Evaluation of integrated weed management techniques and their nuances in Virginia crop production
Beam, Shawn Christopher
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Herbicide resistant weeds are driving implementation of integrated weed management (IWM). A new tactic to manage weeds is harvest weed seed control (HWSC), which targets weed seeds retained on the plant at crop harvest and either destroys, removes, or concentrates them. Research is limited on the effectiveness of HWSC in US cropping systems. For HWSC to be effective it is important to know when and how many seed are shed from a weed species in relation to crop harvest. Research was conducted to quantify when weed seed are shattered from 6 economically important weed species, four broadleaf (redroot pigweed, common ragweed, common lambsquarters, and common cocklebur) and two grass species (large crabgrass and giant foxtail). Results indicate that among summer annuals, broadleaf species retain larger proportions of their seed compared to grass species at the first opportunity for soybean harvest. As harvest was delayed, more seeds shattered from all species evaluated, indicating timely harvest is critical to maximizing HWSC effectiveness. Studies were conducted on grower fields in Virginia to evaluate the effectiveness of HWSC (field residue and weed seed removal). Results indicate that HWSC can significantly reduce populations of Italian ryegrass in wheat and common ragweed in soybean in the next growing season, but reductions were not observed for Palmer amaranth in soybean. Investigating IWM system for common ragweed control in soybean, HWSC was found to be less effective than soybean planting date (i.e. double cropping after wheat) at reducing common ragweed populations. However, the effectiveness of HWSC varied by location. If HWSC adoption were to become widespread, weeds could adapt by shedding seed earlier in the season. Research was conducted by growing Palmer amaranth populations from across the eastern US in a common garden. Currently there are differences in flowering time and seed shatter among Palmer amaranth populations based on the location of the maternal population, indicating potential for adaptation. This research demonstrates that HWSC is a viable option for weed management in US cropping systems but needs to be stewarded like any other weed management tool.
General Audience Abstract
Herbicide resistance in weeds is a growing problem in the US and around the world. Alternative methods of weed control must be adopted to maintain crop yields in the presence of herbicide-resistant weeds. Researchers and extension specialists strongly advise growers to adopt an integrated weed management (IWM) approach. Integrated weed management involves implementing multiple weed control tactics during a growing season. By using multiple methods of weed control within a given season the chances of weeds becoming resistant or adapting to any single tactic is reduced. Harvest weed seed control (HWSC) is a new tactic developed in Australia in response to herbicide resistance. HWSC targets weed seeds retained on the plant at crop harvest. In a normal crop harvest, the combine removes the grain and spreads crop residues (leaves, stalks, and other plant parts), including weed seeds, back across the field. When HWSC is implemented, weed seeds are destroyed (narrow windrow burning, cage mills) or concentrated and potentially removed from the field (chaff carts, direct bale, chaff lining). Thus, HWSC limits the number of weed seeds returned to the soil seed bank. There is limited research on HWSC and its integration with other tactics, in US cropping systems. For HWSC to be effective it is necessary for weed seeds to be retained on the mother plant in sufficient quantities at crop harvest. Research was conducted in Virginia to determine when weed seeds are shattered during the soybean growing season for 6 economically important weed species, four broadleaf (redroot pigweed, common ragweed, common lambsquarters, and common cocklebur) and two grass species (large crabgrass and giant foxtail). The broadleaf species retained >85% of their seed until the first opportunity for soybean harvest (mid-October). In the grass species, more seed shattered prior to soybean harvest with 50% of large crabgrass and 74% of giant foxtail seed being retained at the first opportunity for soybean harvest. When harvest was delayed seed continued to shatter and less was captured using HWSC. This research indicates broadleaf species are more suitable candidates for HWSC than grass species, among summer annuals. Further research on the ability of seed to germinate in relation to when seeds were shed was conducted on redroot pigweed, common ragweed and common lambsquarters. Results indicate that there are variable effects on germination of these species depending on when they were shed. HWSC was implemented on grower fields to assess the impact on weed populations of 3 weed species (Italian ryegrass, common ragweed, and Palmer amaranth). These experiments compared conventional harvest and HWSC (field residue and weed seed removal) when all other management strategies were the same within that field. Italian ryegrass tiller density in wheat varied by location but was reduced up to 69% in the spring following implementation of HWSC. By wheat harvest, HWSC reduced Italian ryegrass seed head density 67% at one location compared to conventional harvest. In soybean, common ragweed densities were reduced by 22 and 26% prior to field preparation and postemergence herbicide applications, respectively, in the HWSC plots compared to the conventional harvest plots. No differences were observed in common ragweed density by soybean harvest. No differences were observed with Palmer amaranth densities at any point during the soybean growing season. This research show that HWSC can reduce weed populations but is variable and additional research is still needed. IWM experiments were established across Virginia to compare soybean planting date (full season or double cropped), + cover crop (cereal rye/wheat or no cover), and + HWSC (field residue removal) to evaluate the best management strategy for common ragweed in soybean. Across all locations, double cropping soybean behind wheat had the greatest impact on common ragweed densities at the end of the first season. The impact of double cropping soybeans on common ragweed population is due to the emergence pattern of common ragweed; majority of common ragweed emerges prior to planting double cropped soybean (mid-June to early-July). HWSC was variable and only reduced common ragweed density at one of three locations. Widespread adoption of HWSC could place a selection pressure on weeds to shatter seed earlier in the season. A common garden experiment was conducted in Blacksburg, VA to assess Palmer amaranth populations collected from central Florida to southern Pennsylvania for differences in flowering time, time to seed shatter, and other phenotypic traits. Results indicate that latitude of the maternal population influences time to first flower with a 0.53 d reduction in flowering time for every degree north in latitude the maternal population was collected from. The strongest predictor of Palmer amaranth flowering time was emergence date/daylength. For every day emergence was delayed the time to first flower was reduced by 0.31 and 0.24 d for female and male plants, respectively. Time from emergence or first flower to first seed shatter was reduced by 0.48 or 0.17 d, respectively, for each day emergence was delayed. These results indicate that differences exist currently among Palmer amaranth populations and the selection pressure of HWSC could push these populations to flower and shatter seed early.
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