Harvest Pressure and Irrigation Approaches on Asparagus Productivity
Spear harvest in 2003, one year after planting, totaled 855 kg/ha (Table 6). After harvest, the different irrigation and ET treatments were imposed. Plots were harvested again in 2004 for either 3 or 6 weeks. There was no difference in yield between treatments harvest in year 1 or year 2 during the first 3 weeks of harvest in 2004 (Table 6). Spear yield was 3768 kg/ha for those harvested first in 2003 and 3381 kg/ha for plots harvested first in 2004 (Table 7). An additional 390 kg/ha was harvested in the final 3 weeks for plots first harvested in 2003.

After one year of the different irrigation treatments, we compared harvest yield for the non-irrigated, drip, or sprinkler irrigated treatments (Table 7). Total and different grades of spear yield in the non-irrigated treatments were less than half that of those irrigated during the previous summer (Table 7). There was no significant difference in yield between those treatments where irrigation was applied from sprinklers or drip systems for any of the size categories in 2004. The interaction between harvest treatments and irrigation methods was not significant.

Applications of varying amounts of applied irrigation water had a significant effect on harvested spear weight (Table 8) during the 2004 harvest period. There was a significantly lower yield in the 0% ET treatments compared to either the 75% or 150% ET treatments for all size classes measured. Since most asparagus growers in the Western U.S. irrigate, when comparing the effects of irrigating at 75% ET verses 150% ET on yield (Table 8) we found that was no difference in total yield between treatments in 2004. There were a greater number of culled spears in 150% ET treatments when compared to the 0% and 75% treatments. This difference was due to an aphid infestation the previous fall that caused an increase in non-marketable spear number and weight.

Effects on Plant Growth
Asparagus plants averaged 7 stems per plant in August and 11 stems per plant in November, 2002. Harvesting asparagus for three weeks in 2003 had no effect on stem number in July or October 2003 (Table 9) and had a similar percent increases in stem number during this period. Stem numbers collected in July 2004 showed that there were significantly more stems per plant when harvest began the second year after planting compared to rows harvested the first year though these differences were no longer apparent in November.

Applying different irrigation amounts had a significant effect on stem numbers in 2003 and 2004 (Table 10). In general as irrigation applications decreased, stems per plant decreased. In most cases, the 0% ET treatment had few stems when compared to the 75% and 150% ET treatments, which were not significantly different from each other.

Irrigation methods also influence fern number (Table 11). There were fewer stems per plant in non-irrigated treatment when compared to drip or sprinkler systems in July and October 2003 and in July 2004. However, by November 2004, the non-irrigated treatments were only different from the drip. There was no difference in stems per plant between the two irrigation application methods in either year.

At the end of each year, fern fresh and dry weight was statistically analyzed to determine significant effects of harvest, ET, or irrigation method treatments. Harvesting asparagus the year after planting or after two years did not influence fern fresh or dry weight in 2003 or 2004 (Table 12). Fern weight was influenced by the amount of irrigation applied in 2003 but not in 2004. The 0% ET had the lowest and 150% ET had the highest weight in 2003 (Table 13). A similar trend of increasing fern weight with increasing water applications was noted in 2004.

Fern dry weight in the fall of 2002 was 0.04 kg/plant. There was no significant difference between plots or replications, indicating uniform plant growth in 2002 (data not shown). Irrigation method had a significant effect on fern fresh and dry weights in 2003 but not 2004 (Table 14). In general, non-irrigated treatments had less fern weight compared to the drip irrigation in 2003 with the sprinkler having an intermediate weight. There was no significant difference however, in fern weight between sprinkler and drip treatments in 2004.

Root fresh weight measured in the spring of 2003 prior to the initiation of the irrigation treatments was 0.37 kg per plant. There was no significant difference between plots or replications, indicating uniform plant growth in 2002 (data not shown). In the spring of 2004, fresh root weight and plant carbohydrate content was evaluated after exposure of the plants to the different harvest treatments, irrigation amounts, and irrigation methods imposed on the plants during the 2003 year. Harvesting asparagus in the year after planting significantly reduced root fresh weight accumulation when compared to waiting until the second year (Table 15). Early harvest pressure did not however affect root CHO content or total available CHO levels in the plants.

Failure to irrigate asparagus (0% ET) resulted in significantly less fresh root weight when compared to either the 75% or 150% ET treatments (Table 16). There was no significant difference in fresh root weight between the 75% and 150% ET treatments. The ET treatments had no significant effect on root CHO content though the 0% ET had less than half the total CHO compared to the 75 or 150% ET due the lower root weight.

Non-irrigated asparagus had the lowest root weight while plants irrigated with drip had the highest root fresh weight (Table 17) and plants irrigated by drip had significantly more root mass than those irrigated by sprinkler. The irrigation application methods had no significant effect on root CHO content though total CHO levels were different from each other.

There was no difference between plots in root distribution in the spring of 2003, with the bulk of the storage roots located in the top 60 cm of the soil profile (data not shown). At the beginning of the 2004 growing season, plots harvested in the first year had fewer roots in the lower portion of the sampled profile than those not harvested until the second year after planting (not shown).
Irrigating asparagus with different amounts of water had a significant effect on root distribution (Figure 3) with both the 75% and 150% ET treatments having an increase in root mass throughout the soil profile when compared to 0% ET. The 150% ET treatment developed fewer roots in the bottom portion of the soil profile (14% of total reported in Table 16) when compared to 75% ET (25%). In addition, root growth in the 75% ET treatments grows more outward and downward than in the 150% or 0% ET treatments.
The irrigation method (Figure 4) also alters root distribution in asparagus. Drip developed a more evenly distributed root distribution throughout the sampled profile. However, in the sprinkler and non-irrigated treatments, a greater percentage (~25%) of the roots were distributed in the lower portion (60-90 cm depth) of the soil profile though both had lower total root mass (see Table 17). The isolines indicated the direction of root growth and indicate that the non-irrigated and sprinkler treatments are closer to the soil surface while plants irrigated with drip have a more downward growth pattern.

Significance of Findings:
Successful asparagus production requires some knowledge of optimal harvest practices, the impact of different irrigation systems, and different irrigation amounts over many production years. This study demonstrates that harvest pressures, irrigation methods, and irrigation amounts do affect the growth and development of young asparagus plants. Due to the young age of the plants and the limited number of treatment years, the early trends suggest that there are benefits and disadvantages associated with the different harvest pressures, irrigation methods, and irrigation amounts on asparagus productivity.

EFFECT OF EARLY HARVEST PRESSURE ON ASPARAGUS PERFORMANCE
Plants harvested one year after planting had a higher total combined yield (5013 kg/ha) at the end of 2004 compared to those where harvest began two year (3381 kg/ha) after planting. This would be expected as one had a total of 9 weeks of harvest (2003 and 2004) compared to 3 weeks (2004 only). Shelton and Lacy (1980) applied harvest pressures of 0, 2, 4, and 6 weeks in the second year after planting in Michigan. The next year 4 additional weeks were added to each harvest treatment (4, 6, 8, and 10 weeks) in the third year after planting. In the fourth year they harvested all treatments for 6 weeks and found that those harvested for 4 and 6 weeks in the second year after planting had a significant reduction in total yield over the three years. Paschold et al. (2002) reported on a field trial of harvest treatments ranging from four to nine weeks in Germany and showed that eight weeks of harvests had a higher total yield than harvests of seven or nine weeks after 10 years of study. Our finding showed that
Plants where harvest began two year after planting had more stems and greater stem dry weight in 2004 than plants harvested one year after planting. Additional spears removal during harvest can leave plants with fewer and smaller buds available for fern establishment after harvest. These findings are in agreement with those of Drost and Wilcox-Lee (1997). The increasing length of harvest can also decrease the amount of time after harvest required for CHO replacement and can significantly affect root growth. Knaflewski and Krzesinski (2002) showed that shorter periods for fern growth and CHO replenishment contributed to a reduction in yield. A longer fern growth period, associated with earlier harvests, improves yields by lengthening the assimilation period. Any delay in fern growth would shorten the duration for CHO accumulation and root growth. For young establishing plants, sufficient time for photosynthesis is needed to ensure that CHO’s are available to grow a large root system, initiate many buds for the next harvest, and provide adequate time for maximum CHO storage. Additional harvest will be made in 2005 and 2006 to determine the long term effect of early harvest pressure on plant productivity. It is clear however, that growers need to carefully monitor crop yield in an attempt to minimize any detrimental influences on future productivity.

THE IMPACT OF IRRIGATION METHOD ON ASPARAGUS PRODUCTIVITY
The choice of irrigation method (drip or sprinkler) had greater impact on root growth than on fern growth or spear yield. Under conditions where drip and sprinkler systems received the same amount of water, we noted a decrease in root weight within the sprinkler treatments due to shallow wetting patterns. Since sprinkler irrigation applies water at higher rates, there is a potential of increased run-off, increased evaporation, and the possibility of development of a restricted root system. Sprinkler irrigated plots may experience some drought stress particularly as plant get larger. Evaluation of the root maps (Figure 3.9) for the sprinkler treatments illustrates that much of the root system was concentrated in the upper portion of the soil profile. Evaluation of soil water content profiles (data not shown) also illustrated that sprinkler irrigation does not wet the soil as deeply and soil moisture is more rapidly depleted than when applying water with drip irrigation. As a result of shallow rooting and faster dry down periods, drought conditions can occur prior to the next irrigation when using sprinkler irrigation systems. In contrast, drip irrigation applies water at slower rates, allowing less run off, less evaporation, and deeper penetration of water through the profile. This promotes more evenly distributed roots and increased the total root weight when compared to sprinkler systems which decreases the risk of a drought response.
Although yields were not significantly different between the drip and sprinkler systems, drip irrigated plants had a higher percentage of large spears. This may be a result of decreased drought stress, improved crown and bud development, or increased rooting depth. Increased numbers of spears in larger categories indicate healthy crown and bud development. Increases in spear size leads to increases in spear weight and yield and over the long-term could improve profitability. Additional evaluation of these treatments during the coming years will help answer these questions.

THE EFFECT OF IRRIGATION AMOUNTS ON ASPARAGUS PERFORMANCE
The seasonal weather conditions in 2003 and 2004 were quite different. In 2003, the fern growth period was hotter and drier (47 days with temperatures in excess of 32°C; 5.5 cm of precipitation; ETo of 54 cm over 96 days) than in 2004 (20 days above 32°C; 7.3 cm of precipitation; ETo of 53 cm over 106 days). Hotter and drier weather conditions increases water use and soils need to supply sufficient water to ensure plant growth and photosynthesis are not adversely affected. Others have noted a decrease in photosynthesis due to drought response brought on by increased temperatures (Pressman et al., 1989).
To accurately calculate ET one needs to have a more precise estimate of the crop coefficient (Kc). The only Kc values for asparagus available are given by FAO (Allen et al., 1998) for a Mediterranean climate and these are not very useful for young asparagus plants or for dry, arid climates. While accurate Kc values were not available for this study or for most asparagus production areas, we attempted to modify existing values to better estimate crop water demands. While our measurements of canopy cover were useful, more frequent measurements are needed during the growing season as the canopy develops. Fern number and height increased in the 75 and 150% ET as the growing season progressed, while there was no change in fern growth in the 0% ET treatment. Since percent canopy closure was taken only after fern establishment early in each year, additional fern measurements before each irrigation event during the summer could improve the Kc values. For different climates, for plantings of different ages and root development, and for use in production agriculture better measures of Kc are needed to help with irrigation scheduling.
Although the average rooting depth was similar for all ET treatments, the distribution of roots throughout the profile varied. Changes in rooting depth within the ET treatments had more influence on scheduling irrigations events than adjusting Kc values. While an average rooting depth was used for scheduling irrigation, from the root maps and distribution profiles, it was clear that rooting depth is more complex because of differences between treatments and changes over the growing season. Due to our sampling approach, it was not possible to collect additional root growth data during the production year. In evaluations of root growth using destructive plant sampling (Wilcox-Lee and Drost, 1991), they reported that there were significant changes in root number and root mass during the summer. Clearly, more work in this area is warranted.
New bud development begins after the asparagus fern is nearly fully established in early summer (Blasberg, 1932; Tiedjens, 1924 and 1926). Although asparagus is classified as drought tolerant, water stress results in sub-optimal CHO production and can place limits on the number and size of buds being initiated (Drost and Wilcox-Lee, 1997; Pressman et al., 1989). Since the 0% ET treatments received no irrigation during the bud development period in either 2003 or 2004, spear yields were significantly less and spear size greatly reduced when compared to the irrigated treatments. Much of this difference is due to the reduction in fern growth due to drought stress which would reduce CHO production and limit bud development. These finding are similar other studies that showed that drought stress decreases bud and spear size (Wilcox-Lee, 1987; Roth and Gardner, 1990; Drost and Wilcox-Lee, 1997; Drost, 1999).
It can be concluded that too little water hinders growth and development (Cannell and Takatori, 1970a) through decreased CHO production limited by water that is essential in photosynthesis (Pressman et al., 1989). Irrigating asparagus at 75 or 150% ET increased spear number and weight in the large and very large categories, increased root growth, improved CHO storage, and increased fern growth when compared to 0% ET. These initial findings agree with Drost (1999) who showed that after four years of irrigation treatments at 80% ET during the fern growth period, spear yields were only 9% higher than when water was applied at 40% ET. However, increasing irrigation from 0% to 40% ET increased yield by 26-31%. There was little difference in yield between the 40 or 80% ET in any one year, though over time differences in yield between the 40 and 80% ET tended to increase. Similar findings may occur in this study during subsequent years. However, when assessing if extra irrigation improves asparagus performance, it was noted that there was no difference in plant performance (root growth or yield) between the 75% and 150 % ET. These results are in agreement with most irrigation amount studies that say a medium application of irrigation is optimal for good asparagus growth and yield (Hanna and Doneen, 1958; Takatori et al., 1970a; Wilcox-Lee, 1987; Roth and Gardner, 1990; Drost, 1996; Drost, 1999).
Fern growth is however responsive to more frequent irrigation events and the high soil water content associated with the 150% ET treatment. This increase in fern growth caused a higher incident of aphid infestation, decreased root mass accumulation slightly, and decreased root distribution. Hartman (1981) noted that increasing irrigation applications resulted in higher fern number than no irrigation. Late summer fern growth stimulates buds to grow rather than stay dormant and be available for next harvest season (Blasberg, 1932; Drost, 1997). Many asparagus growers believe that an increase in fern growth is a good thing and indicates a healthy productive plant. However, fern growth is not an honest indicator of whole plant growth and development (Robb, 1984). In New Zealand, it was noted that treatments receiving higher irrigation amounts had increased fern growth during dry years (Wilson et al., 1996). They also noted that too much irrigation may stimulate excessive fern growth at the expense of storage CHO in the root system, buds, and future yields. Our study showed that the 150% ET treatments had more culled spears (woody, bent abnormally, and early branching) due to increased aphid damage. Plots affected by aphids had the densest fern canopy at the end of the growing season a good environment for aphid populations to develop. Although scientific measurements were not taken on this infestation, it can be noted that increased fern numbers caused by frequent irrigation events lead to higher insect problems and decreased yield due to extra fern growth.

CONCLUSIONS:
Early results of the irrigation and harvest pressure suggest that growers are generally applying too much irrigation water during the fern production period. Since fern growth is easy to observe, growers believe that dense, lush fern is needed for high future yields. This study suggests that excess fern growth does not improve root development, carbohydrate accumulation, or spear yield. Irrigation application method does not appear to influence plant performance or productivity however, we are continuing this project for several more years. Early harvesting (year after planting) increases early yield but could have long-term implications for future productivity. As more information is collected, a better understanding of how plant growth is impacted by harvest may be realized.

Research conclusions:
Objective 1

Implications for Commercial Asparagus Producers
While some cultural practices are similar between Washington and California, differences exist in climate, markets, harvest starting dates and durations, soil types, and other production practices. Washington has a shorter growing season with cooler temperatures but longer summer day lengths, while California’s growing season starts a few months earlier in the year and lasts longer (Benson, 2002). These differences in climate influence crop physiology and affect plant growth, development rates, and CHO accumulation. California historically has had higher yields than Washington (Anon, 2004) but this was not the case is this study when average total yields are compared between states. Washington averaged about 4,200 kg/ha for farms where two years of data were submitted, while California farms averaged about 3,800 kg/ha. This difference is mainly due to the higher plant populations in Washington compared to California, which contributes to higher early yield or to different harvest practices between the two states.
In Washington, asparagus has been grown primarily for the processed market while California product is sold to the fresh market (Mullen, Whiteley, Viss, Goff, and Cancilla, 2002; Benson, 2002). These market differences create some variation in harvesting practices like spear length and when the harvest ends. California producers usually trim spears to 22.5 cm (Mullen et al., 1998) while Washington growers trim spears as short as 18 cm. Increased spear lengths lead to increases in total weight per spear and thus potentially a higher total yield. Longer harvest duration can be influenced by the market. High prices or high demands at the packaging or processing plant will encourage longer harvests while lower prices or no demand will encourage shorter harvest durations.
The soil types also vary between these two growing regions. In Washington asparagus is grown on sandy soils while in California, farms are organic based soils. These soil types have different water-holding capacities with the sandy soils holding the least amount of water. The suggestion for irrigating in the California farms is three to four time during the summer (Mullen et al., 1998) while Washington asparagus growers should apply 10-15 cm at least once every 3-4 weeks during the fern growth period (Dean et al., 1993). No detailed guidelines are available that take into account plant age, stage of plant growth, and future yield considerations. Since production practices vary from farm to farm, the two states and the farms within the states can be compared, much of the variation in results can be attributed to the differences in growers’ philosophies regarding cultural practices.

Harvest Duration and Strategies
Harvest regimes can have a significant influence on asparagus growth and development (Drost and Wilson, 2003). Stored CHO availability in the spring is limited by accumulations from the previous year (Wilson, Sinton, and Wright, 1999). Asparagus growers must balance carbohydrate use for spear harvest with that needed for fern establishment thus allowing healthy fern growth and further development of the crown, buds, and roots. Healthy fern growth and development can increase the root storage capacity (root mass) and storage content (CHO concentration). Increasing the length of harvest or the amount of harvested product means more CHO is used during the harvest period. Longer harvest periods usually mean a shorter CHO assimilation and plant development period, both of which occur primarily after fern establishment (Wilson, Cloughley, Jamieson, and Sinton, 2002b). Extended harvests also reduce long-term total spear weight (Haber, 1932), decrease lifetime yield (Shelton and Lacy, 1980), decrease overall spear quality (Paschold et al., 2002), and contribute to a decline in plant population (Takatori et al., 1970). As little as 10 to 20 days shorter period for fern growth and carbohydrate assimilation period can impact the crop (Knaflewski and Krzesinski, 2002).
The duration and amount harvested in Washington in 2003 was similar for all but Farm D, which harvests for one week longer, resulting in 300 kg/ha more product harvested. This effectively reduces or shortens the photosynthetic period since asparagus requires 4-5 weeks to develop its canopy. A shorter photosynthetic period may result in decreased volume of CHO production and storage. Washington Farm D also had the second highest stand reduction percentage, which may have been caused in part by the longer harvest (Takatori et al., 1970) or an excess depletion in root CHO storage (Drost and Wilson, 2003). Washington Farm D had a 67% increase in root mass, which places it in the middle in this category. It appears that heavier harvest may reduce stands, but losses in crop growth and development can be limited with an establishment of a modest number of spears per meter and healthy fern canopy (Wilson, Cloughley, and Sinton, 2002a). The extra harvest in Washington Farm D appears to have been balanced with modest fern growth; however, this farm did begin and end with the lowest root mass. Other unknown factors during the establishment year of 2002 may have influenced this farm more than the others evaluated.
It is a common practice in Washington to till asparagus fields for weed control. If tillage is done during spear growth, the new spear emergence can be delayed by one week (Dean et al., 1993). Tillage operations at the end of harvest can also delay fern growth and establishment (Putnam, 1972; Wilcox-Lee and Drost, 1991). In Washington, Farm E finished the harvest period about 18 days earlier than most of the other Washington farms and 25 days sooner than Washington Farm D. An 18 to 25 days head start on fern growth for Farm E may be due to differences in cultivation timing, better climatic conditions, or some other unknown factor. In Washington, Farm E an earlier harvest was a contributing factor to maintaining the plant populations and increasing root mass accumulation. With increasing root mass, more storage room is available for CHO, making the potential for future yields to increase (Wilson et al., 2002b). Knaflewski and Krzesinski (2002) demonstrated that the length of time a plant has available after the harvest period and before dormancy in part determines CHO accumulation and plant development. As little as 10-20 days of delay in fern establishment can adversely affect future yield. Others have shown that as little as one week shorter harvest period (8 weeks compared to 9 weeks) can increase lifetime yields and spear quality (Paschold et al., 2002). There appears to be a very small margin between optimal and excessive harvest durations particularly in young plantings.
California asparagus growers also used tillage as part of their weed management strategies. In addition to tillage before and after harvest, most California growers reported that they tilled after fern establishment during the summer. Tillage during the established fern period can damage the fern canopy, induce fern re-growth, and contribute to CHO reductions while tillage in general has been shown to reduce root growth and development (Putnam, 1972; Wilcox-Lee and Drost, 1991).
California had significantly more variation between farms in spear harvest in 2003. Farms D and E harvested the most while Farm A did not take a harvest during the year after planting. By the end of 2003, Farms D and E (1150 kg/ha) had the smallest percent increase in root mass, 8% and –5%, respectively, and the highest percent stand loss, 14% and 18%, respectively. The 5% decrease in root mass on Farm E may be due to excessive tillage (Wilcox-Lee and Drost, 1991), the grower reporting some herbicide injury affecting fern performance, or by root sampling variability. Farm A showed the largest percent increase in root mass during 2003 which may be a result of more CHO available for root growth and development due to the very long fern growth period. Unfortunately, in the spring of 2004, Farm A and 11,000 acres of surrounding land was flooded by a break in the water channel that surrounded Jones Track resulting in over 20 feet of standing water for several months. No further data were available to monitor long-term effects of Farm A’s production decisions.

Irrigation
When irrigation methods used on commercial asparagus farms were considered to determine plant health and root mass accumulation, furrow irrigation appears to benefit asparagus crops. Furrow irrigated farms had the lowest losses in plant population and a higher percentage of root mass in the lower half of the sampled soil profile (Reijmerink, 1973; Drost and Wilson, 2003). Having deeper roots could improve drought tolerance and provide a larger volume of soil to grow new roots. One may expect that as these crops continue to grow the differences in root location and accumulation may change.
Irrigation frequency and duration can also play a role in the root mass development. In Washington, Farm E watered the least frequently and had the highest increase in root mass, while Farms B and C irrigated for the shortest duration (the least amount of water), resulting in the smallest increase in root mass. Deep watering and less frequent irrigations appear to encourage the development of a deeper-rooted asparagus plant. Roots in the top portion of the soil profile are located where water can evaporate or be used by other plants. Shallow-rooted crops or younger plantings can be exposed to drought conditions during hot weather, water shortages, or if there is a malfunction with the irrigation system (Sterrett, Ross, and Savage, 1990).
For the two irrigation systems commonly used in California, it was noted that drip-irrigated farms (D & E) had the highest plant losses during the establishment years. This may be due to higher disease incidence caused by increased irrigation frequency, an increase in weed pressure requiring additional cultivation and herbicide applications, or excessive harvest pressure. Mullen et al., (1998) suggests watering asparagus three times during the summer to reduce disease. Farms D and E exceeded that suggestion by irrigating every two to three days.
Drip-irrigated farms at the beginning of this study had the greatest root mass and this continued through the fall of 2004. Drip irrigation appears beneficial to plant growth and development but the differences are probably explained better by the frequency in irrigation. All of the sampled furrow-irrigated fields were watered only once during the summer, much less frequently than the drip-irrigated fields or what has been recommended (Mullen et al., 1998). When asked how irrigation requirements were determined in California, only Farms D and E reported that soil moisture levels were measured. Other farms reported a set frequency to determine irrigation events. Irrigating based on soil water measurements, rather than recommended frequencies, resulted in higher stand reduction rates in California but increased the overall crop growth and development. However, while reduced irrigation events prevented some stand loss within the first three years after planting, the cost was a decrease in root growth and development. Plants with a smaller root mass may be weakened further when harvested for longer periods in later years and a field may not yield as much over its life span. Reduced watering may save a percentage of the plant stand but irrigating to plant needs will lead to increased lifetime yields and possible greater stand longevity. More careful monitoring of soil moisture may be an important component for increasing root growth and development regardless of irrigation method.

CHO and Root Mass
Some commercial farms are currently monitoring root CHO (www.aspireus.com), and these values are used as an indicator of crop energy availability during harvest and fern growth and recovery over the summer. While CHO content is important, a better understanding of root mass is needed if yield potential is to be predicted (Robb, 1984). The seasonal changes in CHO content for the different farms looked similar, but when root mass was factored into the equation it was clear that not all farms were equal. Farm differences are caused by harvest pressure, irrigation methods, and irrigation amounts. Thus, CHO values are not fully representative of plant growth and development.
California was similar to Washington when comparing the benefits of monitoring and expressing CHO as mg fructose/g root fresh weight (Figures 2.1 and 2.4). However, questions were raised about very high values on some California farms during the late summer. Dehydration decreases root water content while the fructose remained the same, thus giving a higher CHO value. Growers may falsely believe that a field is in good condition going into winter and then when sampling again the following spring, they record much lower CHO levels. AspireUS (www.aspireus.com) does not have a method to compensate for this late summer drought and is not accurate during this period. However, these CHO values can still help growers balance the CHO use between harvest and fern growth periods.
Determining the total amount of CHO/ha should be a more reliable way to predict future yield potential. If the amount of CHO needed for spear growth and fern growth is known, a grower could accurately estimate harvest potential (Wilson et al., 2002b). Knowing the potential yield in kg/ha or spear/ha for each field would be more beneficial than determining the harvesting period by generalized time frames. Although CHO levels were similar in this study, there are differences in the total amount CHO/ha available for future yields and growth between the California farms. These values ranged from 2,500 to 7,500 kg CHO/ha by the end of 2004. Currently asparagus growers adjust the length of the harvest season according to a crop's age and the amount of product harvested in the previous season. As a result of this study, several farms in California are now monitoring root CHO levels, assessing root biomass accumulation, and using this information to better manage plant performance.

Objective 2

Early results of the irrigation and harvest pressure suggest that growers are generally applying too much irrigation water during the fern production period. Since fern growth is easy to observe, growers believe that dense, lush fern is needed for high future yields. This study suggests that excess fern growth does not improve root development, carbohydrate accumulation, or spear yield. Irrigation application method does not appear to influence plant performance or productivity however, we are continuing this project for several more years. Early harvesting (year after planting) increases early yield but could have long-term implications for future productivity. As more information is collected, a better understanding of how plant growth is impacted by harvest may be realized.

We have an additional 2 years before this project is completed and are continuing to collect data on irrigation, plant growth and yields for aspargus irrigated at 0, 75 or 150% ET with drip and sprinkler systems. Final results will be prepared in the spring of 2007.

Participation Summary

Educational & Outreach Activities

Participation Summary:

Education/outreach description:

In 2003, final reports were presented to the Washington and California asparagus industries. Randy Seth Peterson is working on his masters thesis and is expected to complete this in late 2004. Information collected to date has been shared with the Washington, California and Michigan asparagus industies at summer and winter 2003 meetings.

1) Invited Grower Oriented Presentations: (Computer generated and delivered)
Drost, D. December 7, 2004. Improving Asparagus Performance: Growth and Carbohydrate Monitoring in Commercial Asparagus. California Asparagus Growers Meetings. Stockton, CA. (55 persons)
Drost, D. March 2004. Improving Asparagus Performance by Carbohydrate Monitoring: AspireUS. Ontario Canada Asparagus Growers Meetings. Simcoe, Ontario, Canada. (65 persons)
Drost, D. March 2004. Asparagus Growth and Physiology. Ontario Canada Asparagus Growers Meetings. Simcoe, Ontario, Canada. (65 persons)
Peterson, S and D. Drost. December 9, 2003. Monitoring Plant Growth and Carbohydrates in Young Asparagus Fields. California Asparagus Growers Meetings. Stockton, CA. (60 persons)
Drost, D. December 2003. AspireUS: Improving Asparagus Performance by Monitoring Root Carbohydrates. Great Lakes Fruit, Vegetable and Farm Market Expo. Grand Rapids, MI. (75 persons)
Drost, D. December 2003. The Physiology of Asparagus Growth. Great Lakes Fruit, Vegetable and Farm Market Expo. Grand Rapids, MI. (75 persons)
Drost, D. March 2003. AspireUS: Information Support System for Asparagus. Michigan Asparagus Growers Annual Meetings. Hart, MI. (110 persons)
Drost, D. March 2003. Getting to the Root of Asparagus Productivity. Michigan Asparagus Growers Annual Meetings. Hart, MI. (110 persons)
Drost, D. November 2002. Getting to the Root of Asparagus Productivity. Pacific Northwest Vegetable Association Meetings. Pasco, WA. (45 persons)

2) Extension Oriented Papers
Drost, D. December 2003. AspireUS: Improving Asparagus Performance by Monitoring Root Carbohydrates. Great Lakes Fruit, Vegetable and Farm Market Expo. Grand Rapids, MI. www.glexpo.com/abstracts/2003abstracts/asparagus.pdf
Drost, D. December 2003. The Physiology of Asparagus Growth. Great Lakes Fruit, Vegetable and Farm Market Expo. Grand Rapids, MI. www.glexpo.com/abstracts/2003abstracts/asparagus.pdf
Drost, D. April 2003. Asparagus can improve with root carbohydrate testing. The Vegetable Growers News. pg. 28-29.
Drost, D. 2002. Getting to the Root of Asparagus Productivity. Proceedings Pacific Northwest Vegetable Association Annual Meetings. pg. 145-149.

3) Yearly Summaries of Research Efforts
Drost D. and S. Peterson. 2004. Assessing New Asparagus Plantings: Monitoring Root Carbohydrates and Plant Growth. Prepared for California Asparagus Commission.
Drost D. 2003. Assessing New Asparagus Plantings: Monitoring Root Carbohydrates and Plant Growth. Prepared for California Asparagus Commission.
Drost D. 2003. Assessing New Asparagus Plantings: Monitoring Root Carbohydrates and Plant Growth. Prepared for Washington Asparagus Commission.
Drost D. 2002. AspireUS - Washington Asparagus Information Support System. Prepared for Washington Asparagus Commission.

4) Graduate Thesis and associated papers
Evaluating Asparagus Productivity by Assessing Farm Practices, Irrigation Methods, and Harvest Pressure. by Randy Seth Peterson. MS Thesis. 2005. Utah State University. Logan, UT

research papers being developed for publication.

Project Outcomes

Project outcomes:

Farmer Adoption

Objective 1

Growers participating in this project are now more closely monitoring their harvest practices to ensure that they do not over-harvest. In addition, growers are now monitoring the CHO content of asparagus roots to ensure that plants are not over-harvested and that seasonal production and crop management practices optimize CHO accumulation.

At present more than 10,000 acres in North America are using this system.

Growers are also paying more attention to water monitoring and water management as a result of some of this work. Irrigation frequency in drip irrigated asparagus fields are now being more closely monitored to ensure that application amounts meet plant and soil water requirements. While management in furrow irrigated fields is more difficult, growers have increased the number of irrigation events each year from 1 to 3 ensuring that plants are not subjected to water stress events during the fern growth period. Growers using sprinkler irrigation are also trying to apply large quantities of water less frequently to ensure water penetrates to deeper regions in the soil thus helping to increase rooting depth.

Objective 2

It is still too early to assess if the information generated will be adopted by asparagus growers in the US. There has been a great deal of interest but growers are at times slow to adopt new practices. However, several are using the AspireUS CHO monitoring systems to measure changes in root sugars and relating that to spear yield each spring. This information helps them decide if they can harvest longer or if they should stop and allow the plants to grow fern to recharge the root system.

Much of the ET based work is of interest but we have found that we need to know more about root system development, crop coefficient changes during and between years, and water extraction from throughout the soil profile.

Recommendations:

Areas needing additional study

Objective 2

Since 2 years is not adaquate to assess how irrigation amounts or systems impacts long-term plant performance, we will continue to harvest, measure and monitor asparagus plant growth and development until the summer of 2007. Early indicators suggest that there are real differences in root development, root biomass, and plant productivity between drip and sprinkler irrigated fields but that increasing water applications above 75% ET does not improve plant performance. This then would mean that there could be significant savings for growers by reducing water applications without impact plant productivity.

Work will continue in 2004. Information we are collecting will be spear yield, change in root biomass (in replicated trial and field sites in CA and WA), plant growth evaluations during the summer of 2004 and soil moisture monitoring. Information collected will be shared with the California and Washington asparagus industries at summer field days and winter educational meetings. Information will be presented at the ASHS summer meetings in 2004 and the preliminary results published late in 2004.

Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.