Developing Weed Control Methods for Organic Raspberry Production

PROJECT BACKGROUND
Public concern over the use of pesticides in foods has increased the demand for organically grown raspberries in both the fresh and processed food markets. To meet this demand, a complete departure from conventional synthetic pesticides and fertilizers is required.

Raspberries present special challenges to organic production, especially with respect to in row weed control. Weeds compete directly with crops for water, nutrients, space, light and pollination. Due to their tall, sparse stature and slow growth habits, raspberries are not good competitors against fast growing weed species. As raspberries are perennials, the use of many effective organic weed control methods are often precluded. In addition, raspberries have extremely shallow root systems that are easily damaged, and a form or reproduction, suckering, which make in row machine cultivation nearly impossible.

Traditional methods of organic weed control are often associated with being labor intensive and therefore costly. As a result, interest has developed in reevaluating older, effective methods of weed control and adapting their use to large scale commercial operations. In addition, work is proceeding in the development of natural herbicides suitable for use in organic cropping situations.

An older form of weed control receiving renewed interests is mulch. The benefits of mulching are many: water conservation, weed control, temperature modification, protection and improvement of soil structure, reduced nutrient leaching and cleaner fruit. There are also certain problems associated with organic mulches: induced nitrogen deficiency, cooling or heating of soils at the wrong time of year, weed growth if mulches are improperly applied or inadvertently removed, and weed seeds in the mulch itself, all of which may result in low yields and poor fruit quality. There is much left to be investigated regarding proper timing and concentrations of mulch and organic fertilizer applications in raspberries.

A new product that is currently receiving attention as a natural herbicide and fertilizer is corn gluten meal (CGM). CGM is a corn by product with allelopathic characteristics which inhibits root growth in germinating seeds, rendering it useful as a natural, pre-emergence herbicide. It can also serve as a natural, slow release fertilizer source, as it is approximately 10% nitrogen by weight. Little is presently known of the effectiveness of CGM as a herbicide in small fruits. However, results from studies performed on strawberries appear promising, and may be transferable to raspberries. Work is in progress to increase the effectiveness of CGM as a herbicide and develop application methods suitable to mechanized commercial applications.

To succeed in producing high yields of organically grown raspberries in commercial plantings, growers need effective in row weed control strategies that are compatible with organic crop production methods and commercial agricultural technology. To assist in these goals, this study was developed to evaluate the effectiveness of CGM as an in row natural herbicide and fertilizer, alone, and in combination with straw mulch, along with a determination of their impact on raspberry yields and harvest dates.

PROJECT DESCRIPTION AND RESULTS
A sizable perennial weed population composed primarily of dandelions and cinquefoil had established itself prior to the beginning of this study and is depicted on May 11, Graphs 1 and 2. On that date, there were no significant differences in weed cover between treatments. However, the perennial weeds interfered with the study’s ability to determine CGM’s effectiveness as an inhibitor of annual weeds. CGM’s herbicidal activity is to prevent the establishment of weeds by inhibiting root formation of germinating seeds, (Christians, 1993), it has no know herbicidal effect on plants with established root system. To remedy the situation, the perennial weeds were removed after recording the percent weed data on May 11.

[Editor’s note: There are numerous graphs and charts that go along with this report that could not be posted online. If you would like to see these please email us at ncrsare@umn.edu or call us at 800-529-1342. Thanks]

The first significant difference I weed cover between treatments appeared May 18. Graphs 1 and 2 show a considerably higher percent of weeds in the CGM/straw and CGM 97.7 treatment plots over the other treatment groups. CGM 97.7 had a statistically significant higher percent weed cover than the hand weeded, straw only and CGM 195.4 groups. Due to a greater degree of variability within the CGM/ straw group, the same differences were apparent, but not to a significant degree.

This finding is important in that it indicates that CGM at the 195.4 g/sq m rate, 21 days after application, was comparable to the hand weeded and straw only treatments, both of which had continued to receive hand weeding at his point I the study, while the CGM 195.4 plots had not. It is also an indication of a higher degree of effectiveness in weed control of CGM at the 195.4 g/sq m rate, over the lower rate of 97.7 g/sq m. this finding agrees with a 21 day field study with strawberries, where soil incorporated CGM applied at higher rates, 196 g/sq m to 490 g/sq m, increased CGM’s effectiveness as an herbicide and weed cover percentages were reduced by amounts comparable to standard herbicides (Nonnecke and Christians, 1993).

May 29, the CGM 195.4 treatment group showed significantly higher weed cover percentages than the hand weeded and straw only treatments. Both the hand weeded and straw only treatments had received weeding, where as the CGM 195.4 treatment had not. In addition, the straw only and CG

This finding is an indication that CGM’s potency as a herbicide had substantially diminished 32 days after application. This is in close agreement with the reported five week limit of CGM’s effectiveness as a herbicide (Jesiolowski, 1996).

After harvest data analysis was based on data collected from all canes contained in the experimental planting. Analysis of variance (ANOVA) techniques appropriate to the randomized block design and two-tailed, paired t tests were implemented to determine significant differences between treatments at a 5.0% rejection level.

Materials and Procedures
The location of this study was a small fruit grower’s filed in Washington County near Forest Lake, Minnesota. The soil was a mixture of Gotham and Zimmerman loamy sand. The field had been planted in rye in 1994. This was followed by a planting of barefoot, ‘Autumn Bliss’ primocane-fruiting red raspberries in the spring of 1995. During the establishment year, standard cultural practices for fertilization, pest and weed control were followed.

Early spring of 1996, a 10.5 x 12.4 meter section of the raspberry field was staked off and divided into 20 plots, each plot measuring 2.1 x 3.1 meters. The raspberry plants were located in the center of each plot, in north-south running rows, over the 3.1 meters. The experimental design was a randomized block design consisting of five treatments with four replications per treatment. The treatments consisted of:
1) Hand weeded
2) Straw mulch only
3) CGM 97.7 grams/square meter (0.2 lbs/10 sq ft) and straw mulch
4) CGM 97.7 grams/square meter (0.2 lbs/10 sq ft)
5) CGM 195.4 grams/square meter (0.4 lbs/10 sq ft)

A weedy control treatment group was not an option due to grower reluctance to allow the occurrence of unchecked weed growth. Therefore, the hand weeded treatment groups served as the control.

April 27, 1996, prior to primocane emergence, all plots were weeded by hand to remove emerged annual and perennial weeds. CGM was applied in the plant rows to the appropriate plots by incorporating it into the top 5.0 to 7.5 cm of soil. Between April 27 and May 11 a number of perennial weeds, consisting primarily of dandelions and cinquefoil emerged in the experimental planting. As CGM’s usefulness as a herbicide is in inhibiting root growth in germinating seeds, it is not effective in eliminating perennial weeds with established root systems. A determination was made to hand remove the perennial weeds from the experimental plots. Perennial weed removal was performed on May 11. Beginning May 11, the hand weeded plots received weeding on a weekly basis throughout the growing season. The straw only plots also received hand weeding until May 29, at which point 15.0 to 20.0 cm of straw mulch was added. The CGM/straw plots did not receive additional weeding prior to straw mulch placement on May 29. During the growing season straw was added to these plots as needed. With the exception of the perennial weed removal, the CGM 97.7 and CGM 195.4 plots were to receive no further weed removal treatments. Between row cultivation for weed control was performed mechanically and by hand. No additional weed control measures were implemented. Throughout the growing season no remarkable disease or insect problems were encountered. Overhead sprinkler irrigation supplemented natural rainfall during the growing season.

At predetermined time intervals, field measurements were obtained by selecting, at random, five representatives canes from which to record cane height and node numbers, and five representative sites to obtain percent weed and percent plant cover for each of the 20 plots. In addition, a record of the predominant weed species appearing in each plot was made.

Fruit began maturing on July 29, and was harvested from all canes within a plot, every three to seven days, through October 2. A hard frost ended further harvesting. Yield, berry number and average berry weight were recorded for each individual plot. Berry number was obtained by actual berry count. Average fruit weight was determined by dividing the total weight of fruit collected per plot by the actual berry count of the plot.

After harvest, a final field data recording was performed for percent weed cover, percent plant cover and cane height. All canes were later removed from the field by cutting the bases at soil level. Measurements were obtained for cane height (from base of cane to the terminal bud), cane diameter (at 15 cm from the base), primocane fruiting and non fruiting nodes, number of laterals per primocane, fruiting and non fruiting, and number of nodes per lateral, fruiting and non fruiting. For classification purposes, canes were also described as fruiting or non fruiting. A fruiting cane was characterized as having matured, harvestable fruit which had been picked and therefore contributed to the yield figures obtained during the harvest period. A non fruiting cane described either a cane on which no fruit had set or a cane with immature fruit, all of which never reached maturity by the end of the harvesting season and therefore did not contribute to the recorded yield figures. A third classification, total canes, included both fruiting and non fruiting canes.

Two complications occurred during the course of this study. The first problem encountered was erratic stand establishment. Some plots contained an abundance of canes while other plots had only a few. This may have been a weather related problem. The winter of 1995-96 was characterized by long periods of sub-zero, record breaking low temperatures. Winter was followed by very cool spring temperatures into late May. According to the grower, the entire planting was slow to emerge and stunted, with mature canes being 30 to 45 cm shorter than in past years. The second problem occurred on August 9, when the grower’s employees mistakenly weeded the CGM 97.7 and CGM 195.4 test plots. Fortunately, this happened late enough in the course of this study so that determinations could be made of CGM’s effectiveness as a herbicide.

Statistical Analysis:
All field measurements and observations were performed by the same field assistant to ensure consistent data collection throughout the duration of the study. The experiment was laid out in a randomized block design consisting of five treatments with four replications per treatment. The five random measurements within each plot were averaged to arrive at an average measurement per replication. The four replication per treatment averages determined the treatment average for use in tow-tailed, paired t-tests which established the existence of significant differences between treatments at a 95.0% confidence level.

CGM’s Effectiveness against the Predominant Weed Species:
Application of CGM to the test plots occurred April 27. It was expected to be effective as a herbicide until approximately June 1. During this time period the major weed species encountered in the test plots consisted of wild buckwheat, shepherdspurse, lambsquarters, redroot pigweed, and common ragweed. Through observation and the recording of field data, it was determined that CGM was most effective against redroot pigweed, wild buckwheat and shepherdspurse. Lambsquarters, appearing in every plot, was the most prolific weed. Its numbers were reduced by CGM, most noticeably at the 195.4 g/sq m rate, but it continued as the major noxious weed pest. Ragweed started to appear as the effectiveness of CGM started to wane. As ragweed was absent from the CGM test plots for more than a week after it had started in the hand weeded test plots, the possibility exists that ragweed was being repressed by the CGM during that time period.

CGM was effective in controlling annual weeds in the test plots for a period of four to five weeks. Furthermore, its effectiveness as a herbicide was more noticeable at the 195.4 g/sq m rate than at the 97.7 g/m rate.

Percent Plant Cover, Growing Season Data:
Graph 3 compares the average percent plant cover of each treatment by date.

Throughout the growing season the straw treatment plots consistently displayed the greatest amount of percent plant cover. The straw only plots also contained the highest number of canes per plot. August 29, both CGM 97.7 and CGM 195.4 show significantly less plant cover than the straw only plots. On October 5, CGM 195.4 again displays significantly lower percent plant cover than the straw only treatment group. Other than these few occasions, there were no significant differences in plant cover between treatments.

A problem which affected the percent plant cover was erratic stand establishment. Some plots contained an abundance of canes while others only a few. The three poorest stands were located in CGM treated plots. It was felt that poor stand establishment was not a treatment effect as it was not consistent for any particular treatment. For example, CGM 97.7 contained the plot with the lowest number of canes (three), and the plot with the highest number of canes (31). The most likely cause for the problem was the weather. A recent study experienced failed cane emergence and lower cane counts with ‘Autumn Bliss’ at two northerly locations, suggesting that ‘Autumn Bliss’ root systems may not be as cold hardy as other northern grown cultivars (Prive, Sullivan, Proctor and Allen, 1993). The winter of 1995-96 was characterized by long periods of sub-zero, recording breaking low temperatures, followed by very cool spring temperatures into late May the entire planting was slow to emerge and stunted.

After May 29, CGM 97.7 and CGM 195.4 were similar in percent weed cover, with no significant differences between he two treatments reported throughout the balance of the study. This indicates that the higher degree of weed control gained by the higher concentration of CGM 195.4 over CGM 97.7 early in the season, was rapidly lost as weeds were allowed to emerge in the no longer hostile soil environment. Furthermore, these findings suggest the necessity of implementing additional weed control measure, such as additional CGM, mulching or hand weeding.

From May 29, to the end of the growing season, CGM 97.7 and CGM 195.4 displayed significantly higher percentages of weed cover over the other three treatment groups for the majority of the reporting dates. An exception to this occurred August 9, when the CGM 97.7 and CGM 195.4 plots were accidentally weeded, on this date there were no significant differences between the CGM 97.7 and CGM 195.4 and the other treatment groups. From August 16 through August 29, there continued to be no significant differences between the CGM 97.7 and CGM 195.4 and the straw only and CGM/straw groups. After these dates, significant differences reappeared. It is interesting to note that on July 31 weed percentages in these plots before weeding and after thirteen weeks of study, were between 18.8% and 20.0%.Within six weeks after weeding, the weed population rebounded to even higher levels of 21.4% and 22.4%.

The straw only and the CGM/straw plots showed similar wed cover percentages for the majority of the season, although weed percentages were slightly higher for the CGM/straw group. An exception to this was the time period of May 29 through July 5, where the CGM/straw plots were significantly higher in percent weed cover than the straw only plots. This was primarily due to the lower cane height and uneven raspberry plant spacing in the CGM/straw treatment group early in the growing season, which resulted in less straw cover in the plant rows and a greater opportunity for weeds to establish themselves. Once the CGM/straw group reached sufficient size, more straw mulch was added and the significant differences disappeared.

The only treatment to rival the hand weeded plots in percent weed cover was the straw only group. Table 1 displays percent weed cover by treatment during the growing season and provides an average from May 11 through July 31, before the accidental weeding. During that time period, there was no significant difference between the hand weeded plots and the straw only plots in percent weed cover. When comparing the two treatment averages over the entire season, significant differences develop. This was primarily due to the weeds that had emerged through the straw becoming increasing larger in size, and thus encompassing a larger area in the raspberry rows, which translated into higher weed cover percentages. This suggests that when using straw mulch for in row weed control, benefits may be obtained by conducting a midseason, hand harvest of weeds.

As there was no weedy control in this experiment, it was not possible to properly evaluate the effectiveness of the CGM 97.7 treatment group over the weed population.

In comparing percent weed cover to percent plant cover between treatments, the variable stand establishment appeared to have an effect on percent weed cover. While raspberries are not generally considered good weed competitors due to their growth habits, in most cases, the plots with the greatest cane numbers had the highest percent plant cover and the lowest percent weed cover. Good stand establishment proved to be important in the straw mulched plots. Plots where there were only a few canes, or where the canes were not evenly placed in the row, were often in need of straw replenishment, as there was little plant material to hold the straw mulch in the plant row when strong winds developed. This problem often left bare spots of soil which weeds were quick to exploit. The CGM/straw plots were more problematic in this respect due to the lower cane densities. This aspect of erratic stand establishment showed up in the early season as the significant differences in the percent weed cover between the CGM/straw and the straw only treatments. The straw only plots, with their greater cane numbers, were more successful in retaining straw cover and experienced less weed competition as a result.

Average Can Height and Cane Diameter, Growing Season Data:
Graph 4 compares the average can height of each treatment by date throughout the growing season.

Throughout the growing season, there were no significant differences in cane height between hand weeded, straw only and CGM/straw treatments, with the exception of the May 29 and June 6 where the straw only canes were significantly taller than the hand weeded canes. The straw only canes also registered significantly taller than the CGM 97.7 and CGM 195.4 canes on May 29. When cane height recording began on May 11, the straw only plots were ahead of the other plots in height, they retained this position until July 5, at which point CGM/straw and hand weeded canes began to surpass them. Mid and late season there were a number of significant differences in cane height, with the CGM 195.4 and CGM 97.7 treatments displaying significantly shorter canes than the hand weeded, straw only and CGM/straw treatments. Between CGM 195.4 and CGM 97.7 there were no significant differences in average weekly cane height.

Superior cane height early in the season appears to have advantaged the straw only plots over the CGM/straw plots with respect to weed control. The higher proportion of taller plants in the straw rows enabled the placement of a deeper layer of straw to exclude weed species. The slower emergence and lesser plant height of the primocanes in the CGM/straw plots necessitated a thinner layer of straw which was not as effective in controlling weeds, as many were able to punch their way through the lighter straw layer. Also, with smaller a slower plant numbers in the CGM/straw plots, the wind was more likely to blow straw off of the plant rows as there was little or nothing to hold the straw in place. This was not a significant problem in the more densely populated straw only plots.

Harvested Cane Data, Average Cane Height:
Graph 5-8 compare average cane height by treatment and type.

The data from the canes harvested at the end of the season correlated fairly closely with the weekly cane height data. The average cane height of the total canes and the fruiting canes were similar, with the CGM/straw and hand weeded treatments being significantly taller than the other three treatment groups. A comparison of the non fruiting canes found the CGM/straw canes were significantly taller than the hand weeded and the CGM 195.4 canes.

Cane Diameter:
Graphs 9-11 compare cane diameter by treatment and cane type.

The CGM/straw treatment group displayed the largest cane diameters for total, fruiting and non fruiting canes. In comparing total canes, the CGM/straw group was significantly larger than the CGM 195.4, CGM 97.7 and straw only treatments. In fruiting canes, the CGM/straw group displayed significantly larger cane diameters over the CGM 97.7 and straw only treatment groups. For non fruiting canes, CGM/straw canes were significantly larger than the straw only, CGM 195.4 and hand weeded treatments. The hand weeded treatment group was significantly larger in cane diameter than the straw only treatment group for total canes.

While not considered major yield components in raspberries, benefits relating to cane height and cane diameter have been found. Cane diameter has been associated with an increased carbohydrate supply within plants, which suggests an indirect effect on cane yield (Crandall, Chamberlain and Biderbost, 1974). In addition, cane diameter and cane height were found to be factors related to the yield potential of over wintering canes (Dale, 1986). Recent research with primocane fruiting red raspberries found that study location reporting the largest cane diameters also reported the highest yields (Prive, Sullivan, Proctor and Allen, 1993). The combination of CGM and straw mulch may have been beneficial in producing the greater cane heights and diameters in the CGM/straw group over the other treatments. A balanced C:N ratio favors carbohydrate storage in plant tissue, and straw mulch may lead to an imbalance in this ratio, necessitating the addition of supplemental nitrogen in such planting systems (Preece and Read, 1993). The 10% nitrogen content in the CGM may have prevented C:N imbalance in the CGM/straw treatment group. Benefits of straw mulch include the conservation of moisture and the lowering of soil temperatures which help keep the crop root zones from becoming too hot. If roots overheat, excessive respiration occurs and exhausts stored food and reduces growth (Preece and Read, 1993). The significant differences in cane height and diameter between CGM 97.7 and CGM 195.4 and the CGM/straw treatment group may have been in part, due to greater moisture availability and cooler soil temperatures experienced by the CGM/straw group as a result of the straw mulching material. In addition, the CGM 97.7 and CGM 195.4 treatment groups experienced greater competition for moisture and nutrients than the CGM/straw group. The combined treatment effects of the CGM, providing early season weed control and supplemental nitrogen, and the advantageous timing and placement of the straw mulch, providing additional weed control, cooler soil temperatures and greater moisture retention, are plausible reasons for the superior cane height and diameter evidenced in the CGM/straw plots. In addition, such benefits may translate into higher yields and fitter canes in future years.

It should be noted that the CGM 195.4 treatment groups displayed greater cane diameter and height in its fruiting canes than the straw and CGM 97.7 treatments. This may be due to the higher amount of nitrogen this treatment received from the CGM.

Percent Fruiting Canes to Total Canes:
CGM 97.7 + Straw, 79.59%
Cultivated, 78.79%
Straw , 67.39%
CGM 97.7, 60.00%
CGM 195.4 , 53.97%

In comparing percent fruiting to non fruiting canes, CGM/straw, with the lowest cane density, and the hand weeded canes displayed the highest percentages, 79.59% and 78.79% respectively. Straw only, with the highest cane density, brought 67.39% of its canes to fruiting, followed by CGM 97.7 at 60.00% and CGM 195.4 at 53.97%. The percent of fruiting canes to non fruiting canes followed the pattern of the average cane heights. Those treatments with the greatest average cane heights, CGM/straw and hand weeded, displayed the highest percent of fruiting canes. The straw only treatment group was intermediate in both average cane height and percent fruiting canes. CGM 97.7 and CGM 195.4 displayed the lowest cane heights and the lowest percentage of fruiting canes.

Node Counts, Growing Season Data:
Graph 12 compares the average node numbers by treatment and date.

Data gathered during the growing season revealed significant differences in node counts between treatments. Overall, the hand weeded plots consistently displayed higher node counts than the other treatments, followed closely by the straw only plots. CGM/straw was intermediate in node counts, followed by the CGM 97.7 and CGM 195.4 treatments.

Data from Harvested Canes:
Nodes were divided into three groups, total nodes, fruiting nodes and non fruiting nodes and were compared over two canes categories; total canes (combined fruiting and non fruiting canes) and fruiting canes. Graphs 13-18 compare treatments for average node number per cane, percent fruiting nodes to total nodes, and node numbers by treatment and type.

The data from the harvested canes followed the field data fairly closely and offered further opportunity to differentiate nodes numbers between total canes and fruiting canes. The data is described first for all canes and then fruiting canes. Under each group, node counts were further divided into three sub-groups; total nodes (combined fruiting and non fruiting nodes), fruiting nodes and non fruiting nodes.

Node Counts, Average Nodes per Cane—Total Canes:
The average node count per cane in the hand weeded and straw only treatment groups was significantly greater than the other treatments. Average nodes/cane: hand weeded 26.26 nodes/cane, straw only 25.86 nodes/cane, CGM/straw 23.57 nodes/cane, CGM 195.4 23.16 nodes/cane and CGM 97.7 22.26 nodes/cane.

Average Nodes per Fruiting Cane:
Hand weeded can straw only treatments continued to display significantly greater node counts over the CGM 195.4 and CGM 97.7 treatments. Average nodes per fruiting cane: hand weeded 27.50 nodes/fruiting canes, straw only 27.08 nodes/fruiting cane, CGM/straw 25.41 nodes/fruiting cane, CGM 195.4 25.12 nodes/fruiting cane, and CGM 97.7 24.45 nodes/fruiting cane.

Average Fruiting Nodes per Cane – Total Canes:
The average fruiting nodes per cane of the hand weeded treatment group, were significantly greater than all other treatments. Straw only, CGM/straw and CGM 195.4 were similar in average fruiting nodes per cane. Also, the straw only treatment fruiting nodes per cane averages were significantly higher than those of the CGM 97.7 treatment group. Due to the greater variability within the CGM/straw treatment group, the difference was not significant between CGM/straw and CGM 97.7. Average fruiting nodes/cane: hand weeded 9.85 fruiting nodes/cane, CGM/straw 8.18 fruiting nodes/cane, CGM 97.7 6.80 fruiting nodes/cane.

Average Fruiting Nodes per Fruiting Cane:
Hand weeded canes contained significantly greater numbers of fruiting nodes per cane over the CGM 195.4 and CGM 97.7 canes. CGM/straw and straw only treatments were not significantly different from either group. Average fruiting nodes/fruiting cane: hand weeded 10.88 fruiting nodes/fruiting cane, CGM/straw 9.87 fruiting nodes/fruiting cane, Straw only 9.61 fruiting nodes/fruiting cane, CGM 97.7 9.48 fruiting nodes/fruiting cane, CGM 195.4 9.44 fruiting nodes/fruiting.

Average Non Fruiting Nodes per Cane – Total Canes:
The straw only canes displayed a significantly higher number of non fruiting nodes per cane over four treatments. CGM/straw displayed the lowest number of non fruiting nodes per cane.

Average Non Fruiting Nodes per Fruiting Cane:
Straw only and hand weeded treatments had the greatest number of non fruiting nodes per fruiting cane. Node numbers from the straw only treatment were significantly greater than the CGM 195.4, CGM 97.7 and CGM/straw treatments.

Percent Fruiting Nodes to Non Fruiting Nodes – Total Canes
Hand weeded , 37.50%
CGM 977.7/straw, 34.72%
CGM 195.4, 33.45%
Straw only, 31.50%
CGM 97.7 , 30.51%

Percent Fruiting Nodes to Non Fruiting Nodes – Fruiting Canes
Hand weeded , 39.58%
CGM 97.7/straw, 38.85%
CGM 97.7 , 38.79%
CGM 195.4 , 37.59%
Straw only , 35.50%

Percent of nodes fruiting is an important yield component in primocane fruiting re raspberries (Hoover, Luby and Bedford, 1986). In this study the hand weeded canes displayed the highest averages in fruiting and non fruiting nodes per cane, along with the highest percentages of fruiting nodes to non fruiting nodes and most importantly in yield per cane. The straw only and CGM/straw treatments were very similar to the hand weeded canes with respect to node averages. Comparisons between the two treatments show the straw only treatment as higher in average nodes per cane, for both total and fruiting canes, while the CGM/straw treatment group was higher in the average number of fruiting nodes per cane and the percent of fruiting nodes to non fruiting nodes. The totals for CGM 195.4 were consistently higher than those of CGM 97.7, and both treatments were consistently below those of the other treatments, often to a significant degree. For percent fruiting nodes to total nodes in fruiting canes CGM 195.4 and CGM 97.7 were both higher than the straw only treatment groups.

Yield Data
Total Yield (grams):
The hand weeded plots obtained the highest yields at 2556.59 g, followed by straw only at 1796.29 g, CGM/straw with 1231.03 g, CGM 195.4 at 836.17 g and CGM 97.7 with the lowest yield of 788.08 g.

Average Yield per Cane:
Graph 19 compares average yield per cane by treatment.

Hand weeded and CGM/straw treatments displayed the highest per cane yield. The average yield of the hand weeded treatment was significantly greater than the CGM 97.7, CGM 195.4 and straw only treatments. Yield per cane totals: hand weeded 38.45 g/cane, CGM/straw 29.60 g/cane, straw only 20.28 g/cane, CGM 195.4 16.51 g/cane and CGM97.7 14.87 g/cane.

Average Yield per Fruiting Cane
Graph 20 compares the average yield per fruiting cane between treatments.

The hand weeded and CGM/straw treatments were statistically equal. The per cane yield of both treatments was significantly greater than the CGM 195.4 and CGM 97.7 yields. There was no significant difference between the CGM/straw groups and the straw only group. The yield of the hand weeded treatment was significantly greater than the yield of the straw only treatment group. It should be noted that variability within the hand weeded treatment plots was quite high, while variability in the straw only and CGM/straw groups were low in comparison. Yield per fruiting cane totals: hand weeded 57.43 g/fruiting cane, CGM/straw 33.92 g/fruiting cane, straw only 29.80 g/fruiting cane, CGM 195.4 25.71 g/fruiting cane, and CGM 97.7 19.11 g/fruiting cane.

Yield per Fruiting Node – All Canes
Graph 21 compares fruit yield per fruiting node between treatments of total canes.

All treatment groups were statistically equal with the exception of the hand weeded group yielding significantly more fruit per node over the CGM 97.7 treatment group. Variability within treatments was higher for CGM 97.7 and CGM 195.4 and lowest in the straw only and CGM/straw treatments. Yield per fruiting node totals: hand weeded 3.89 g/node, CGM/straw 3.40 g/node, straw only 2.50g/node, CGM 195.4 2.06 g/node and CGM 97.7 1.75 g/node.

Berry Number per Fruiting Node – All Canes
Graph 22 compares berry number per fruiting node between treatments on total canes.

All treatments were statistically equal with the exception of the hand weeded treatment group producing significantly more berries per fruiting node than the CGM 195.4 treatment group. Berry number per fruiting node was highest for the hand weeded and CGM/straw groups, intermediate for the straw only treatment, and lowest in the CGM 97.7 and CGM 195.4 treatments.

Yield per Fruiting Node – Fruiting Canes Only
Graph 23 compares the number of berries harvested between treatments in fruiting canes.

Canes were determined to have matured fruit which contributed to the yield figures as evidence by the empty receptacles remaining on the plant.

The hand weeded and CGM/straw treatment groups were statistically equivalent. The CGM/straw, straw only, CGM 195.4 and CGM 97.7 were also statistically equal. However, the CGM 97.7 treatment group yielded significantly less than the hand weeded treatment group. Variability was highest in the hand weeded and CGM 97.7 treatments and lowest in the straw only and CGM/straw treatments. Yield per fruiting node totals: hand weeded 5.03 g/fruiting node, CGM/straw 3.47 g/fruiting node, straw only 3.04 g/fruiting node, CGM 195.4 2.74 g/fruiting node and CGM 97.7 1.93 g/fruiting node.

Berry Number per Fruiting Node – Fruiting Canes Only
Graph 24 compares berry number per fruiting node between treatments on fruiting canes.

The hand weeded treatment group displayed significantly greater berry numbers per fruiting nodes over the CGM 97.7 and CGM 195.4 treatment groups. The CGM/straw and the straw only treatments were statistically equal to all treatments.

Average Berry Weight by Treatment
Graph 25 compares average berry weight between treatments.
Graph 26 displays the average berry weight by treatment and harvest date.

The hand weeded and straw only treatments displayed significantly greater average berry weight over the CGM 97.7 treatment group. There were no significant differences between the other treatment groups. There was very little variability in average berry weight in the hand weeded and straw only treatments, while variability was high in the CGM/straw and CGM 195.4 treatment groups. Average berry weight: hand weeded 2.55 g/berry, straw only 2.37 g/berry, CGM 195.4 2.50 g/berry, CGM/straw 2.47 g/berry and CGM 97.7 2.12 g/berry.

The hand weeded treatment group displayed the highest yield, yield per cane, yield per node, berry number/node and highest average berry weight. Although, with the exception of total yield, the figures of the hand weeded treatment group were not statistically significant over the straw only and CGM/straw groups. The straw only treatment was second in total yield; however, the straw only plots contained almost twice as many canes as the CGM/straw plots. When the data was divided into its components, the CGM/straw displayed a higher yield per cane over the straw only treatment. In either case the two treatments were statistically equal in all yield components.

The CGM 97.7 and CGM 195.4 displayed the lowest figure in all categories and generally showed significant differences in comparison to the hand weeded treatment group.

Variability within treatment groups was generally highest for the hand weeded, CGM 97.7 and CGM 195.4 treatments, and lowest in the straw only and CGM/straw groups. An exception to this was the average berry weight where CGM/straw and CGM 195.4 displayed greater variability over the other treatments.

Harvest Data
Graphs 26-28 display raspberry yield information through the harvest season by treatment

Harvesting began August 29. All treatments had at least one plot contributing to the harvest but all yields were very low. The hand weeded treatment group was the first to have all four test plots producing mature fruit. This occurred September 10. September 13, all straw only plots were yielding, followed by the CGM/straw on September 18. CGM 195.4’s four plots were yielding on September 23, while it took until October 2, before CGM 97.7 reached the same status. Between August 29 and September 13 yields were fairly small, but increased substantially after that date. Harvest ended on October 2, due to a hard freeze two days later.

Is should be noted that the CGM/straw was very slow in maturing its fruits, and was actually behind CGM 97.7 and CGM 195.4 in total cumulative yield weight until September 23, at which point its yields rapidly increased. Towards the end of the harvest season, October 2, the straw only and, to a greater extent, the CGM/straw groups increased yields at a faster rate then the hand weeded treatment group, but there was not enough time for them to obtain the yields produced by the hand weeded plots.

The highest yields occurred in the same order the treatments were able to bring all four plots into production. The hand weeded plots obtained the highest yields at 2556.59 g, followed by straw only at 1796.26 g, CGM/straw with 1231.03 g, CGM 195.4 at 836.17g and CGM 97.7 with the lowest yield of 788.08g.

Graph 26 displays the changes in berry weight from one harvest date to another and between treatments. It is interesting to note the overall pattern of the decline in berry weight from first to final harvest. Also, a comparison of Graphs 26 and 27 reveal average harvested berry weight decreasing as harvested berry numbers are increasing.

In comparing the yield component data of yield per cane, number of fruiting canes, yield per node, berry number, and berry weight to the specific treatments, it becomes apparent that while the hand weeded treatment displayed superior numbers over the other treatment groups, the CGM/straw and straw only treatments were not significantly different form the hand weeded plots in most categories studied. In addition, these treatments tended to offset shortcomings in various yield components through an increase in other yield components, the CGM/straw treatment, with lower cane numbers, offset this deficiency by bringing a higher percentage of canes to fruiting, and a higher yield per cane. The straw only treatment had a lower percentage of fruiting canes and a smaller yield per cane, which were offset by greater cane numbers. The CGM/straw group, with a comparatively low total node count, remedied the shortage through a thriftier use of nodes, as evidence by a higher ratio of fruiting to non fruiting nodes and a higher yield per node. A lower average berry weight was offset through the production of higher berry numbers. The straw only treatment balanced smaller berry numbers with higher average berry weights.

In comparing treatment effects, the hand weeded plots experienced little weed pressure, and the lack of mulch probably created higher soil temperatures with less moisture retention (Trinka and Pritts, 1002, Preece and Read, 1993). Canes in the straw only treatment group were under more weed pressure than the hand weeded plots, but less than the CGM/straw group. In addition, the straw only treatment group was subjected to more crowding than the other treatments, due to higher cane numbers. The straw mulch served to decrease soil temperature and increase soil moisture (Preece and Read, 1993). The CGM/straw group received the highest weed pressure, but the least crowding of canes. And like the straw only group due to mulching, probably experienced lower soil temperatures and higher soil moisture. The CGM/straw group also received supplemental nitrogen due to the 10% nitrogen content of the CGM (Christians, 1993). The hand weeded and CGM/straw group contained the tallest canes with the largest diameters. The straw only canes were considerably shorter and had the smallest cane diameters.

Prior to harvest, it appeared that anyone of the three treatment groups, hand weeded, straw only and CGM/straw, were in a favorable position to be the highest yielding group. Due to high weed pressure, it was unlikely that the CGM 97.7 and CGM 195.4 treatments would be in a position to rival the yields of the other treatments (Lawson and Wiseman, 1976). The determining factor, as to which treatment group produced the highest yields, was the group first to bring a consistent supply of fruit to maturity before the weather ended harvesting.

A determination as to what induced the hand weeded plots to mature greater numbers of fruits earlier than the other treatment groups are not easily forthcoming. It has been noted that a benefit of straw mulch is in the conservation of water, through the reduction of moisture evaporation from the soil, furthermore, mulch aides in reducing irrigation requirements (Preece and Read, 1993). The irrigation requirements of the experimental lots were controlled by the grower, who based these needs on the entire field, not on the needs of the experimental plots. It ahs been observed that increasing the amount of water during June and July delayed the date of first harvest in ‘Autumn Bliss’ (Prive, Sullivan, Proctor and Allen, 1993). Between the three treatment groups, hand weeded straw only and CGM/straw the hand weeded treatment group, with its bare soil, would have been least affected in this respect. The straw only treatment group would have retained moisture longer than the hand weeded plots, due to the reduced evaporation rate caused by the straw mulch, but would have more rapidly depleted soil moisture over that of the CGM/straw group, due to greater water demand created by the larger number of canes. The CGM/straw treatment group required the least amount of water due to decreased evaporation created by the straw and the smaller number of canes. There may be a correlation between the experimental finding that increased water during June and July delays the first harvest date in ‘Autumn Bliss’ and the results obtained in this experiment, although further study would be required to substantiate this supposition.

The results obtained in this study support evidence that CGM is effective in controlling annual weeds for a period of approximately 22 to 32 days. Furthermore, CGM at a rate of 195.4 grams per square meter was more effective in weed control than CGM at the lower rate of 97.7 grams per square meter. The data also demonstrated that on application of CGM alone is not enough to successfully control weed populations throughout the growing season in raspberries. This study found that a combination of CGM and straw mulch can be successful in controlling weeds in raspberries to a substantial degree. There was an observed tendency of CGM/straw mulched raspberries to develop taller and larger diameter canes than the other treatment groups studied, and an inclination to be slower in maturing fruit. The results also indicate that ‘Autumn Bliss’ primocane fruiting red raspberries do not appear to be adversely affected by the use of CGM. The number of positive results obtained in this first year study warrant further investigation, though continuing research, to more thoroughly evaluate the effects of CGM, and the combination of CGM and straw mulch, in primocane fruiting red raspberries, for the purpose of developing functional methods of natural, in row weed control suitable for commercial raspberry growers.

Conclusion:
The results obtained in this experiment determined that CGM was effective in controlling weeds in primocane fruiting raspberries for a period of approximately 22 to 32 days. And that CGM was more effective at higher rates of application. CGM is also a slow release nitrogen source, being 10% nitrogen by weight. If concentrations of CGM are increased for weed control, concentrations of nitrogen will also be greater. Depending upon the concentration of nitrogen which actually becomes available to the plant throughout the growing season, there could be negative or positive consequences on fruit yield. If the nitrogen supply is too high or too low, yield will be adversely affected. Further studies to determine the best concentrations of CGM to achieve an optimum balance between weed control and the crop’s nitrogen requirement should be pursued. A determination of the time period over which CGM is effective as a herbicide would also be in order. In addition, as soils vary in their ability to retain nutrients and herbicides, it would be advisable to test CGM over a wide range of soil types to determine the best concentrations for use in different soil situations.

Primocane fruiting raspberries have a relatively long growing season, which requires a high degree of long term weed control. Other small fruit crops, such as summer fruiting raspberries and strawberries, produce fruit much earlier in the growing season. Due to their shorter time requirements, one or two applications of CGM alone may prove very effective as weed control in these crops.

In this experiment, the hand weeded treatment group resulted in the highest raspberry yield. However, with this higher yield a considerable amount of labor was required to keep the plots weeded throughout the growing season. The straw only treatment group required three weedings prior to placement of the straw mulch, while the CGM/straw treatment group required one application of CGM, and no weeding. There were extra expenditures for GCM, but as CGM contains 10% nitrogen by weight, there was a corresponding saving in fertilizer. The straw mulch, and its application, was also an added expense, but much less labor intensive and costly than the hand cultivation required by the hand weeded plots. When cost and crop yield factors are considered, the CGM/straw and the straw only treatment groups have the potential of providing a substantial profit over the hand weeded treatment group. In addition, as all treatments were grown without the use of synthetic herbicides, a price premium may be demanded in the market place for organically grown produce.

An interesting aspect brought to light through this study was the greater cane height and cane diameter of the CGM/straw canes. As cane height and diameter may be related to potential yield in future years, one is reminded that the general overall health of the raspberry planting should receive more consideration than what it often times does. There is a tendency to focus on one year’s yield at a time, but decisions based upon such narrow viewpoints may adversely affect yield in future years. A case in point, cane density is considered an important yield component and one often strived for in many plantings.

This study showed that the most crowded plants, the straw only treatment, had shorter canes and smaller cane diameters, an indication of some stress to the plant. In addition, crowding can increase disease incidence by reducing air flow and drying of leaves, leaving the plant more vulnerable to attack by bacterial and fungal infections. Also, stressed plants may be more appealing and vulnerable to damage by insect pests and nematodes. As raspberry are perennials, a rapberry stand is expected to last for a number of years, so it is in the growers interest to see that it remain healthy. In addition, costs to reestablish a raspberry field is considerable in terms of materials and labor costs, and loss of yield during the establishment year. Any extension in the life expectancy of a high yielding field will save the grower money. With this in mind, long term experiments to determine the best balance between healthy plants and a consistent and acceptable yield would be beneficial in terms of long term costs to the grower, and ultimately the consumer.

In conclusion, the results obtained from this study suggest that CGM and a combination of CGM and straw mulch may prove to be very beneficial treatment options for use in organic crop production. However, further studies over a number of growing seasons is suggested to substantiate this conclusion.

OUTREACH
Three grower education activities were carried out regarding this project:
- A grower field day was held at North Star Gardens on July 7, 1996, attended by 50-60 individuals. Approximately half of the attendees took advantage of an opportunity to visit the research plot, where Jeanine Allen and Dr. Hoover provided an overview of the project, and showed the trial in progress.
- A formal presentation of research results was provided in a session of the Minnesota Fruit and Vegetable Growers Association annual conference. The presentation was attended by 60-70 growers. In addition, the conference organizers featured a guest speaker from Iowa who used CGM in strawberry establishment. The grower showed slides and indicated how he annually used CGM in establishing large commercial strawberry plantings using the 40lbs/1000 ft2 rate.
- Finally, a written summary of the project has been prepared for a future issue of MFVGA newsletter; the newsletter goes out to over 2500 individuals in the region.