Weed management should focus on combining different methods to prevent and control weed populations.
“[This is] not only in the short term but also in the long term,” says Dr Suzette Bezuidenhout, acting scientific manager of Cedara’s crop protection unit.
She adds that cultural weed management practices are important. These include production practices that improve crop competitiveness such as cover crops in combination with conservation tillage.
Allelopathy is a natural process whereby a plant produces one or more biochemicals that influence the germination, growth, survival, and reproduction of other plants.
Bezuidenhout explains that allelopathic cover crops release allelochemicals into the environment and can be used to enhance weed management.
Researchers are constantly conducting field and tunnel experiments to evaluate the weed control abilities of various cover crops and cultivars in combination with the application of herbicide.
Recently, researchers evaluated the effects of two cover crops, Italian ryegrass (Lolium multiflorum) and stooling rye (Secale cereale), without herbicide use, on the growth of maize and yellow nutsedge (Cyperus esculentus) in the field.
The trial involved three control treatments, namely weed residue left on the soil surface, herbicide application, and weed control by hoeing.
In a tunnel experiment, oats, stooling rye and three cultivars of ryegrass were used to evaluate their influence on maize and yellow nutsedge growth and development.
The field experiment examined the desiccation times of cover crops on their weed-control abilities by spraying them with glyphosate four and two weeks before planting and at planting.
Minimum-till maize was planted into the residue with selected spraying of pre- and post-emergence herbicides, and its growth and development were evaluated.
In the first field experiment, maize emergence and growth were delayed in the presence of residues of both cover crop species, and especially in annual ryegrass residues.
C. esculentus growth was significantly inhibited in the area between the maize planting rows by the cover crops for the first 14 days after maize emergence. This growth-suppressing effect diminished after 28 days.
In the tunnel experiment, maize and C. esculentus growth were suppressed, especially by the root residues of the cover crops. The annual ryegrass cultivar Midmar was the most suppressive.
The field experiment indicates that spraying annual ryegrass at planting reduced weed and maize growth the most.
Adequate weed control was not achieved by applying only post-emergence herbicides.
Combining annual ryegrass residues sprayed at planting with only post-emergence herbicides applied later in the season resulted in the lowest maize yields.
Weed growth can be reduced by the allelochemicals leached from cover crop residues, but to achieve prolonged, effective weed control, a farmer needs to apply herbicide and retain mulch on the soil surface.
“More research is needed to establish principles of cover crop weed management in order to define its role in a weed management strategy,” says Bezuidenhout. “The use of cover crops for weed control should therefore be considered a tool that is supplementary/complementary to standard weed-control practices aimed at managing weed populations in the long-term.”
Source: Bezuidenhout, SR. ‘The use of allelopathy in a weed management strategy’. Department of Agriculture and Environmental Affairs, KZN. Retrieved from kzndard.gov.za/research-reports.