Biological Control of Weeds in Corn and Soybean with Dwarf Brassica Smother Plants

Final Report for ANC92-013

Project Type: Research and Education
Funds awarded in 1992: $0.00
Projected End Date: 12/31/1995
Matching Non-Federal Funds: $36,000.00
ACE Funds: $67,000.00
Region: North Central
State: Minnesota
Project Coordinator:
Donald Wyse
University of Minnesota
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Project Information

Summary:

In 1994 and 1995, field experiments were conducted in four geographical regions throughout Minnesota to evaluate the effectiveness of dwarf-Brassica smother plants for weed control in corn and soybeans. Additionally, the effects of dwarf-Brassica plants on crop growth and development and grain yields were examined under a variety of soil types and environments. In 1994, three seeding rates of dwarf-Brassica were evaluated. Two early season harvests were conducted for each crop at each site. Broadleaf and grassy weeds, dwarf-Brassica, and corn or soybeans were counted and harvested within specific, defined plot areas for each of the two early season harvests. In general, dwarf-Brassica plants were quite competitive with both corn and soybeans early in the crop growing season. Often, all three seeding rates of dwarf-Brassica resulted in smaller and/or fewer corn and soybean plants and sometimes delayed the crop’s development. The visual differences were more apparent in corn than soybeans. Generally, the stunting effect only lasted until the middle of the growing season. The effectiveness of dwarf-Brassica for weed control was quite variable between sites in 1994. In some instances at least the highest seeding rate of dwarf-Brassica provided significant early season weed control.

In 1994, late-season weed harvests were conducted near the end of the crop growing season at the three locations with the heaviest weed populations. Interestingly, dwarf-Brassica provided the best and most consistent weed control at the location with the greatest weed populations. In 1994 there were no apparent positive or negative effects of dwarf-Brassica on corn or soybean grain yields. However, the presence of weeds significantly decreased grain yields of corn and/or soybeans at three of the five locations. Therefore, it is possible that by providing weed control, the dwarf-Brassica may have had an indirect, but positive, effect on maximizing grain yields. It is important to note that growing conditions were optimal for corn and soybeans in Minnesota in 1994, and weed pressure was fairly light at four of the five experiment locations, which was due to past weed control practices and favorable growing conditions. The experiments conducted at four locations in 1995 were similar to the 1994 studies except that another dwarf-Brassica seeding rate was added that was twice the highest seeding rate used in 1994.

Additionally, the 1995 experiments were conducted in fields with high weed populations to most accurately assess the ability of dwarf-Brassica to control weeds in corn and soybeans. In 1995, climate was the most important factor affecting the performance of dwarf-Brassica. The 1995 growing season was very different from 1994 and was characterized by a cool to normal spring followed by a hot dry early summer, based on 30-year averages. The dwarf-Brassica smother plant we have been developing is a cool season plant that typically will germinate well under the cool spring conditions often found in Minnesota, and then will near the end of its life cycle by mid-summer. The average temperatures in mid-June of 1995 were several degrees above normal, while precipitation was below normal. The hot, dry weather experienced in mid-June occurred during the beginning of the dwarf-Brassica reproductive growth phase which is when most plants are especially vulnerable to environmental stress. Because the growth and development of dwarf-Brassica was delayed or arrested during mid-June at most sites in 1995 it was not able to compete very well with weeds found at these locations.

In 1995 the presence of weeds significantly decreased grain yields at all of the corn experiment locations and three out of four soybean locations. The dwarf-Brassica seeding rate generally had no effect on crop grain yields except at the south central Minnesota location where all but the lowest seeding rate of Brassica decreased corn grain yields. This site was the one location where dwarf-Brassica was the least affected by the high early season temperatures. Dwarf-Brassica was able to achieve maximum ground cover at this location and was fairly competitive with corn but did not suppress weed growth enough to eliminate yield reductions due to weeds. Based on the past two years of field research, climate had the greatest impact on the ability of dwarf-Brassica to compete with and control weeds. Additional research is needed to further determine the effectiveness of dwarf-Brassica for weed control in corn and soybean systems in Minnesota.

Project Objectives:

1. Develop dwarf-Brassica smother plants for weed control in soybeans and corn through classical plant breeding.

2. Evaluate the effectiveness of dwarf-Brassica smother plants for controlling weeds and reducing soil erosion.

3. Identify the factors that could impede the adaptation of the dwarf-Brassica smother plant system by corn and soybean producers.

Research

Research results and discussion:

Objective 1. Continue to develop, through classical plant breeding, dwarf-Brassica smother plants for weed control in soybeans and corn.

Throughout 1993 and 1994, ongoing plant breeding research continued in both the field and greenhouse to further improve the ability of dwarf-Brassica plants to control weeds. Field research was conducted to determine the cause of the apparent increased height of the dwarf-Brassica line being used in this project. Over time, the dwarf-Brassica height has become significantly taller. Our results indicated that at least part of the increase in plant height was due to a genetic shift in the population. This occurred because there was a selective advantage favoring taller plants that could produce more seed under field conditions. A potential problem with dwarf-Brassica is that it can produce only small amounts of seed. This is because dwarf-Brassica has a short life cycle that limits its growth and development. If smother plants such as dwarf-Brassica are to offer a viable alternative to conventional methods of weed control, large quantities of inexpensive seed will needed annually. To address this problem, plant breeding research was conducted in both the greenhouse and the field to continue to improve the growth characteristics of dwarf-Brassica for use as a smother plant. Seed from new dwarf-Brassica lines that appeared promising due to rapid germination, dwarf stature, broad leaves and high seed production were identified and reserved for use in seed increase and future evaluation in field trials. The last component of the breeding program in 1994 was to screen large numbers of commercially available and experimental Brassica spp. under field conditions to identify lines that might be of future use in the dwarf-Brassica smother plant breeding program.

The objectives of the 1995 dwarf-Brassica breeding program were to continue to evaluate new dwarf-Brassica lines for weed control in corn and soybeans and to increase seed supplies of the most promising dwarf-Brassica lines identified in 1994. Seed supplies for the new dwarf-Brassica lines were limited so those lines that were evaluated in small plot studies in 1995. The new dwarf-Brassica lines provided statistically greater weed control compared to control plots containing no Brassica, but their ability to control weeds was similar to the dwarf-Brassica type we developed and have been using for the past two years in the large scale field studies. In 1995 we attempted to make further plant breeding selections for shorter plant height from the most promising plant material identified in 1994, but this was not successful since the hot, dry weather we experienced in June occurred shortly after these dwarf-Brassica lines germinated and resulted in death of the dwarf-Brassica plants.

Objective 2. Evaluate the effectiveness of dwarf-Brassica smother plants for controlling weeds and reducing soil erosion.

In 1993, dwarf-Brassica field experiments were established but discontinued because of poor growing conditions at the experiment locations. The first year of large scale field evaluation was changed from 1993 to 1994. In 1994 and 1995 large scale field studies were conducted in several regions of the state to evaluate the effects of dwarf-Brassica on the growth and development of corn and soybeans and to determine the effectiveness of dwarf-Brassica for controlling weeds. These field studies were located both on farmer/cooperator fields and at University of Minnesota Agricultural Experiment Stations.

In 1994, dwarf-Brassica plants were quite competitive with both corn and soybeans early in the growing season. Often, all three seeding rates of dwarf-Brassica resulted in smaller and/or fewer corn and soybean plants and sometimes delayed the crop’s development. The visual differences were more apparent in corn than soybeans. Generally, the stunting effect only lasted until the middle of the growing season. The one exception occurred in southeast Minnesota, at the Charles Priebe farm, where the dwarf-Brassica delayed corn growth and development throughout most of the growing season.

The effectiveness of dwarf-Brassica for weed control was quite variable between sites in 1994. In some instances at least the highest seeding rate of dwarf-Brassica provided significant early season weed control. In general, dwarf-Brassica was more effective at controlling grassy weeds than broadleaf weeds. However, broadleaf weed populations were quite low at most of the sites in 1994. From results obtained in 1994, dwarf-Brassica appeared to have the potential to provide early season weed control in corn and soybeans, but its weed control ability was variable and somewhat unpredictable. In 1994, late season weed harvests were conducted near the end of the crop growing season at the three locations having the greatest weed populations. At the south central Minnesota site, dwarf-Brassica provided significant late season weed control in corn. Of the five geographic locations, this site had the greatest weed populations. It was very encouraging that the dwarf-Brassica appeared to provide the best and most consistent weed control at the site having the greatest weed populations.

In 1994, none of the seeding rates of dwarf-Brassica had a significant effect on corn or soybean grain yields. The presence of weeds significantly decreased grain yields in two of the five corn experiments and three of the five soybean experiments. It is possible that by providing weed control, the dwarf-Brassica may have had an indirect, but positive, effect on maximizing grain yields. Because there were no long lasting negative effects observed when dwarf-Brassica was grown with corn or soybeans in 1994, we were optimistic that dwarf-Brassica might be suitable for use as a smother plant for biological weed control.

The experiments conducted in 1995 were similar to the 1994 studies except that another dwarf-Brassica seeding rate was added that was twice the highest seeding rate used in 1994. We wanted to assess whether a higher dwarf-Brassica seeding rate would result in improved weed control without adversely affecting crop growth and development. The dwarf-Brassica seeded in 1995 was of the same genetic background as the Brassica used in 1994. Since weed populations were moderate to fairly light in 1994 we wanted to ensure that our 1995 experiment locations would have at least moderate to heavy weed pressure to best assess the effectiveness of dwarf-Brassica for weed control. We worked with our farmer cooperators and Experiment Station personnel to select areas that had high weed populations in the past. In 1995 we collaborated with three of the four farmer cooperators from 1994 and added experiments in southwestern Minnesota but discontinued studies in west central Minnesota.

In 1995, climate was the most important factor affecting the performance of dwarf-Brassica. In 1995 we experienced a cool, wet spring that delayed normal planting dates at two of our locations. The cool spring was followed by an unusually hot and dry early summer at all of our experiment locations. The hot temperatures that occurred in June throughout the state had a dramatic effect on the growth and establishment of the dwarf-Brassica, which is a cool season plant that typically germinates and begins growth under cool temperatures and has completed its life cycle by the time the temperatures begin to rise in mid-summer. The hot and droughty conditions in 1995 occurred when the dwarf-Brassica was just beginning to flower. Most plants are very susceptible to environmental stresses when they are entering their reproductive stage of growth. Adverse weather conditions during this stage of growth may have severe and irreversible negative effects. The hot dry weather arrested normal growth of the dwarf-Brassica at two of the experiment locations. The corn experiment in southern Minnesota at Waseca was discontinued because the heat killed the dwarf-Brassica. The hot weather did not harm the corn, soybeans or weed species nearly as much as it affected the dwarf-Brassica. Because of this, dwarf-Brassica did not compete as much with either corn or soybeans as compared to 1994. Most importantly, the dwarf-Brassica generally was not able to compete with, and thereby control, the weed species predominant at most sites. The predominant weeds found at the 1995 locations were foxtail species, which are grassy weeds. The hot, dry early summer weather favored the growth and development of the foxtail species and gave the growing advantage to these weeds over the dwarf-Brassica. Corn and soybeans were also more heat tolerant than dwarf-Brassica, and all sites received much needed rainfall in time to prevent yield losses due to early season environmental stress.

In 1995, weed control ratings were scheduled at monthly intervals to evaluate the ability of dwarf-Brassica to control weeds. Because of the hot weather experienced early in the growing season, the monthly ratings continued at only two of the four locations for the remainder of the growing season. The first weed control comparisons were made when the corn was growing vegetatively but approaching tassel development, and when the soybeans were beginning to flower. At the time of the first weed control ratings in corn, the medium-high seeding rate of dwarf-Brassica provided a level of weed control that was statistically greater than corn growing without the dwarf-Brassica smother plant at both locations. Unfortunately, the 40-45% weed control obtained from the most optimal dwarf-Brassica seeding rate would be unacceptable to most farmers. The highest seeding rate treatment added in 1995 proved to be somewhat more competitive with both crops but did not improve weed control efficacy. At the time of the first weed control evaluation in soybeans, weed populations were generally still low to moderate. Weed populations in plots containing dwarf-Brassica were not statistically different from weed populations in soybeans grown without Brassica. By mid-season, weed populations had increased and the medium and medium-high brassica seeding rates resulted in improved weed control of 77-88%.

Near the end of the crop growing season, late-season weed harvests were conducted at the two locations where dwarf-Brassica survived the early season heat. Weeds were harvested from the experimental plots when the crops had reached physiological maturity but the weeds were still actively growing. The dwarf-Brassica did not provide statistically significant late season weed control in either corn or soybeans, but this was not completely unexpected since dwarf-Brassica typically has completed its life cycle by late in the crop growing season. Dwarf-Brassica has the best potential for controlling weeds early in the growing season before the crop canopy has formed, but it is evident that above-normal temperatures early in the growing season can drastically affect and possibly arrest the growth of dwarf-Brassica.

In 1995, the presence of weeds significantly decreased grain yields at all of the corn experiment locations and at three of the four soybean locations. At the south central Minnesota location all seeding rates but the lowest rate of dwarf-Brassica resulted in significant reductions in corn grain yields. This site was the one location where dwarf-Brassica was the least affected by the high early season temperatures. Dwarf-Brassica was able to achieve maximum ground cover at this location and was fairly competitive with corn but did not suppress weed growth enough to eliminate yield reductions due to weeds. Dwarf-Brassica seeding rates had no effect on soybean grain yields at any of the experiment locations in 1995.

After the past two years of field and greenhouse research we have gained information about the optimal dwarf-Brassica seeding rate to potentially control weeds without being overly competitive with corn or soybeans. Dwarf-Brassica may have the potential to work as a smother crop for corn and soybean production systems, but conditions need to be optimal for its growth and development early in the growing season. If dwarf-Brassica is harmed by environmental stress early in the growing season it may not compete well with the typical weed species found in most corn/soybean production systems.

Objective 3. Identify the factors that could impede the adaptation of the dwarf-Brassica smother plant system by corn and soybean producers.

After two years of large scale field research, it appears that dwarf-Brassica may have possible application for corn and soybean systems in Minnesota but its weed control ability is unpredictable and possibly only moderate at best. Factors that could impede the adaptation of this smother plant system by corn and soybean producers include practical issues involved with planting, such as method of seeding and equipment to use. Also, dwarf-Brassica must be seeded very shortly after the crop is planted, and this requires additional labor and other resources at a very busy time during the growing season. Since the dwarf-Brassica developed by the University of Minnesota has been selected to have a short life cycle, there are no special management needs at the time of grain harvest.

Research conclusions:

If dwarf-Brassica smother plants can effectively control weeds in corn and soybean production systems, some positive benefits would include reduced herbicide cost and reduced herbicide load in the environment. However, because of the performance of dwarf-Brassica in 1995 we cannot be certain that dwarf-Brassica smother plants can provide reliable weed control. To date, we have not conducted an economic analysis to compare the cost of adding dwarf-Brassica to a corn or soybean system to the potential herbicide savings. This is primarily because of the generally poor performance of dwarf-Brassica in 1995. Although we have not specifically evaluated the effects of dwarf-Brassica on soil erosion, since dwarf-Brassica can provide substantial ground cover when growing conditions are favorable, it appears there may be potential to reduce soil erosion in areas where soil erosion is of concern.

New Hypotheses

To improve the dwarf-Brassica smother plant system we need to continue to develop dwarf-Brassica lines with characteristics that match the requirements for weed suppression. In order to accomplish this, we need a better understanding of how we can improve dwarf-Brassica so it can be a better competitor with weeds, especially weedy grasses. Continued plant breeding research must be followed by field evaluation of weed control abilities of new dwarf-Brassica lines. Additionally, there is a need to research different types of smother plant systems for corn and soybean producers. For long term sustainability, a perennial cover/smother crop system may be preferable over an annual system. At the University of Minnesota we have already formed a multi-disciplinary Cover Crop Research Group to continue to research and develop new cover crop systems. In 1995 we screened a large number of potential cover crop species for weed control in corn and soybeans. We selected some of the more promising species, including dwarf-Brassica, and will continue to evaluate them in field studies in 1996. Over the past two years we have gained a lot of new information about the dwarf-Brassica smother plant and will use this information to further advance our cover/smother crop systems research at the University of Minnesota.

Economic Analysis

To date, we have not had the opportunity to conduct an economic analysis of the dwarf-Brassica smother plant system. At this point it would be premature to conduct an economic analysis as the weed control effectiveness has been somewhat variable.

Farmer Adoption

All four farmers who participated in this project in 1994 are cash grain farmers. The Charles Priebe farm in Waseca is located in southeastern Minnesota on a Webster clay loam soil. The Elton and Larry Goman farm in Jeffers is located in southwestern Minnesota on a Webster Clay loam. The Keith Stroman farm in Alberta is located in west central Minnesota on a Forman clay loam soil. The two locations on University of Minnesota Agricultural Experiment Stations were in south central Minnesota at Rosemount, on a Waukegan silt loam soil and in west central Minnesota at Morris on a McIntosh silt loam. The climate in all five locations in 1994 was characterized by slightly above normal temperatures and precipitation from April through October of 1994, based on 30-year averages. In 1995 we collaborated with three of the four farmer cooperators from 1994. They were Elton and Larry Goman in southwestern Minnesota on a Webster-Delft clay loam soil and Charles Priebe in southeastern Minnesota on a Webster clay loam soil. In 1995 we conducted experiments at University of Minnesota Agricultural Experiment Station locations in Rosemount, MN on a Waukegan silt loam soil and in southwestern Minnesota at Lamberton on a Ves loam soil.

Farmer Evaluation/Testimonials

In 1994, Charles Priebe of Waseca, MN said he “did notice the stunting (due to dwarf-Brassica) in the corn plots and the soybean plots seemed weedy.” Keith Stroman of Alberta, MN said the dwarf-Brassica “didn’t seem to do much good for controlling weeds” and Larry and Elton Goman said they “couldn’t really seem to see any differences (in weed control).” It should be emphasized that weed pressure at both the Stroman and Goman farms was light in 1994, and there were, in fact, few visual differences apparent between experimental treatments. In 1995, Charles Priebe of Waseca, MN said “The soybean plots looked very weedy.” The 1995 corn experiment at the Charles Priebe farm was the study that had to be discontinued because the dwarf-Brassica was killed by the early season heat. Elton Goman of Jeffers, MN said he “didn’t think it (dwarf-Brassica) did a whole lot for weed control” and that he “can’t see it being a viable option (for weed control) at this point.”

Producer Involvement

In 1994, four farmer-cooperators were involved with the dwarf-Brassica project at the University of Minnesota. Three of the five locations for these experiments were on-farm, with the remaining two locations being on University of Minnesota Agricultural Experiment Stations. In 1994 there were no conferences, workshops, or field days associated with this research project since it was still in the early experimental stages. In 1995 three farmer-cooperators were involved with this project. All three farmers had cooperated with us in 1994. We also conducted experiments at two University of Minnesota Agricultural Experiment Stations. We had planned to conduct a summer field day demonstration at the Southwest Minnesota Agricultural Experiment Station but because the dwarf-Brassica was severely stressed by the early hot and dry weather we were not able to include this as part of their summer field day.

Changes in Practice

Since the dwarf-Brassica smother plant concept is still in a relatively experimental stage of development, we cannot report any specific changes in practice at the present time.

Operational Recommendations

Because of the variable performance of dwarf-Brassica after two years of field and greenhouse experiments we cannot make any specific recommendations to farmers in terms of day-to-day operations.

Participation Summary

Educational & Outreach Activities

Participation Summary

Education/outreach description:

After two years of field studies no scientific or news articles have been published to date on the effectiveness of dwarf-Brassica for weed control in corn and soybeans, primarily because the data have been somewhat variable. We had planned to conduct a field demonstration at the Agricultural Experiment Station in southwestern Minnesota, but this site was one of the two locations where dwarf-Brassica was severely affected by the hot dry conditions in early summer.

Project Outcomes

Recommendations:

Areas needing additional study

The areas needing additional study beyond the scope of the current project are related to the need to better understand how to make dwarf-Brassica better able to compete with weeds. We need to combine classical plant breeding techniques with biotechnology to develop dwarf-Brassica and other smother plants that will provide weed control with a wide range of agricultural crops. Through these techniques it may be possible to improve dwarf-Brassica by (a) increasing its ability to suppress weed growth, (b) be less competitive with the crop, (c) terminate its life cycle even sooner and (d) be even better able to reduce soil erosion.

Additionally, we hope to be able to address the larger question of identifying other types of cover/smother crop systems for corn and soybeans. Ideally, a cover/smother crop for corn and soybean systems in Minnesota would be a perennial, winter annual or biennial plant that (a) establishes easily, (b) has good winter hardiness, (c) has minimal competition with the main crop, (d) controls weeds, (e) fixes nitrogen, (f) reduces soil erosion, (g) provides snow catch, and (h) reduces disease incidence.

Developing more sustainable crop production systems is a priority for the College of Agricultural, Food and Environmental Sciences at the University of Minnesota. Within the Department of Agronomy and Plant Genetics, we are planning to form a Farmer/Researcher Cooperative that will enable us to link up research needs identified by both agricultural researchers and crop producers, with potential cooperators. Establishing such a Cooperative will allow us to move quickly from the development of new ideas to evaluation in actual crop production systems and will promote dialogue between farmers and agricultural scientists in Minnesota.

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.