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. / Doctor of Philosophy / 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.

<p>  </p> <p>Giant ragweed (<em>Ambrosia trifida</em> L.) is an annual broadleaf plant capable of emergence throughout the cropping season, opportune colonization of disturbed soil, rapid biomass accumulation, and a propensity to evolve mutations that endow resistance to herbicides, all of which contribute to giant ragweed being one of the most challenging weeds to control in row-crop production. Many soybean growers rely on acetolactate synthase (ALS)-inhibiting herbicides such as cloransulam for control of giant ragweed prior to its emergence, though the spread of biotypes resistant to ALS inhibitors can render these herbicides largely ineffective. Mesotrione inhibits the 4-hydroxyphenylpyruvate dioxygenase (HPPD) enzyme, and applications of this herbicide have recently been approved for use in mesotrione-resistant soybean varieties. Field experiments demonstrated that preemergence applications of mesotrione resulted in greater control of giant ragweed populations segregating for ALS-inhibitor resistance than several other commonly used herbicide combinations. Where mesotrione was applied, giant ragweed biomass was reduced by an average of 84% relative to the nontreated, while treatments without mesotrione increased biomass by an average of 34% by suppressing competition from other weed species. Additionally, both soil- and agar-based bioassays demonstrated that combinations of mesotrione and metribuzin can be synergistic for control of giant ragweed. </p> <p>Cloransulam was shown to result in strong selection for giant ragweed individuals with ALS-inhibitor resistance, increasing the proportion of resistant plants that emerged at one field site from 15% to greater than 90% after a single preemergence application. This selection pressure was reduced when mesotrione was co-applied with cloransulam. However, no herbicide combination, including sequential applications of non-ALS-inhibiting herbicides, consistently resulted in a resistance frequency similar to the baseline if an ALS inhibitor was applied preemergence. Resistance to cloransulam and other ALS inhibitors is expressed in giant ragweed plants possessing at least one mutant (Trp574Leu) <em>ALS</em> allele. The distribution of this allele in one field violated the Hardy-Weinberg Equilibrium, despite the fact that <em>ALS</em> is a nuclear gene and the Trp574Leu mutation does not incur a fitness penalty. We suspected that the inheritance of this mutation may be linked with a gene or genes responsible for self-incompatibility (SI) in giant ragweed, and that linkage drag was disrupting pollination in resistant plants. This research provided evidence that giant ragweed does possess SI, as greater pollen retention, pollen tube growth, and seed set were observed in cross-pollinated plants compared with self-pollinated plants. Non-Mendelian inheritance of the Trp574Leu mutation was documented in crosses between plants from three different giant ragweed populations, indicating that the mutant <em>ALS</em> allele may be linked with an SI allele common to many plants because of a shared resistant ancestor.  In crosses between plants from one population, production of resistant F1 seeds was 33% greater on average compared with the expectation under Mendelian inheritance. </p> <p>Collectively, this research demonstrated that mesotrione may become a highly effective tool for control of giant ragweed in soybean. Applications of mesotrione can also reduce the selection for an increased frequency of ALS inhibitor-resistant biotypes induced by cloransulam, though a more robust weed management strategy may be necessary to maintain the long-term viability of ALS inhibitors.  The need for sound weed management practices is underscored by the impact of the linkage of SI and <em>ALS</em> genes, which may be encouraging a more rapid spread of herbicide-resistance than was previously anticipated.</p>

Harvest weed seed control (HWSC) methods concentrate, remove, or destroy weed seeds captured by the combine during harvest. Furthermore, chaff lining uses a chute fitted on the back of a combine to concentrate chaff and weed seed therein into a narrow line. Since chaff amount increases with crop yield, studies aimed to determine how varying crop yield and the associated chaff amount will affect chaff lining control of select weed species, while also examining subsequent crop performance. Objective 1 of this work focused on wheat chaff lining (WCL), and objective 2 studied soybean chaff lining (SCL). Weed species of interest included wild mustard (Sinapis arvensis L.) and Italian ryegrass (Lolium perenne ssp. multiflorum L. Husnot) in WCL and Palmer amaranth (Amaranthus palmeri S.) and common ragweed (Ambrosia artemisiifolia L.) subject to SCL. Each weed species was evaluated in separate experiments, and the SCL experiment included an additional factor of with and without a cereal rye cover crop treatment. Chaff lines mimicked harvest across a range of wheat and soybean yields, with equal weed seed additions (based on existing fecundity and seed shatter phenology data) to each chaff line. A conventional harvest (control) and an outside-the-chaff-line treatment were included, where total fecundity or weed seed rain occurring prior to harvest based on weed species were broadcast respectively. Inhibition of crop and weed emergence as a function of yield and the associated chaff amount was also investigated in the greenhouse. Crop yield across treatments at the field scale (accounts for both chaff lines and outside-the-chaff-line), was not affected in double-crop soybean following WCL and full-season soybean following SCL. Field scale wheat yield in WCL compared to conventional harvest was not different, increased, or decreased in 8, 3, and 1 site-years, respectively. WCL reduced total weed emergence over the combined double-crop soybean and winter wheat growing seasons by 43-54% at the field scale. SCL reduced common ragweed emergence in cereal rye by 64% and 85% in 2 of 3 locations across the soybean growing season. The cover crop did not reduce common ragweed emergence while it was growing, but residual mulch in soybean reduced emergence by 39%. No differences were observed in Palmer amaranth emergence during cereal rye growth, however cereal rye decreased total emergence by 41%. In 6 of 7 Palmer amaranth location-years, SCL decreased field scale weed emergence in soybean by 81%. These results indicate chaff may create an unfavorable environment for weed seed emergence. In both WCL and SCL, greater amounts of chaff caused larger reductions in weed emergence. Objective 3 focused on quantifying the above-ground biomass breakdown of soybean plants into chaff, straw, and seed fractions as they are processed and dispensed by various harvesters. Depending on HWSC system, chaff and straw residues may also be destroyed, removed, or concentrated. Therefore, chaff and straw nutrient composition was analyzed to evaluate the nutrient and economic consequences of HWSC. Our results show average soybean harvest index is 0.57:1. Furthermore, chaff and straw residues equal 13.4% and 68.5% of the seed weight, respectively. Using 5-year average fertilizer prices (2017 – 2021), replacement of N, P, K and S in chaff, straw, and the combination of both residues costs USD 1.58, USD 5.88, and USD 7.46, respectively. / Master of Science in Life Sciences / In conventional wheat and soybean production, the primary means of weed control is herbicides. If herbicide use is not diversified, a repeated selection pressure drives weeds to evolve resistance to such chemistries. Producers and researchers alike are constantly looking for new ways to combat weeds and herbicide-resistant issues. Originally developed to control nuisance weeds in Australia, harvest weed seed control (HWSC) offers promise in aiding our current herbicide resistance crisis. To further explain HWSC, it is important to know the harvesting mechanism. Many of the row crops including corn, soybean and wheat are harvested using a combine. Combines cut below or tear off plant material to capture the grain or seed which is processed via a threshing system and separated into three fractions: the seed, chaff, and straw. The grain is allocated to a storage bin and eventually removed from the field. In conventional harvest, the remaining crop residue is spread evenly behind the combine across the field to ensure a balanced distribution of organic matter, nutrients, and residue across the field. There is however more than chaff and straw being dispersed. Weeds present in the field at harvest whose seed is retained at crop maturity and at an elevation above the combine header height will inherently end up inside the combine. HWSC are methods intended to capitalize on the combine capturing weed seeds during harvest. Many HWSC approaches to managing weed seed exist, including destruction, removal and concentration of weed seed. Most of this research focuses on only one method of HWSC, chaff lining. Chaff lining utilizes a chute fitted onto the back of the combine and concentrates weed seed and the chaff fraction only into a narrow line behind the combine. Although not directly known, chaff may inhibit future weed emergence within the line due to a mulching effect, intraspecific competition, a greater degree of rotting and increased seed predators. The chute is inexpensive to construct, and there are no additional labor requirements at harvest making it an appealing HWSC option. There is a limited amount of research on chaff lining in North American cropping systems making it a prime HWSC candidate for this thesis. We were curious if chaff lining could benefit wheat and soybean farmers and if crop yield and the associated chaff amount deposited in chaff lines would have any impact on crops planted and weeds placed in lines. Our results indicate chaff lining does not cause field scale yield consequence in double-crop and full-season soybean following wheat and soybean chaff lining, respectively. The effect of wheat chaff lining on wheat field scale yield was variable, but only caused a yield decrease in 1 of 12 experimental locations. Reductions in weed emergence in chaff lining systems compared to conventional indicate chaff may alter the environment to be unfavorable for weed seed emergence. The final objective of this thesis investigates the economic cost of nutrient loss among HWSC systems. Using average fertilizer prices, the cost to apply N, P, K and S concentrated or lost during HWSC in chaff, straw, and the combination of both residues is USD 1.58, USD 5.88, and USD 7.46, respectively.

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