Cover crop effects on horseweed (Erigeron canadensis) density and size inequality at the time of herbicide exposure

Proactive integrated weed management (IWM) is critically needed in no-till production to reduce the intensity of selection pressure for herbicide-resistant weeds. Reducing the density of emerged weed populations and the number of larger individuals within the population at the time of herbicide application are two practical management objectives when integrating cover crops as a complementary tactic in herbicide-based production systems. We examined the following demographic questions related to the effects of alternative cover-cropping tactics following small grain harvest on preplant, burndown management of horseweed (Erigeron canadensis L.) in no-till commodity-grain production: (1) Do cover crops differentially affect E. canadensis density and size inequality at the time of herbicide exposure? (2) Which cover crop response traits are drivers of E. canadensis suppression at time of herbicide exposure? Interannual variation in growing conditions (study year) and intra-annual variation in soil fertility (low vs. high nitrogen) were the primary drivers of cover crop response traits and significantly affected E. canadensis density at the time of herbicide exposure. In comparison to the fallow control, cover crop treatments reduced E. canadensis density 52% to 86% at the time of a preplant, burndown application. Cereal rye (Secale cereale L.) alone or in combination with forage radish (Raphanus sativus L.) provided the most consistent E. canadensis suppression. Fall and spring cover crop biomass production was negatively correlated with E. canadensis density at the preplant burndown application timing. Our results also show that winter-hardy cover crops reduce the size inequality of E. canadensis populations at the time of herbicide exposure by reducing the number of large individuals within the population. Finally, we advocate for advancement in our understanding of complementarity between cover crop– and herbicide-based management tactics in no-till systems to facilitate development of proactive, herbicide-resistant management strategies.