Cover crop programs, termination methods and timings, and suppression mechanisms on weed growth and competition

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Virginia Tech

Herbicide resistance, regulations on pesticide use, and cost of pesticides are all challenges for managing weeds in production agriculture. The use of cover crops (CC) has emerged as a promising integrated weed management tool to aid in weed suppression. There are many questions concerning the best management practices to reap the most benefits from CC. Research was conducted to determine if the application of a pre-plant herbicide as well as the type of CC planted would increase CC biomass and subsequent winter weed suppression. Early planting and selecting a cereal rye or a cereal rye-containing mixture are the most important factors to obtain the greatest CC biomass production. Additionally, the combination of a CC and a pre-plant herbicide increased weed suppression compared to a no CC (winter fallow) treatment or CC without a pre-plant herbicide. The difference in Palmer amaranth emergence between a rolled cereal rye CC or one that is left standing was also examined along with termination timing to achieve different CC biomass levels. Overall, greater CC biomass suppressed more Palmer amaranth, but treatments of rolled or standing or termination timing did not affect weed suppression consistently. Light penetration data also showed that greater CC biomass led to a decrease in light penetration through the CC canopy, which could be a factor in reducing Palmer amaranth emergence particularly at the greater CC biomass accumulation levels. Additionally, studies were conducted to investigate the effect of cereal rye CC termination timing (i.e., "planting green" being CC terminated at the time of soybean planting or "planting brown" being CC terminated 2 weeks prior to planting) on Palmer amaranth suppression, as well as to determine how termination timing influences herbicide program optimization. A delay in emergence and growth rate of Palmer amaranth was documented in the CC containing plots when compared to the no CC plots, but no differences were observed between the termination timings. Additionally, significantly lower Palmer amaranth densities were observed under CC containing plots when compared to the no CC treatments. Within CC treatment options, the most economical option was planting green with a single postemergence herbicide application, but overall, no CC treatments were more economical programs. Finally, research was conducted to understand weed and corn competition for nitrogen when hairy vetch + cereal rye CC was present. A range of side dress nitrogen fertilizer rates, weedy versus weed free herbicide programs, and CC versus no-CC treatments were compared. Overall, yield did not differ among treatments. Ear leaf and grain nitrogen was generally greater under weed free, CC, and when fertilized at or above yield goals respective of location. Despite these findings, early season weed control in corn is still necessary to achieve maximum potential yield. These studies indicate that CC biomass is consistently the most important factor for achieving weed suppression, and that CC results can vary in response to environmental and management effects. More research is therefore necessary to evaluate the effects of CC over greater periods of time.

cover crops, planting green, planting brown