Optimizing Weed Suppression Via Cover Crop and Herbicide Programs in the Mid-Atlantic

Abstract

Many Virginia farmers include cover crops in their corn (Zea mays L.) and soybean (Glycine max (L.) Merr.) production due to government subsidies and agronomic benefits. Progressive farmers have optimized cover crop management for specific goals, such as weed suppression and nitrogen fertilizer reductions. Waiting to terminate the cover crop until cash crop planting, so-called "planting green," is often part of the management and is an evolution from the traditional termination timing prior to cash crop planting. To scientifically scrutinize these production systems, three research objectives were developed. These objectives included determining if these systems lead to greater biomass accumulation, increase overall weed suppression, and reduce herbicide inputs. The goal of our first experiment was to determine if a hairy vetch (Vicia villosa R.) and cereal rye (Secale cereale L.) mixture, where the cereal rye was selectively terminated in March, performed better than a hairy vetch monoculture for biomass accumulation and weed suppression. Results indicated that hairy vetch monocultures typically produced greater hairy vetch biomass throughout the season, had greater nitrogen contents, and provided similar weed suppression. Overall, a hairy vetch monoculture cover crop can substitute for a hairy vetch + cereal rye mixture, when the cereal rye is selectively terminated, while providing similar or greater benefits to the following cash crop. Our second objective compared 1-, 2-, and 3-pass herbicide programs initiated either two weeks prior to, or at, corn planting paired with either hairy vetch, hairy vetch + cereal rye, or a winter fallow to determine if herbicide input reductions are possible. We determined a reduction from a 3- pass to a 2- pass herbicide program is possible, however, at least a 2-pass program is needed for season-long weed suppression. Also, herbicide programs that terminated the cover crop at planting (i.e. planted green) tended to provide as good or better weed suppression compared to cover crops terminated prior to planting. While cereal rye is a very popular species for cover cropping, farmers have reported nitrogen immobilization and planting problems because of its extensive biomass and high C:N ratio. Due to this, farmers are interested in substituting black oat (Avena strigosa S.) for cereal rye. Therefore, we conducted two experiments to compare cover crop characteristics. In corn, cereal rye, cereal rye + hairy vetch, black oats, and black oat + hairy vetch were compared while in soybean, black oat and cereal rye treatments were compared. Our results for both experiments indicated that cereal rye and black oat monocultures are similar in terms of lignin content and C:N ratio at cash crop planting. However, black oat treatments typically produced less biomass and suppressed fewer weeds compared to cereal rye treatments. This trend was also seen when comparing the black oat and hairy vetch mixture to the cereal rye and hairy vetch mixture. Ultimately, regardless of cash crop, farmers may still prefer to use a cereal rye monoculture or mixture as their cover crop. Overall, the results of these experiments show that hairy vetch monocultures have the potential to increase weed suppression, nitrogen output, and biomass accumulation compared to hairy vetch and a grass cover crop mixture. However, farmers who include a grass species may lean towards cereal rye because of its benefits over black oats.