Abstract
Herbicide resistance is a growing concern in dry bean (Phaseolus vulgaris L.) production, which can be mitigated through the adoption of integrated weed management practices. The main objectives of this thesis project were to 1) evaluate the agronomic and economic impacts of combining cover crops and herbicides for weed control in dry beans, 2) evaluate changes in soil health (mainly soil active carbon and microbial biomass) after cover cropping, and 3) evaluate the impacts of cover crops alone for weed control in an organic dry bean system.
Chapter 1 of the thesis is a general introduction and Chapter 2 addresses objective 1, presenting field studies conducted in 2023 and 2024 in Parma and Kimberly, ID, USA. Three fall-planted cover crops (barley, Hordeum vulgare L.; triticale, Triticosecale rimpaui L.; and wheat, Triticum aestivum L.) were planted in October, with a control of no cover crops. Cereal cover crops were either terminated chemically with glyphosate or harvested for forage in May. Three pre-emergence herbicides, one post-emergence, and a nontreated control were applied as herbicide treatments. All three cover crops in Parma, either hayed or chemically terminated, reduced weed biomass by 65 to 80 % and 14 to 54 % in Kimberly compared to the control. All herbicide treatments (except dimethenamid-p + EPTC) reduced total weed biomass by 25 to 75 % in Parma and 33 % to 76 % in Kimberly compared to the nontreated control. Hayed cover crop treatments reduced dry bean density in Parma up to 38 % but had no impact on seed yield. Chemically terminated cover crop treatments, on the other hand, reduced dry bean stand density in Kimberly up to 37 % and seed yield up to 49 %, which was a result of high forage production in 2024. Economic analysis revealed that most cover crops and herbicide combinations were profitable (except for a few combinations where dry bean seed yield was reduced). This study showed the potential of integrating cover crops and herbicides for effective weed management in dry beans.
Chapter 3 focuses on objective 2, where soil samples were collected before and after planting cover crops from the same field studies in Parma and Kimberly, ID in 2023 and 2024. Soil samples were analyzed for permanganate oxidizable carbon (POXC) and phospholipid fatty acid (PLFA) tests to evaluate active carbon and microbial biomass changes respectively after cover cropping. Major changes in soil health were not expected but were conducted for completeness of the study. Post-cover crop active carbon increased in all cover crops and herbicide treatments in Parma, with the highest increase observed in the ‘no cover crop, dimethenamid-p + EPTC’ treatment (up to 39 %). In contrast, post-cover crop active carbon in Kimberly decreased across all treatments (up to 43 % less). Similarly, post-cover crop total microbial biomass increased in all cover crop treatments by 68 % in Parma, while in Kimberly, only barley increased microbial biomass by 14%. These factors suggest that short-term changes in soil health might depend on site-specific factors and might not be achieved in most cases.
Chapter 4 of the thesis addresses objective 3, where a field study was conducted on an organic farm in 2024 in Shoshone, ID, USA. The study included four cover crop treatments (barley, triticale, wheat, and winter pea) arranged in a randomized complete block design with four replications. Winter pea was planted as a control treatment. Cereal cover crops suppressed weed biomass by 60 to 74% compared to winter peas at the time of cover crop termination. However, weed suppression was similar between cereal crops and winter peas after they were incorporated into the soil. Similarly, there were no differences in dry bean stand density and seed yield among the cover crop treatments. Soil samples were also collected before and after planting cover crops to conduct POXC and PLFA tests. Soil tests showed that post-cover crop active carbon decreased in all cover crop treatments by 43 to 61 %. Total microbial biomass increased in wheat (by 18 %) and triticale (by 16%), while it decreased in barley (by 31 %) and winter peas treatment (by 23 %). The study demonstrated that cereal crops could be used as cover crops in dry bean production without concerns regarding their negative impacts on stand densities and seed yield. However, changes in soil health might not be observed in a short time.