1992 Annual Report for ANC92-013
Biological Control of Weeds in Corn and Soybean with Dwarf Brassica Smother Plants
Summary
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.
Methods:
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.
Results:
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 only 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 having the
heaviest weed populations.
Interestingly, dwarf-Brassica provided the best and most consistent weed control at the location
having 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 thirty-year averages.
The dwarf-Brassica smother plant being developed is a cool season plant which typically will
germinate well under the cool spring conditions often found in Minnesota, and then will be
nearing 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.
Farmer Adoption and Impact:
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.
Areas Needing Further Study:
To improve the dwarf-Brassica smother plant system, development of a dwarf-Brassica line with
characteristics that match the requirements for weed suppression should be continued. In order to
accomplish this, a better understanding of how Brassica competes with weeds, especially weedy
grasses, is needed. 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 plants systems for corn and soybean producers. For long-term
sustainability, a perennial cover/smother crop system may be preferable over an annual system.