This Is AuburnElectronic Theses and Dissertations

Integrating Cover Crops and Herbicides for Weed Control in Soybean and Cotton




Kumari, Annu

Type of Degree

PhD Dissertation


Crop Soils and Environmental Sciences


In mid-South, southeastern, and northeast United States soybean production regions, the evolution of herbicide-resistant weeds has become a significant management challenge for growers. The issue of rising herbicide costs for managing herbicide-resistant weeds is also a growing concern, leading to the utilization of cover crops as an integrated weed management strategy for addressing these challenges. Field experiments were conducted at two locations in Alabama in 2022 to evaluate winter cereal cover crops including a mixture, and herbicide system integration in soybean. Treatments included five cover crops: oats, cereal rye, crimson clover, radish, and a cover crop mixture. Cover crops were evaluated for their weed-suppressive characteristics compared to a winter fallow treatment. Additionally, four herbicide treatments were applied: a pre-emergence (PRE) herbicide, a post-emergence (POST) herbicide, PRE plus POST herbicides, and a non-treated (NT) check. The PRE herbicide was S-metolachlor, the POST treatment contained a mixture of dicamba and glyphosate. The PRE plus POST system contained the PRE application followed by POST application. Our results show that cereal rye and the cover crop mixture provided weed biomass reduction compared to all cover crop treatments across both locations. Furthermore, we observed greater soybean yield following the cereal rye cover crop than the winter fallow treatment at one location. POST and PRE+POST herbicide treatment resulted in greater weed biomass reduction and improved soybean yield than the PRE herbicide treatment alone and NT check at both locations. A field study conducted in Alabama at three locations from autumn 2021 through the crop harvest in 2022 aimed to evaluate the combined effect of cover crop residue and herbicides for weed control and improved cotton lint yield. The experiment was conducted in split plot design with main plots consisting of six cover crop treatments: cereal rye, crimson clover, oats, radish, cover crop mixture, and winter fallow. The subplots included four herbicide treatments: 1) PRE, pendimethalin + fomesafen, 2) POST, dicamba + glyphosate + S-metolachlor, 3) PRE followed by POST, and 4) NT check. Cover crops, excluding radish, exhibited greater weed biomass reduction than winter fallow with corresponding herbicide treatments of either PRE, POST, or PRE+POST as compared to control (winter fallow and NT check). Considering PRE+POST treatment, cereal rye, crimson clover, oats, and cover crop mixture provided >95% weed biomass reduction as compared to control. Cereal rye outperformed and showing higher weed biomass reduction than radish relative to control. PRE+POST herbicide treatment resulted in greater lint yield than other treatments. Cereal rye resulted in a greater lint yield than winter fallow at one out to three locations. In conclusion, integrating herbicides along with the incorporation of high residue cover crops such as cereal rye, is an effective weed management strategy to control troublesome weeds. A greenhouse experiment was conducted to evaluate the germination and growth response of troublesome southeastern weeds to various cereal rye residue levels. Trays having Palmer amaranth, sicklepod, ivyleaf morningglory, and large crabgrass seeds mixed with organic garden soil were covered uniformly by four different biomass of cereal rye residue. The following field experiment was conducted at two locations in Alabama in a split-plot design, with the main plot factor being four seeding rates of cereal rye to obtain various cereal rye biomass. In the sub-plot factor, a preemergence herbicide flumioxazin and NT check were considered. The greenhouse results illustrated that Palmer amaranth, sicklepod, and large crabgrass showed decreased seed germination and lesser weed biomass under higher biomass of cereal rye. In both greenhouse and field conditions, germination of ivyleaf morningglory was not decreased with increasing cover crop biomass. Palmer amaranth was most responsive to germination, which decreased with increasing cover biomass, due to their small seed size. Cereal rye biomass with Palmer amaranth counts was strongly negatively correlated with a coefficient of 0.83, while weakly negatively correlated for ivyleaf morningglory with 0.49. In conclusion, increasing biomass of cereal rye residue is effective in suppressing Palmer amaranth seed germination. Flumioxazin herbicide treatment showed 90-95% control while NT check exhibited approximately 30-40% control of Palmer amaranth and ivyleaf morningglory. Growers often report their preferences of cover crop seeding rates below the standard recommendation to reduce the cost of implementation. Despite the extensive research conducted on cover crops, there continues to be a wide range of suggested seeding rates for winter annual cover crops in the southern region of the United States. Moreover, as cover crop mixtures are gaining popularity due to multiple benefits, finding a balance between managing competition and potential tradeoffs between overall biomass production and seed costs can be accomplished by adjusting the seeding rates of each species in a mixture. A collaborative research trial was conducted in 2020 and 2022 across various states in the southern United States. Five legumes species were evaluated: hairy vetch, crimson clover, common vetch, winter pea, and berseem clover at four seeding rates. Each leguminous cover crop was planted separately with or without cereal rye in a mixture at a seeding rate of 33.63 kg ha-1. The findings from this study showed that incorporating cereal rye within a mixture with legume cover crop resulted in a significant increase in cover biomass compared to using the legume cover crop alone across all locations. However, the trial did not indicate any considerable impact of the seeding rate on cover crop biomass production. Averaged across locations, we found that crimson clover and hairy vetch produced greater cover biomass than berseem and common vetch when planted as monocultures.