Production management systems that incorporate smother crops to control weeds and/or supply plant nutrients may reduce the transport of agrichemicals to surface and ground waters. Alternatives to agrichemicals will be adopted by producers when they effectively replace those chemicals without reducing crop yield, disrupting management practices, and diminishing farm profitability. This project investigated if annual medics, used as a living smother crop, would control weeds, not adversely effect corn productivity, supply N to corn, increase soil quality, and be similar in cost to synthetic chemical application.
Field experiments were conducted in South Dakota and Iowa using three species of annual medic (barrel medic, Medicago trunculata; burr medic, Medicago polymorpha; and snail medic, Medicago scutellata). The first of several experiments evaluated the effect of medic planting date and rate on corn production and weed control.
Medics were planted at two planting dates (about 2 weeks before average planting date for the area and at corn planting) and two broadcast planting rates (15 and 30 lb seed per acre). The crop year in 1995 was cool and wet, and medic grew aggressively except in drainage areas. The 1996 growing season was warmer and drier and medic biomass was about 70% less than in 1995. Medic planted early had more biomass and ground cover than medic planted at the same time as corn but all medic senesced by mid to late July. Medic reduced total weed biomass (predominant grass weeds were yellow and green foxtail, predominant broadleaf weed was common lambsquarters) in both 1995 and 1996. Corn yield was very poor in 1995 at both Brookings and Sioux Center sites and was reduced by medic. In 1996, the barrel medic broadcast at high rates reduced corn yield by about 30% compared to the weed-free control. However, when medic was banded into the interrow areas and alachlor was banded in the row, yields were similar to areas where only herbicide was applied.
In a second experiment, the effect of medic on soil quality and N cycling were evaluated. Medic increased water infiltration from about 1.5 inches per minute (bare soil) to about 6 inches per minute (medic planted at 30 lb per acre). Nitrogen mineralization measured from mid-July to mid-August increased by about 50% in medic plots compared to areas without medic. However, nitrogen credit from medic the following year was minimal. Corn yield was not influenced in this study.
In economic analyses using enterprise budgets, results were similar (within 5%) when comparint the cost to produce a bushel of corn (assuming no N credit for the medic) using broadcast methods for medic alone or alachlor alone, or a band combination of medic (interrow) and alachlor (row). It is important to note that banding would reduce herbicide application by 50% compared to a broadcast application.
The information gained in this research indicates that annual medics may have a very important niche in achieving more sustainable, environmentally benign crop production systems. Early in the growing season, medic was competitive with corn for soil N. However, after the medic senesced, this N was subsequently mineralized and made available for plant uptake. When using medic, enough N must be present in the soil so corn does not become N deficient. In addition, using medic in conjunction with other agrichemicals, especially in sensitive areas such as acres coming from CRP, would benefit the soil water profile, reduce soil erosion, and reduce N losses in agronomic systems.
Production management systems that incorporate smother crops to control weeds and/or supply plant nutrients may reduce herbicide and fertilizer applications, chemicals that often pollute both surface and ground water in environmentally sensitive areas. Alternative input systems for agrichemicals will only be adopted by producers if crop productivity can be maintained, fits within current management practices (or management that is considered feasible by the producer), controls weeds or has other beneficial aspects, and is cost effective. The testable hypotheses for this SARE research grant were that annual medics, when used as a living smother crop, would control weeds, do not adversely effect corn productivity, supply nutrients (primarily nitrogen) to corn (Zea mays L.), increase soil quality, and be similar in cost to synthetic chemical application.
Three species of annual medic (barrel medic, Medicago trunculata; burr medic, M. polymorpha; and snail medic, M. scutellata) were evaluated in field trials on experimental and producer farms in South Dakota and Iowa. In the first set of studies, medics were planted at two planting dates (about 2 weeks before average planting date for the area and at corn planting) and at two broadcast planting rates (15 and 30 lb seed per acre). The crop year in 1995 was cool and wet and medic grew aggressively except in drainage areas. The 1996 growing season was warmer and drier and medic biomass was about 70% less than in 1995. Medic planted early had more biomass and ground cover than medic planted at the same time as corn, and all medic senesced by mid to late July. Medic reduced total weed biomass [predominant grass weeds were yellow and green foxtail (Setaria glauca (L) Beauv. and S. viridis (L.) Beauv.), predominant broadleaf weed was common lambsquarters (Chenopodium album L.)] in both 1995 and 1996. Corn yield was very poor in 1995 at both Brookings and Sioux Center sites and medic reduced yield even further. In 1996, the barrel medic broadcast at high rates reduced corn yield by about 30% compared to the weed-free control. However, when medic was banded into the interrow areas and alachlor was banded in the row, yields were similar to areas where only herbicide was applied.
The effect of snail medic on soil quality (water infiltration and N cycling) was evaluated. Medic increased water infiltration from about 1.5 inches per minute (bare soil) to about 6 inches per minute (medic planted at 30 lb per acre). Nitrogen mineralization measured from mid-July to mid-August increased by about 50% in medic plots compared to areas without medic. However, nitrogen credit from medic the following year was minimal. Corn yield was not influenced by medic in this study.
In economic analyses using enterprise budgets, the results were similar (within 5%) when the cost to produce a bushel of corn (assuming no N credit for the medic) using broadcast methods was compared to medic alone or alachlor alone, or a band combination of medic (interrow) and alachlor (row). Banding would reduce herbicide application by at least 50% compared to a broadcast application.
The information gained in this research indicates that annual medics may have a very important niche in achieving a more sustainable, environmentally benign crop production system. Planting medic as a cover crop may be very appropriate in sensitive areas such as land coming out of CRP because it can reduce soil erosion, change N cycling so that the N available when the crop requires it, and reduce herbicide applications through banding.
1. Quantify weed suppression by three annual medic species.
2. Measure corn productivity in medic-corn production systems.
3. Determine the impact of medic on soil quality parameters and N contribution to soil.
4. Compare the economics of corn production in conventional and annual medic-based systems.
Weed suppression and corn productivity B small plots. Field experiments were conducted during 1995 and 1996 at Brookings, SD (Barnes clay loam; fine-loamy, mixed, superactive, Udic Haploboroll) and Sioux Center, IA (Galva silty clay loam; fine-silty, mixed, mesic, Typic Hapludoll). Soil organic matter averaged 2.5% at Brookings and 4% at Sioux Center. In the fall of 1994 and 1995, about 150 lb N acre as ammonium nitrate was added at Brookings and at Sioux Center, liquid dairy manure (1994) and ammonium nitrate (1995). The previous crop was soybean at both sites.
Treatments at each location in 1995 included two medic cultivars (a burr medic, ‘Santiago’; and a snail medic, ‘Sava’), two seeding rates (15 and 30 lb/acre), and two planting dates (two weeks prior to ‘normal’ planting date of corn and at corn planting). In 1996, the treatments had three medic cultivars (‘Santiago’, ‘Sava’, and a barrel medic, ‘Caliph’). Plots were about 45 ft long and 4 corn rows wide. Row width was 30 inches at Brookings and 40 inches at Sioux Center.
In 1995, early medic plots were planted by spreading the medic seed with a drop seeded and lightly raking the seeds into the top 0.5 inch of soil on 27 April and 28 April for Brookings and Sioux Center, respectively. Hybrid field corn ‘Sexhauer 560’, 100 day maturity rating, was planted 22 May at about 25,000 seeds/acre. At Sioux Center, ‘LOL 5501ED’, 104 day maturity rating, corn was planted 18 May at 30,000 seeds/acre. Immediately after corn planting, medic was overseeded as a broadcast application as reported above. In 1996, medic was planted 29 April at Brookings and 23 April at Sioux Center as the early date. Hybrid field corn ‘Pioneer 3751’, 97 day maturity rating, was planted on 13 May at Brookings at 26,000 seeds/acre. At Sioux Center, ‘LOL 522’, 107 day maturity rating, was planted 6 May at 30,000 seeds/acre.
Ground cover measurements were taken at each site the third week of June using a transect method. In the third week of June, July, and August, weed density by species (taken only in June) and biomass of weeds (separated into grass and broadleaf species), corn, and medic were taken at three sites within each plot from the row and halfway into the interrow.
Chlorophyll readings from twenty corn plants in each plot. Readings were taken midway between the midrib and leaf edge of the last fully expanded corn leaf in June 1996 and at ear leaf after mid-silk in July 1995 and August 1996 using a hand-held chlorophyll meter (SPAD 502 chlorophyll Meter, Minolta Camera Co., Ltd., Japan).
Corn was hand-harvested from 25 ft of row in the center two corn rows 12 October at Brookings and 3 October at Sioux Center. Corn was dried to constant weight, weighed, shelled, and reweighed. Corn stover was harvested from the same sample plots and after drying, used to calculate total corn biomass. In 1996, corn was harvested the 15 October at Brookings and 5 October at Sioux Center.
The plots were arranged in a randomized complete block design with four replications. Analysis of variance was performed on data collected at each site. Significant differences were determined with an F-test and mean separation was done using LSD values (p<0.05). Regression analysis was performed when appropriate.
Corn yield and economic analysis B producer plots. On-farm research was conducted during 1996 and 1997 in Sioux Center, IA. Soil type was a Galva silty clay loam with a soil organic matter content of 2.8% and pH of 6.6. All field operations except medic planting and corn harvesting were done as part of the large field operations by the producer. Prior to corn planting, 100 lb N/acre was applied.
Hybrid field corn ‘LOL 492’ was planted 7 May, 1996 and ‘Ciba Max 101’ was planted 11 May, 1997. Both hybrids were rated at 105-day maturity. Corn planting rate was about 30,000 seeds/acre on 35-inch row spacing.
Four weed management treatments were examined. ‘Caliph’ was banded at corn planting in a 12 inch band over the corn row at 6 lb/acre in the row; alachlor applied in a 12 inch band at a rate of 2.5 lb ai/acre; alachlor banded in a 12 inch band over the corn row at 2.5 lb ai/acre and ‘Caliph’ banded in the interrow at 6 lb/acre; and a weedy control. Alachlor granules and ‘Caliph’ each cost about $6/acre and both can be applied using dry herbicide boxes that are common on corn planters.
Broadleaf and grass densities were taken at three sites in each plot in 1996. Corn was hand-harvested from the middle two rows by about 20 ft long area in 1996 dried to constant weight, shelled, and weighed. In 1997, a plot combine was used for harvest.
Each plot was four corn rows wide by 60 ft long. Plots were arranged in a randomized complete block design with three replications in 1996 and four replications in 1997. Analysis of variance was performed on data collected and differences were determined with an F-test. Mean separation was done using LSD values (p<0.05). Enterprise budgets for corn following soybeans were developed for the corn-medic, corn-herbicide, and corn-medic-herbicide systems by modifying a standard corn/soybean budget prepared by Iowa State economists (based on ISU File A1-20) (Table 1).
Soil quality measurements. The objective of this study was to determine the influence of ‘Sava’, a snail medic type, on corn production, nitrogen cycling, and soil quality. Treatments were interrow plantings of medic at 0 and 30 lb seed/acre, with two nitrogen fertilizer rates, 0 and 150 lb N/acre. The treatments were replicated four times. The experiment was located at the SDSU research farm in Aurora, SD on a Brandt silty clay loam, and consisted of a randomized complete block design (plots 10 by 30 ft). There were four treatments: corn with no fertilizer; corn with fertilizer; corn with medic and fertilizer; and corn with medic and no fertilizer. Crop management followed the SDSU Best Management Practices Guidelines, and fertilizer was applied to appropriate plots based on soil analysis from spring soil testing.
Soil samples were collected monthly to 60 cm at 15-cm increments. Soils were dried and ground, and nitrate and ammonia concentration determined by extracting 10 grams of soil with 1 M KCl and analyzed on a Wescan Ammonia Analyzer with a zinc nitrate reducer (Model 360, Deerfield, IL).
Five 2.5-cm diameter soil cores at 0 to 5 cm were sampled randomly from each plot at monthly intervals, sieved to 2 mm and stored in polyethylene bags at 4°C until processed. Soil microbial biomass and activity determinations were determined by the chloroform fumigation/incubation method, fumigating three replicate samples of 10 g soil (dry weight equivalent) at 60% water holding capacity, accompanied by three non-fumigated controls. Samples were incubated for 10 days in Mason jars with CO2 traps of 2 M NaOH. The CO2 traps were titrated with 0.1 M HCl to determine CO2 respired. The daily respiration rate from the controls is an indicator of microbial activity; the size of microbial biomass is calculated by subtracting the control from the fumigated respiration rate and dividing by a constant Kc.
Potential mineralizable nitrogen was determined by extraction of lab incubated samples at 4, 14 and 28 days. Samples of 100 g of soil (dry weight equivalent) were incubated at 100% water holding capacity at 25°C, then sub-sampling and analyzing soil for total inorganic N at 4, 14 and 28 days with the Wescan analyzer described above. These time points will yield a daily estimation of nitrogen released at ideal conditions for nitrogen mineralization. Percent soil organic matter C and N and plant biomass C and N was determined with an Europa 20-20 ratio mass spectrometer.
In-field nitrogen mineralization was determined by monthly leaching of intact cores fitted with an ion exchange resin bag. Cores were 5-cm diameter, 18 cm length incubated in the field with a nylon mesh bag of ion exchange resins fit into the bottom of the core. Cores were leached monthly with 0.01 M CaCl2 and resin bags extracted with 2 M KCl and analyzed for nitrate and ammonia as described above.
Water infiltration was determined weekly with a double ring infiltrometer. This was done by driving a 30 cm diameter section of thin vent pipe 15 cm into the crop interrow, and a 90 cm diameter collar into the soil around the 30 cm ring; then watering both rings to 2.5 cm water depth and timing water infiltration into the soil. The second ring prevents the confounding effect of horizontal water flow.
Oats were planted in the plots the year following corn and yield and nitrogen content were examined to determine fertilizer replacement value.
Data was analyzed using ANOVA analysis using LSD for mean separation.
Weed suppression and corn productivity B small plots.
1995 Medic-Weed Data. In 1995, rainfall during April and May at Brookings was 630 and 383% above and the average temperature was 60 and 20% below the 30 year average, respectively. Rainfall in April and May at Sioux Center was about 165% above and average temperatures were 28 and 12% below the 30 year average, respectively. Temperatures from June through October at both locations were near normal. These statistics indicate that spring conditions in 1995 at both locations were cool and wet.
Corn was planted about 14 days later than “ideal” at Brookings and about 7 days later than “ideal” at Sioux Center. Due to the later corn planting date, the early medic planting date was about 3 weeks prior to corn planting at both locations.
Medic biomass in June 1995 was similar among cultivars and between sites within a planting date (Tables 2 and 3). Doubling the seeding rate from 15 to 30 lb/A generally did not double medic biomass. The early planting of medic resulted in an average medic biomass of about 500 lb/A compared to 40 lb/A for plots planted in mid May.
Green foxtail was the dominant grass at both sites in 1995. Grass biomass was reduced ‘Santiago’ medic when planted early. At Brookings, grass biomass reduction in the low planting rate was 66% whereas in the high planting rate biomass was reduced by 93%. At Sioux Center, grass biomass was less than at Brookings and medic did not significantly reduce biomass. However, when using regression analysis, it was noted that a negative correlation was observed between medic and grass biomass at Brookings (r=-0.82; p=0.004) and at Sioux Center (r=-0.55; p=0.07) in June. In July, the same trend was noted with decreasing grass biomass as medic biomass increased at Sioux Center (r=B0.77; p=0.009).
Broadleaf weed species at Brookings were Pennsylvania smartweed (Polygonum pennsylvanicum L.) and common lambsquarters whereas at Sioux Center, velvetleaf (Abutilon theophrasti Medicus), redroot pigweed (Amaranthus retroflexus L.) and common lambsquarters were most prevalent. Broadleaf biomass at Brookings averaged about 390 lb/A in June and was reduced by all medic treatments except ‘Santiago’ planted with corn at 15 lb/A. At Sioux Center, broadleaf biomass in June was only 10% of the amount observed at Brookings and was not influenced by medic treatment. In July, a negative correlation between medic and broadleaf biomass was noted (r=-0.88; p=0.001) at Sioux Center. At Brookings, broadleaf biomass was about 60% less than the weedy control in the 15 lb/A early planted ‘Sava’ treatment and 30 lb/A late planted ‘Santiago’ treatment.
Medic in all treatments senesced by the third week in August at both sites. Grass and broadleaf biomass generally increased from the July to August sampling and no medic effect was noted on the biomass of either weed category.
1995 Corn data. In June, corn was at the 2-leaf growth stage and biomass was similar in the weed-free and weedy treatments. Corn biomass was reduced in all medic treatments by 50% to 90% at all three sampling dates (Table 4). Negative correlations were found between corn and medic biomass ranging from r=-0.74 (p=0.01) to r=-0.89 (p=0.004) in both June and July at both locations. Although medic had senesced by August, corn in the medic treatments did not recover and lagged well behind weed-free corn plots.
The relative greeness index averaged 49 in the weed-free treatments and 31 in the weedy treatments in July at Brookings (Table 4). All corn in medic treatments had greeness index values that were 56% lower than corn in the weed-free plots. Relative greeness is highly correlated to N concentration in leaf tissue. Therefore, the low index numbers for corn suggest that medic was competing with corn for N. Because the greeness index of corn grown with medic is lower than the weedy check, it also suggests that medic was a more efficient N scavenger or had other effects on corn growth than weed competition alone. This same trend was seen in the medic treatments at Sioux Center (Table 4).
Growth stage of corn in July also was influenced by medic. In weed-free plots, corn was at about the 6-leaf at both locations. Corn in medic plots ranged from 3.3 leaves at Brookings in the ‘Santiago’ early seeded medic plots to an average of 5.3 leaves in ‘Sava’ late planted medic plots (data not shown).
Corn grain yield in medic treatments was about 80% less than yield in the weed-free plots at Brookings for all treatments (Table 5). At Sioux Center, yield was reduced 87% in early planted medic treatments and 60% less in late planted medic treatments when compared to the weed-free treatments. Corn stover biomass was reduced at both locations but not to the extent that was observed in grain.
1996 Medic-weed data. In 1996, wet conditions were observed at both Brookings and Sioux Center. At Brookings, rainfall was about 310% above normal from May through September and about 109% above normal for this same time at Sioux Center. Temperatures were near normal during the growing season.
Corn was planted at the “ideal” time at Brookings (13 May) and Sioux Center (6 May). The early medic planting date was 2 weeks prior to corn planting at both locations (29 April, Brookings; 23 April, Sioux Center). Therefore unlike 1995, medic did not have as much early growth before corn planting.
In 1996, the barrel medic, “Caliph”, had three to six times more biomass than the other cultivars. In June, early planted “Santiago” and ‘Sava’ averaged about 35 lb/a regardless of planting rate, whereas ‘Caliph’ averaged about 150 lb/a. In July, medic biomass ranged from 55 to 700 lb/a. This was quite variable between cultivar, site, planting date, and planting rate. It appeared that the medic did well in select plots but with no real consistency. Doubling rates or planting earlier than corn did not favor increased medic biomass. By August, no medic was present in the plots. While both 1995 and 1996 were both wetter than normal, 1996 had warmer temperatures. It appears that the cooler temperatures observed in 1995 favored medic growth.
In 1996, the most common weed species at Brookings were wild buckwheat (Polygonum convolvulus L.) and green foxtail. Common lambsquarters and green foxtail were the most common weed species at Sioux Center. The amount of medic growth in most plots was not enough to reduce broadleaf biomass at any time during the growing season. In July, there was a negative correlation between grass biomass and overall medic biomass (r=-0.68; p=0.007) at Sioux Center. Grass biomass was not reduced by medic in June or August at either site.
Corn data B 1996. In June at Brookings, corn biomass averaged about 55 lb/a among all treatments with very little difference accounted for by medic treatment (Table 8). In July, corn biomass increased 10 times or more when compared to June sampling and was more than twenty times more than the amount recorded in 1995. In August, corn biomass was about 5400 lb/a. Biomass generally was not influenced by medic treatment although in plots with high medic and grass biomass, significant decreases in biomass could be seen.
The relative greeness index was slightly lower in ‘Caliph’ treatment than in the weedy treatments in July. However, in August the greeness index was similar across all treatments.
Corn biomass at Sioux Center in July and August was over 5 times the biomass recorded in 1995. Corn grown with the ‘Caliph’ medic cultivar showed the greatest biomass reduction and this reduction was observed in both July and August. The relative greeness index generally was lower in ‘Caliph’ treatments in June although not statistically different from the weed-free check.
Relative greenness is an indirect measure of N in the leaf. It appears in plots with medic, corn was experiencing N competition early in the season. It appears that N competition was due largely to medic in the plots since weedy plots had similar weed pressures but higher greeness index.
Corn grain yield was only about 16% greater in the weed-free plots in 1996 when compared with 1995 at Brookings (Tables 5 and 9). However, corn yields in medic and weedy treatments were about 5 times greater in 1996 compared to 1995. Yields were reduced in three of the four ‘Caliph’ medic treatments when compared to the weed-free and weedy treatments at both locations (Table 9).
Yield in the weed-free treatment at Sioux Center was 45% greater in 1996 compared to 1995 whereas yields in the medic treatments increased an average of 8 times. As at Brookings, ‘Caliph’ appeared to reduce yield more than the other treatments.
The data from 1996 appear to be inconsistent with 1995 results. In 1996, medic did not seem to influence yield although early in the season corn biomass was somewhat reduced. The weather conditions in 1996 were moist and warm and corn grew very well as evidenced by no yield reduction even in the weedy check. However, medic did not grow particularly well. In 1995, medic flourished with large quantities of biomass present in July and this must have hindered the growth and productivity of corn.
In this experiment we concentrated on how medic affected weed biomass and corn growth and productivity. Medic grown as a broadcast treatment stunted corn growth and reduced both corn yield and weed biomass, except when medic biomass was low. Corn was yellow and spindly when medic biomass was high. Therefore, the broadcast treatment with no applied N may not be an agronomically viable alternative in cropping systems. Both weeds and corn were present in these plots and it is difficult to separate weed from medic interference. Medic appeared to compete with corn for nitrogen as shown by reduced leaf greeness. However, allelopathic effects from medic on corn also may help to explain some of the observed reductions in growth and yield.
Results from producer fields and economic analysis. In a producer’s field, medic was seeded as a band into the interrow area of corn. This tactic was chosen because of the injury observed in the broadcast treatments. Green foxtail and velvetleaf were the two troublesome weeds in this field. To control the grass weeds in the corn row, alachlor was band applied. The application rate was one-third the amount that would have been applied as a broadcast application. There was no grass present in the row areas in observations taken in June, however velvetleaf was present (Table 10). In June, relative greeness of corn with and without medic were similar. Corn yield was not reduced by medic in 1996 and only slight reductions in yield were measured in 1997.
The cost of production using alachlor as a banded treatment or medic with alachlor were similar (Tables 11, 12, and 13). The cost/bu were about 25% less than the amount estimated by the sample ISU budget (Table 1). The major reduction can be seen in herbicide costs (from $30/a in the conventional system to $6/a in the alachlor-medic system). Giving medic an N credit of 45 lb/a reduced the cost of production somewhat over not giving an N credit. But because N is relatively inexpensive ($0.19), the cost reduction is slight. If N price were to increase and the estimated N credit is correct, the medic-alachlor system would have an even more favorable balance.
A major factor that needs to be overcome for this system to be adapted by producers is the weed-free field concept. Medic will suppress but not stop weed growth. An economic threshold approach to weed management would tend to show this system as a viable approach to sustainable weed and soil management.
Soil quality measurements.
The presence of ‘Sava’ snail medic in the corn interrow increased the water infiltration rate up to four times (Table 14). The impact of medic on infiltration may have been the result from the medic providing (1) a cover on the soil surface and thereby reducing raindrop impact ; (2) medic has most of its roots concentrated in the surface 3 cm of soil and thereby helping to stabilize surface soil aggragates; or (3) the medic stems provided a path where water can flow down into the canopy and soil or a combination of all three. The impact of medic on water infiltration is significant because it means that medic reduces runoff and erosion. Increased infiltration also increases plant available water. In 1998 medic had senesced prior to infiltration measurement therefore the medic appears to have a long term effect on infiltration and is not just observed when medic topgrowth was present.
The impact of medic on the N cycle was observed during the year that medic was planted and in the subsequent year when oats were planted. During the year of corn, fertilizer addition increased N mineralization (Table 15) from mid-July to mid-August. This is due in part to the uptake of fertilizer nitrogen and subsequent release upon decomposition.
This increased mineralization most likely resulted from immobilization of fertilizer that was subsequently mineralized later in the season. Mineralization was further increased by adding medic. This increase could be caused by medic fixing N that was subsequently mineralized or medic utilizing fertilizer N that was subsequently mineralized. It is likely that medic utilized fertilizer N because when fertilizer was not added medic did not increase N mineralization.
Medic and N fertilizer impacted corn yields (Table 16). If adequate amounts of N were added then medic did not influence corn yields. However, if N was limiting, then medic reduced corn yields in one out of two years.
In the second year of the rotation additional fertilizer was not applied. This study was conducted to evaluate the residual effects of the medic on a subsequent crop. Within an N rate medic did not influence oat yield or total N amount contained in the grain. These findings suggest that the amount of N fixed by medic was minimal. However, medic did influence the %N in the grain. In fertilized-medic treatments, oats had a higher %N.
These results show that medic impact on the N cycle was limited to changing when the N was available for plant growth. During the year that medic was planted this resulted in increased mineralization and during the second year of rotation, this resulted in increased %N or grain quality. In addition, medic increased water infiltration rates that should reduce runoff and erosion and may result in more water available in the profile later in the season. This research showed that medic did not eliminate the need for applying nitrogen fertilizer.
Medic was shown to have some impact on many facets within a farming system. Medic suppressed grass weed biomass and did not impact corn yield when used in combination with N fertilizer. The information gained in this research indicates that annual medics may have a very important niche in achieving a more sustainable, environmentally benign crop production system.
Planting medic as a cover crop may be very appropriate in sensitive areas such as land coming out of CRP because it reduces soil erosion, change N cycling so that the N is available when the crop requires it, and as an interrow planting to reduce herbicide applications when banded. If herbicide use were by cut by 50% in corn by using medic, it would reduce herbicide use by 6.4 million lb in South Dakota, 23 million lb in Iowa, and 15 million lb in Minnesota. In 1990, about 206 million lb of alachlor was used in corn production in the United States and made up 28% of the total herbicide used in corn. Using a medic system would reduce this amount significantly. In addition, atrazine and other broadleaf herbicides would need to be band applied to allow for medic growth. A 50% reduction in atrazine use would result in a reduction of nearly 23 million lb of atrazine in the Corn Belt states alone. The monetary reduction in herbicide input would be from $30/a to $6/a, only 20% of today’s cost.
Other benefits that would be realized if medic were used as an interrow smother crop in corn production include increased water infiltration (up to 25% more water saved in the soil profile), reduced N loss, decrease in soil erosion potential, and improved water quality. All of these factors favor the use of medic in corn production.
Base line summary. Specific enterprise budgets are given for several scenarios in the results section of this report. In addition, there are several other possibilities that can occur. When both alachlor and ‘Caliph’ are used in combination, corn yield increased from 122 to 132 bu/a. This increase in yield had a slightly lower cost/bu than alachlor or medic alone (Table 18). If a nitrogen credit is give for ‘Caliph’ then costs are reduced further. The highest cost of production occurred in the weedy control due to a 15% yield reduction.
Other less tangible benefits have not been included in on this analysis. For example, these include a reduced potential for soil erosion because the medic can provide soil cover in early June when corn does not cover much area. Another benefit is the N credit in the second year. Also, since spring-applied N is tied up in medic biomass, less N is available for leaching or runoff into water supplies. Another benefit that is difficult to quantitatively assess is the increased water infiltration that was observed. With less water running off the soil surface, more water can be stored in the soil profile that may be available for corn during the critical stages of anthesis and silking.
Demonstration plots were set up in two growers’ fields, one in South Dakota and one in Iowa. These were successful demonstrating that medic could be used in a producers field to supplement some of the herbicide and N applications. Further research needs to be conducted to further understand the effect of medic on N cycling, the effect on other weed complexes and how cultivation fits into this system.
One large factor in implementation of this system is the grower’s perception of what the ideal production field should look like. Producers have a much lower acceptance level of weeds in their fields due to possible harvest problems if weeds are left uncontrolled, future problems with the weed due to increased numbers of weed seeds in the soil, and grain quality or dockage problems. The use of medic is economical, reduces herbicide usage and cost, and has positive impacts on soil quality. But until economic threshold concept for using herbicides is accepted (or as some producers describe it as the “dirty field” syndrome), the use of medic in large scale production fields will be a concept to be further researched and not a standard viable agronomic practice.
Involvement of Other Audiences
Several field days that highlighted these medic experiments were given during the course of the grant. Dordt field days averaged about 35 growers per year from 1995 to 1998. At a Hay Expo at Dordt, a poster of the positive aspects of medic was given. The number of growers that participated was near 1,500. Tours at the Brookings site averaged over 70 growers per year and at Beresford about 200 growers participated in field days where medic plots were highlighted.
In 1996, two presentations were given at Sustainable Agricultural Society Meeting in Aberdeen, SD. The audience of about 200 consisted of growers, policy makers, and research scientists.
Educational & Outreach Activities
Dordt College Voice featured an article on the medic project in December 1996 with a circulation of about 25,000 copies.
Smeltekop, H.E., D.E. Clay, S.A. Clay, and R. Vos. 1998. The effect of a medic cover crop on soil quality. ASA abstract. 90:213.
Vos, R. and S.A. Clay. 1998. Allelopathic effects of three annual medics. ASA abstract. 90:115.
Smeltekop, H. 1998. Bereford field days. SDSU Exp. Sta. Farm. Approx. 200 producers.
Vos, R., S.A. Clay, and D.E. Clay. 1997. Allelopathy of three annual medics. North Central Weed Sci. Soc. Proc. Abstract. 52:135-136.
Smeltekop, H.E., D.E. Clay, S.A. Clay, and R. Vos. 1997. The impact of annual medic cover crops on soil quality. SDSU Soil/Water Research Annual Report. SDSU Agric. Exp. Sta. Report TB99. Circulation: 225 copies.
Smeltekop, H.E., D.E. Clay, S.A. Clay, and R. Vos. 1996. The effect of medic on soil quality. Symposium A21st Century Agriculture B Creating a Sustainable Future@ co-sponsored by SARE and the Northern Plains Sustainable Agriculture Society to approx. 200 people, Aberdeen, SD.
Vos, R. Poster presented at Hay Expo sponsored by Wallace’s Farmer at Dordt College. Approx. 1,500 growers in attendance.
PhD Dissertation. The use of medic in corn production for the suppression of weeds. R. Vos. Copy will be sent upon completion
M.S. Thesis . Annual medic cover crop impact on soil quality. H. Smeltekop. Copy will be sent upon completion
Areas needing additional study
In order to develop a successful annual medic management system for production agriculture, additional research will have to be conducted. Results from these experiments indicate that the response of corn and weeds to annual medic varies from location to location and year to year. In addition, somewhere between 6 to 15 lb of medic appears to be the ideal seeding rate. When planted at 30 lb, the amount of medic biomass did not increase of over the amount observed at the 15 lb rate. The planting date at Brookings may be more critical than at Sioux Center due to cooler temperatures and shorter growing season. It also appears to be critical that with whatever medic is chosen, that band seeding to interrow areas and the application of N fertilizer needs to be used to limit the impact of medic on corn growth.
Further response evaluation needs to be conducted on other weed species and weed complexes. At both the South Dakota and Iowa site, green and yellow foxtail and common lambsquarters were predominant. However, in the North Central region of the US, several other grass and broadleaf weed species need to be suppressed in production fields. These responses indicate that it may be difficult to develop widespread recommendations for medic seeding rates and planting dates as soil types, weed species, and climate conditions change. Local recommendations for specific medic genotypes will need to be determined. Additional research could be used to identify medic genotypes that are less competitive with corn than barrel medic (‘Caliph’) but still provide good ground cover early in the growing season. Replicated experiments that use medic banded in interrow areas with corn rows treated with appropriate herbicide(s) need to be further evaluated in this system. In addition, time of removal using cultivation in interrow areas or contact type herbicides in row may result in a more reliable system especially in cool, wet years when the length of the medic growing season may provide too much competition for corn. Plant breeding programs could be used to develop specific medic genotypes that could be used in cropping systems as most of the medic genotypes that have been developed are used for forage.
Another important area that must be addressed before this technique is adopted is the concept of economic thresholds for weeds in production size fields. While researchers have done quite a bit of research recently on the bioeconomic threshold of weed densities in corn, this concept has not been widely accepted by producers. One of the problems not addressed is scale of the economic threshold data. Typically, economic threshold research is conducted in small plots where moderate levels of green foxtail or velvetleaf in corn have been shown to be acceptable. Usually yield is not reduced until high weed levels are reached. Producers have a much lower acceptance level of weeds in their fields due to possible harvest problems if weeds are left uncontrolled, future problems with the weed due to increased numbers of weed seeds in the soil, and grain quality or dockage problems. The use of medic is economical, reduces herbicide usage and cost, and has positive impacts on soil quality. But until the “dirty field” syndrome is overcome, the use of medic in large scale production fields is still a concept and not a practice.