Field experiments were conducted in 2017 and 2018 at two locations in Indiana to evaluate the influence of cover crop species, termination timing, and herbicide treatment on winter and summer annual weed suppression and corn yield. Cereal rye and canola cover crops were terminated early or late (2 wk before or after corn planting) with a glyphosate- or glufosinate-based herbicide program. Canola and cereal rye reduced total weed biomass collected at termination by up to 74% and 91%, in comparison to fallow, respectively. Canola reduced horseweed density by up to 56% at termination and 57% at POST application compared to fallow. Cereal rye reduced horseweed density by up to 59% at termination and 87% at POST application compared to fallow. Canola did not reduce giant ragweed density at termination in comparison to fallow. Cereal rye reduced giant ragweed density by up to 66% at termination and 62% at POST application. Termination timing had little to no effect on weed biomass and density reduction in comparison to the effect of cover crop species. Cereal rye reduced corn grain yield at both locations in comparison to fallow, especially for the late-termination timing. Corn grain yield reduction up to 49% (4,770 kg ha–1) was recorded for cereal rye terminated late in comparison to fallow terminated late. Canola did not reduce corn grain yield in comparison to fallow within termination timing; however, late-terminated canola reduced corn grain yield by up to 21% (2,980 kg ha–1) in comparison to early-terminated fallow. Cereal rye can suppress giant ragweed emergence, whereas canola is not as effective at suppressing large-seeded broadleaves such as giant ragweed. These results also indicate that early-terminated cover crops can often result in higher corn grain yields than late-terminated cover crops in an integrated weed management program.

Synthetic-auxin herbicides are often applied for horseweed control before soybean planting. However, certain days of planting interval must be maintained before soybean planting, depending on the product and rate used, because of potential crop phytotoxicity. Halauxifen-methyl is a new synthetic-auxin herbicide for horseweed control in preplant applications in soybean. Field experiments were conducted in 2015 and 2016 in Indiana to evaluate soybean phytotoxicity in response to applications of halauxifen-methyl (5 g ae ha−1) at five preplant intervals (0, 1, 2, 3, and 4 weeks before planting [WBP]). In 2015, soybean phytotoxicity was not observed for any of the preplant intervals at any of the sites. In 2016, 0% to 15% phytotoxicity was observed at 14 d after planting (DAP) when halauxifen-methyl was applied at planting, 1 WBP, and 2 WBP at different sites. Soybean phytotoxicity was expressed in the unifoliate leaves only at 14 DAP. However, the first trifoliate did not show any injury symptoms at 21 DAP from any preplant application timing. Preplant application intervals for halauxifen-methyl did not affect soybean stand counts or grain yield in any site-year. Therefore, field results indicated that halauxifen-methyl applied alone can cause slight soybean phytotoxicity in preplant applications. In growth-chamber bioassays, reductions in soybean biomass, plant length, and emergence were accentuated at 30 C, compared with 20 or 15 C, when halauxifen-methyl was applied at 20 or 40 g ae ha−1. These results contradict the currently held paradigm in which lower temperatures generally increase crop phytotoxicity levels to herbicide soil residual.

Synthetic auxin herbicides such as 2,4-D and dicamba are often utilized to control broadleaf weeds in preplant burndown applications to soybean. Halauxifen-methyl is a new synthetic auxin herbicide for broadleaf weed control in preplant burndown applications to corn, cotton, and soybean at low use rates (5 g ae ha–1). Field experiments were conducted to evaluate efficacy and weed control spectrum of halauxifen-methyl applied alone and in mixtures with 2,4-D (560 g ae ha–1), dicamba (280 g ae ha–1), and glyphosate (560 g ae ha–1). Glyphosate-resistant (GR) horseweed was controlled with halauxifen-methyl applied alone (90% control) and in mixtures (87% to 97% control) 35 d after treatment (DAT). Common ragweed was controlled 93% with halauxifen-methyl applied alone and 91% to 97% in mixtures 35 DAT. Halauxifen-methyl applied alone resulted in poor giant ragweed control 21 DAT (73% control); however, mixtures of halauxifen-methyl with 2,4-D, dicamba, or glyphosate controlled giant ragweed (86% to 98% control). Halauxifen-methyl alone resulted in poor redroot pigweed control (62% control) 21 DAT; however, mixtures of halauxifen-methyl with dicamba, 2,4-D, or glyphosate controlled redroot pigweed (89% to 98% control). Halauxifen-methyl controls GR horseweed and common ragweed applied alone and in mixtures with other synthetic auxin herbicides and glyphosate. Furthermore, mixing 2,4-D or dicamba with halauxifen-methyl can increase the weed control spectrum in preplant burndown applications.