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. / Doctor of Philosophy / Herbicide resistant weeds are a major challenge for farmers across the globe. With the increased number of weed species resistant to multiple herbicides and the restrictions on pesticide use, farmers need more tools to control weeds. The use of cover crops (CC) to suppress weeds can be a viable integrated weed management tool for farmers.
Although there are multiple benefits associated with CCs, there are also many drawbacks. CCs are an additional input cost for farmers, and require a greater level of management when compared to conventional systems. There are also many questions concerning best management practices in order to reap the benefits of CCs. Previous research indicates CC biomass is the most important factor in achieving weed suppression. Research trials were conducted at Virginia Tech to determine whether CC species as well as the application of preplant herbicide at the time of CC planting would affect CC biomass production as well as weed suppression. Cereal rye-containing plots produced more biomass overall compared to hairy vetch alone or crimson clover and earlier plantings accumulated greater CC biomass. Additionally, planting earlier was significant for CC biomass accumulation while applying a pre-plant herbicide was not. Weed suppression varied by species but was more successful when both a CC and pre-plant herbicide were applied as compared to no CC and no pre-plant herbicide application.
Termination timing and mechanism of CC termination are both known to impact potential CC benefits. Many farmers roll their CC at termination, but it is unclear whether rolling is necessary for weed suppression benefits and soybean yield. Palmer amaranth counts were collected at four and six weeks after planting, as well as yield data across a range of cereal rye CC biomass levels, rolled or left standing at termination, and planted green (CC terminated at the time of planting) versus brown (CC terminated 2 weeks prior to planting). Light penetration measurements were also collected to observe changes in light availability through a CC canopy. Overall, CC treatments reduced Palmer amaranth emergence when compared to no CC, and suppression increased with greater CC biomass. Yield did not differ among treatments, therefore CC management can be tailored to weed suppression efforts. There were clear patterns of light penetration reduction as CC biomass increased. This reduction in light penetration could be part of the reason for the decreased Palmer amaranth emergence with greater CC biomass. Overall, the most important factor affecting weed suppression is accumulation of greater CC biomass.
Research was conducted to determine differences in Palmer amaranth suppression due to CC termination timing, as growers have experimented with planting green while traditionally, planting brown or no CC at all was more common. Palmer amaranth groups were created and followed throughout a 10-week period to examine density and growth rates, which allowed various herbicide programs to be simulated and compared. 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. The most effective programs varied by year, but amongst CC options, planting green with a single POST herbicide was optimal, but overall, no CC treatments were more economical due to the expenses associated with CC seed and planting. If a grower is planning on using a CC, the recommendation is therefore to plant green, as lower costs were associated with this practice.
In addition to weed suppression benefits, the potential for leguminous CC such as hairy vetch to provide nitrogen for the cash crop is another reason that farmers may plant CCs. However, the effect of a CC on weed competition for nitrogen is still unclear, as the nitrogen released by the CC can also stimulate weed emergence while the CC itself is physically suppressing weeds through its biomass. A study was established to evaluate the effects of CC or no CC, weedy or weed free herbicide programs, and six nitrogen side dress fertilizer rates on yield. Overall, greater concentrations of nitrogen in the ear leaf and in grain were found under CC, weed free, and when fertilized-to-yield goal for the respective locations. Yield was not responsive to treatment effects, and agronomic optimum analysis indicated that a side dress nitrogen to yield goal is still the best option even under CC to achieve optimal yields.
Use of CCs is another tool for farmers to implement in their weed control programs. Proper management of CCs can result in increased weed suppression and provide other benefits not examined in this document. However, varying results by location calls for further research to explore the intricacies of CC management and its effect not only on other weed species, but also on other major cash crops.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/117306 |
| Date | 04 January 2024 |
| Creators | Sias, Cynthia |
| Contributors | Plant Pathology, Physiology and Weed Science, Flessner, Michael L., Reiter, Mark S., Cahoon, Charles W., Thomason, Wade E. |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Language | English |
| Detected Language | English |
| Type | Dissertation |
| Format | ETD, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
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