After termination, cover crop residue can suppress weeds by reducing sunlight, decreasing soil temperature, and providing a physical barrier. Experiments were implemented to monitor horseweed suppression from different cover crops as well as two fall-applied residual herbicide treatments. Results suggest that cover crops, other than forage radish in monoculture, can suppress horseweed more consistently than flumioxazin + paraquat or metribuzin + chlorimuron-ethyl. Cover crop biomass is positively correlated to weed suppression. Subsequent experiments were designed to determine the amount of weed suppression from different cover crop treatments and if carbon to nitrogen (C:N) ratios or lignin content are also correlated to weed suppression or cover crop residue thickness. Results indicate that cereal rye alone and mixtures containing cereal rye produced the most biomass and suppressed weeds more than hairy vetch, crimson clover, and forage radish alone. Analyses indicate that lignin, as well as biomass, is an important indicator of weed suppression. While cover crops provide many benefits, integrating cover crops into production can be difficult. Hairy vetch, a legume cover crop, can become a weed in subsequent seasons. Multiple experiments were implemented to determine germination phenology and viability of two hairy vetch cultivars, Groff and Purple Bounty, and to determine when viable seed are produced. Almost all germination occurred in the initial cover crop growing season for both cultivars. Both cultivars had <1% of viable seed at the termination of the experiment. These results indicate that seed dormancy is not the primary cause of weediness. / Master of Science in Life Sciences / Cover crops are grown in the time between cash crop production, such as corn or soybeans. These crops are not grown for profit but mainly for environmental benefits such as reducing erosion and increasing soil organic matter and water infiltration. Another benefit of cover crops is the ability to suppress weeds. Cover crops can suppress weeds while they are actively growing by competing for resources such as light, water, and nutrients. After the cover crops have been terminated, or killed prior to cash crop planting, the residue can form a mulch layer on the soil surface which acts to suppress weeds by reducing the amount of sunlight that reaches the soil surface, decreasing soil temperature, and providing a physical barrier to slow weed growth.
Horseweed is a problematic weed for growers to control and the number of herbicide options that growers can utilize is decreasing due to herbicide resistance. This weed has small seed and multiple germination periods, which cover crops have the ability to target. Experiments were designed to compare horseweed suppression from different cover crop monocultures and mixtures with suppression obtained from two fall-applied residual herbicide programs. The cover crop species used were cereal rye, crimson clover, hairy vetch, and forage radish. The cover crops were planted and herbicides applied in the fall. Data collected included horseweed counts, visible suppression ratings, and horseweed biomass taken in the following corn or soybean growing season. All cover crop treatments suppressed horseweed as compared to the nontreated check, with the exception of forage radish alone. The fall-applied herbicides did not perform as well as the cover crops. Results indicate that integration of cover crops is a viable tactic for horseweed management.
As cover crop biomass increases the level of weed suppression also increases. Experiments were implemented to measure the level of weed suppression and to determine if the composition of the cover crop residue is important in weed suppression. Monocultures and mixtures of the same four cover crop species listed above were grown prior to corn and soybean production. At cover crop termination, samples were taken to determine biomass, carbon to nitrogen (C:N) ratio, and lignin content. Cereal rye and mixtures containing cereal rye provided > 55% weed suppression 6 weeks after cover crop termination. Analyses also indicated that lignin, as well as biomass, is an important predictor of weed suppression after termination.
While cover crops have many benefits, there can be some complications. Hairy vetch is a legume cover crop species that has the ability to suppress weeds but can also become weedy in subsequent crops. Experiments were performed to track germination and seed viability of two hairy vetch cultivars, Groff and Purple Bounty as well as determine when seeds are added to the soil seedbank. Over the course of the experiment, Groff had greater germination than Purple Bounty by 30% in the initial germination periods. Both cultivars had <1% of seed still viable at the end of the experiment. Also, both cultivars produce viable seed in mid-June. The results from these experiments indicate that seed dormancy is not the primary cause of weediness in hairy vetch and that if proper termination occurs prior to mid-June, seeds will not be added to the soil seedbank.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/82041 |
| Date | 07 February 2018 |
| Creators | Pittman, Kara |
| Contributors | Plant Pathology, Physiology, and Weed Science, Flessner, Michael L., Barney, Jacob, Hagood, Edward S. |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Detected Language | English |
| Type | Thesis |
| Format | ETD, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
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