Horseweed (Conyza canadensis) Suppression from Cover Crop Mixtures and Fall-Applied Residual Herbicides

Abstract

Horseweed is a problematic weed to control, especially in no-tillage production. Increasing cases of herbicide resistance have exacerbated the problem, necessitating alternative control options and an integrated weed management approach. Field experiments were conducted to evaluate horseweed suppression from fall-planted cover crop monocultures and mixtures as well as two fall-applied residual herbicide treatments. Prior to cover crop termination, horseweed density was reduced by 88% to 96% from cover crops. At cover crop termination in late spring, cereal rye biomass was 7,671 kg ha–1, which was similar to cereal rye–containing mixtures (7,720 kg ha–1) but greater than legumes in monoculture (3,335 kg ha–1). After cover crops were terminated in late spring using a roller crimper, corn and soybeans were planted and horseweed was evaluated using density counts, visible ratings, and biomass collection until harvest. Forage radish winterkilled, offering no competition in late winter or biomass to contribute to horseweed suppression after termination. Excluding forage radish in monoculture, no difference in horseweed suppression was detected between cereal rye–containing cover crops and legumes (crimson clover and hairy vetch) in monoculture. Likewise, horseweed suppression was similar between monocultures and mixtures, with the exception of one site-year in which mixtures provided better suppression. In this experiment, the cover crop treatments performed as well as or better than the fall-applied residual herbicides, flumioxazin+paraquat and metribuzin+chlorimuron-ethyl. These results indicate that fall-planted cover crops are a viable option to suppress horseweed and can be an effective part of an integrated weed management program. Furthermore, cover crop mixtures can be used to gain the benefits of legume or brassica cover crop species without sacrificing horseweed suppression.

Effects of fall-planted cereal cover-crop termination time on glyphosate-resistant horseweed (Conyza canadensis) suppression

“…At Isabella 2018, all cover-crop treatments, with exception of WWH, reduced horseweed density 46% to 56% compared with NC (Table 5). Similarly, previous studies have found cover crops reduced horseweed density 80% at the time of termination compared with NC (Wallace et al 2019;Pittman et al 2019). At Isabella 2019, horseweed densities in the planting-green treatments were 66% lower compared with the early termination treatments (Table 5).…”

Do diverse cover crop mixtures perform better than monocultures? A systematic review

“…The five cases where the mixture performed significantly better than the monoculture fell in four categories: cover crop productivity, weed suppression, N retention, and crop yield promotion. All five were situations the mixtures were of legumes with non-legumes and were planted at rates exceeding substitutive rates-anywhere from 123 to 210% of the full rates used for the monocultures (Kaye et al, 2019;Pittman, Barney, & Flessner, 2019;Teasdale & Abdul-Baki, 1998) -causing the effect of plant mixing to be confounded by the effects of seeding rate. The elevated performance of the cover crop mixtures in these five cases could be more a result of elevated seeding rate than the actual mixing of different cover crop 3 26 13 12 6 5 16 11 119 Note.…”

Alternative performance targets for integrating cover crops as a proactive herbicide-resistance management tool

“…Fall-sown cover crops provide direct competition with weed species that have overlapping life cycles, and cover crop residue left on the soil surface after termination can indirectly influence the recruitment and growth rates of summer annual weed species (Ryan et al 2011b;Teasdale et al 2005). Recent studies have demonstrated that integrating cover crops can increase suppression of glyphosateresistant weeds, including fall-and spring-emerging horseweed [Conyza canadensis (L.) Cronquist] (Cholette et al 2018;Pittman et al 2019;Wallace et al 2019) and summer annuals Palmer amaranth (Amaranthus palmeri S. Watson) and waterhemp [Amaranthus tuberculatus (Moq.) Sauer] (Loux et al 2017;Montgomery et al 2018;Wiggins et al 2015).…”