Master of Science / Department of Agronomy / Dallas E. Peterson / Auxin herbicides have been widely used for broadleaf weed control since the mid-1940’s. With new auxinic herbicide-resistant traits in corn, soybean, and cotton, use of these herbicides is likely to increase. Glyphosate-resistant Palmer amaranth (Amaranthus palmeri) and common waterhemp (Amaranthus rudis) are two primary problematic weed species that will be targeted with dicamba and 2,4-D in the new systems. No-till double-crop soybean after winter wheat harvest is a popular cropping system in central and eastern Kansas, however, management of glyphosate resistant Palmer amaranth has become a serious issue. Field experiments were established near Manhattan and Hutchinson, KS, in 2016 and 2017, to compare seventeen herbicide treatments for control of Palmer amaranth and large crabgrass (Digitaria sanguinalis) in dicamba/glyphosate resistant no-till double-crop soybean after winter wheat. Herbicide programs that included a residual preemergence (PRE) treatment followed by a postemergence (POST) treatment offered greater Palmer amaranth control 8 weeks after planting when compared to PRE-only, POST-only and burndown-only treatments. All treatments that contained glyphosate POST provided complete control of large crabgrass compared to less than 43% control with PRE-only treatments. Soybean grain yield was greater in programs that included PRE followed by POST treatments, compared to PRE-only and burndown-only treatments. A second set of field experiments were established in 2017 near Manhattan and Ottawa, KS to evaluate dicamba and 2,4-D POST efficacy on Palmer amaranth and common waterhemp. Five rates of dicamba (140, 280, 560, 1121, and 2242 g ae ha⁻¹) and 2,4-D (140, 280, 560, 1121, and 2242 g ae ha⁻¹) were used to evaluate control of the Amaranthus spp. Each experiment was conducted twice at each location. Dicamba provided better Palmer amaranth and common waterhemp control than 2,4-D across the rates evaluated. Control of Palmer amaranth was 94% and 99% with dicamba rates of 1121 and 2242 g ae ha⁻¹, respectively, but 2,4-D never provided more than 80% control at any rate. The highest rates of both dicamba and 2,4-D provided greater than 91% common waterhemp control, but control was less than 78% with all other rates of both herbicides. Palmer amaranth and common waterhemp control did not exceed 73% with the highest labelled POST rates of either dicamba or 2,4-D. Auxinic herbicide-resistant traits in corn, soybean, and cotton offer new options for controlling glyphosate-resistant Palmer amaranth and common waterhemp, however proper stewardship is vital to maintain their effectiveness.

Field and greenhouse research was conducted in 2009 and 2010 on herbicide applications enabled by the integration of Optimum GAT crop traits providing for resistance to glyphosate and certain ALS-inhibiting herbicides. The herbicide concepts were evaluated for control of several winter and summer annual weed species, as well as the effect of the resulting weed control on grain yield of Optimum GAT soybean. The combination of chlorimuron + rimsulfuron did not provide sufficient efficacy on the winter annual grass species little barley and annual bluegrass. Factors contributing the sub-lethal activity include: 1) a relatively low inherent sensitivity of the species to these herbicides, 2) a significant reduction in herbicide efficacy with increases in weed plant height, and 3) a lack of herbicide enhancement with more aggressive foliar adjuvants. The tank-mixture of glyphosate with chlorimuron + rimsulfuron was frequently necessary to achieve a maximum herbicide activity above 90% on annual bluegrass and little barley. Optimum GAT herbicide treatments including chlorimuron + rimsulfuron + flumioxazin in field experiments provided the greatest control of horseweed and common waterhemp in glyphosate-susceptible and -resistant populations. The addition of chlorimuron + rimsulfuron to glyphosate and 2,4-D improved horseweed control above glyphosate and 2,4-D applied alone even as weed height increased with applications made closer to soybean planting. However, removal of competitive vegetation with herbicide combinations including chlorimuron + rimsulfuron selected for emergence of ALS-resistant common waterhemp. Inclusion of flumioxazin with chlorimuron + rimsulfuron was beneficial for control of common waterhemp when applied 7 days before planting. However, chlorimuron + rimsulfuron + flumioxazin provided only 80% control of common waterhemp in a glyphosate-resistant population which demonstrates opportunity for improvement in herbicide concepts enabled by Optimum GAT. Grain yield of Optimum GAT soybean was greatest for herbicide treatments which provided effective weed management throughout the growing season which were the herbicide treatments applied the closest to soybean planting (7 days before planting). Optimum GAT herbicide concepts for corn include chlorimuron + thifensulfruon + tribenuron, chlorimuron + rimsulfuron, and rimsulfuron + tribenuron + mesotrione. These herbicides provided similar to slightly increased control of annual morningglory (Ipomoea spp.) in comparison to glyphosate alone. The addition of atrazine increased the consistency of control of annual morningglory for any herbicide treatment with additional residual activity at 28 days after treatment. Optimum GAT enabled herbicide concepts can improve control of some problematic weed species, including some glyphosate-resistant weed populations, compared to current herbicide tactics that rely primarily on glyphosate for weed control in commercial glyphosate-resistant soybean and corn. However, the integration of postemergence soybean herbicides beyond the ALS chemistry is necessary to provide a broader spectrum of weed control when considering the challenges of managing both glyphosate- and ALS-resistant weed species that are becoming more frequent in commercial fields.

Field and greenhouse experiments were conducted in 2008, 2009, and 2010 to evaluate the efficacy of glyphosate combined with other broadleaf herbicides on herbicide-resistant and -susceptible weeds in Illinois. In the field, the addition of lactofen or fomesafen to glyphosate provided the greatest consistency and control of problematic target weeds including common waterhemp and giant ragweed. Applications of glyphosate tank-mixtures at EPOST provided 10% greater control of giant ragweed compared with the same treatments at POST. The addition of a tank-mix herbicide with glyphosate provided the greatest impact on weed control when applied to glyphosate-resistant common waterhemp with control increasing by 67% compared with glyphosate applied alone. The addition of a tank-mix herbicide with glyphosate had little impact on control of PPO-resistant and herbicide-susceptible common waterhemp. Generally, the addition of an adjuvant to herbicide mixtures with glyphosate did not influence weed control 14 DAT compared with no additional adjuvant. However, the addition of PO-HSOC and SO-HSOC to glyphosate tank-mixtures containing fomesafen and cloransulam increased annual morningglory control compared with no additional adjuvant. In the greenhouse, PPO-inhibiting herbicides (fomesafen, lactofen, flumiclorac, fluthiacet) applied with glyphosate resulted in additive and antagonistic responses depending on the PPO-inhibitor used and the common waterhemp biotype. Generally, glyphosate tank-mixtures applied to PPO-resistant and herbicide-susceptible common waterhemp biotypes resulted in a greater frequency of antagonism than that of a glyphosate-resistant biotype. The efficacy of glyphosate tank-mixtures was also influenced by environmental conditions shortly before and after the herbicide application. Glyphosate tank-mixtures usually resulted in greater efficacy on common waterhemp when applied at 24 C with 75% RH than at 32 C with 40% RH, regardless of tank-mix herbicide or common waterhemp biotype. Although this research supports an improvement in control of glyphosate-resistant common waterhemp with the addition of PPO-inhibiting herbicides to glyphosate, the overall herbicide efficacy was still somewhat variable with evidence for environmental conditions that may alter efficacy of the tank mixtures. Thus, the use of preemergence, soil residual herbicides are still justified to reduce reliance on these postemergence herbicide mixtures for weed control in soybean and ultimately deter further selection of common waterhemp populations resistant to glyphosate and PPO-inhibiting herbicides.

Master of Science / Department of Agronomy / Kassim Al-Khatib / Field experiments were conducted near Manhattan, KS in 2005 and 2006 and Sabetha, KS in 2005 to determine the efficacy of S-metolachlor tank mixed with fomesafen on common waterhemp in soybean. Preemergence treatments included S-metolachlor + fomesafen at 0.91 + 0.22, 1.21 + 0.28, 1.52 + 0.36, and 1.82 + 0.43 kg ha-1 and S-metolachlor + metribuzin at 0.55 + 0.14 kg ha-1. These treatments were applied alone or followed by a postemergence glyphosate application at 0.88 kg ha-1. Ratings were taken 2, 4 and 8 weeks after treatment. The study showed that S-metolachlor + fomesafen gave excellent early season control of common waterhemp at both Sabetha and Manhattan. S-metolachlor + fomesafen at the 1.52+0.36 kg ha-1 rate gave greater weed control than S-metolachlor + metribuzin. A separate study was conducted to determine the competitiveness and fitness of a protox-resistant common waterhemp biotype. Protox-resistant and protox-susceptible biotypes of common waterhemp were grown under noncompetitive and competitive arrangements in the greenhouse. In the noncompetitive study a single plant of both biotypes was planted in 15-cm-diam pots. Photosynthesis, leaf area, and plant biomass were measured 10, 20, 30, and 40 day after transplanting (DATP). In general, photosynthesis rate and plant biomass was similar between biotypes. However, the protox-resistant biotype had higher leaf area then the susceptible biotype at 20, 30, and 40 DATP. Under competitive conditions, a replacement series study, photosynthesis, leaf area, plant height, and plant biomass were measured 7, 14, 21, and 28 DATP. In general protox-resistant and –susceptible common waterhemp values were similar 28 DATP. Relative crowding coefficient values 28 DATP were 0.86, 0.89, 1.09, and 1.13 for photosynthesis, leaf area, plant height, and plant biomass, respectively. Suggesting, protox resistance did not change the ability of common waterhemp to grow normally under competitive conditions.

Renewed interest in hemp (Cannabis sativa L.) for cannabinoid production has highlighted critical knowledge gaps for growers. The impacts of weed interference on hemp yield are largely unstudied despite causing serious economic losses in most cropping systems. Furthermore, understanding factors determining cannabinoid content of hemp will be crucial for effective production. To evaluate the effects of weed competition on hemp yield and cannabinoid content, hemp growing in plasticulture was subjected to competition from 5, 3, 1, or 0 waterhemp (Amaranthus tuberculatus (Moq.) Sauer) plants. Hemp biomass and cannabinoid content were not significantly affected by weed competition. Yields in weedy and weed-free treatments were generally comparable, although unexpectedly high variation limited the ability to detect subtle differences between treatments. Waterhemp biomass was significantly reduced by competition from hemp in comparison to hemp-free controls. Suppression of weed growth by hemp and lack of significant yield loss from weeds suggests that hemp can be highly competitive and grown successfully without herbicides in certain circumstances. Abundance and documented phytotoxicity of hemp secondary metabolites suggest a potential for allelopathic activity. While incorporated hemp residue was not more effective than a maize control at reducing plant growth, hemp residue and extracts effectively inhibit seed germination. A laboratory assay showed that crude extracts of hemp can reduce the germination of a Brassica napus L., a bioindicator. Furthermore, a greenhouse experiment showed that small amounts of hemp residue on the soil surface can effectively reduce and delay the germination of waterhemp. Together, these results show that hemp residue has the potential to be incorporated with a practice such as chaff-lining for enhanced control of germinating weed seeds.

Weed management options in agronomic crop production have been severely limited by widespread populations of weeds resistant to herbicides, including waterhemp [Amaranthus tuberculatus (Moq.) Sauer (syn. rudis)] resistant to foliar applications of herbicides that inhibit protoporphyrinogen oxidase (PPO; EC 1.3.3.4) activity (PPO-R). Herbicides within this site of action (WSSA site of action #14) remain efficacious when soil-applied to PPO-R waterhemp populations. Therefore, the continued use of these herbicides for soil-residual control of PPO-R waterhemp, especially in soybean production, is paramount with limited postemergence herbicides that remain effective. An improved understanding of the selection for PPO-R waterhemp would provide information to help minimize future loss of residual PPO-inhibiting herbicide activity. Five studies, consisting of 14 experiments, were conducted to improve our understanding of the selection for herbicide-resistant individuals. Soil-residual herbicides have been suspected to select for herbicide-resistant individuals; however, this phenomenon has never been observed experimentally in field conditions. This dissertation provides direct evidence from greenhouse and field experiments that significant selection pressure can occur from soil-residual herbicides; however, this selection for resistance could be delayed when using full commercial herbicide rates and effective herbicides from multiple sites of action. Also, the frequency of heterozygous individuals (RS) and PPO-inhibiting herbicide efficacy on RS individuals is a factor in the selection for herbicide resistance; however, current information on the these topics is limited. To provide additional information on RS individuals, a large-scale genotypic and phenotypic screen of multiple PPO-R waterhemp populations was conducted. It was determined that RS individuals were less frequent than expected and PPO-inhibiting herbicide efficacy on RS individuals was population-dependent. Finally, the hormetic effects of soil-residual herbicides have been paradoxically implicated as a means of both mitigating and exacerbating the selection for herbicide resistant biotypes; however, limited information was available on the hormetic effects of soil-residual PPO inhibitors. Greenhouse and growth chamber experiments were conducted to improve understanding of hormesis and soil-residual PPO-inhibiting herbicides. Experiments indicated that PPO-inhibiting herbicides may exert a limited hormetic effect on waterhemp germination below doses that cause a phytotoxic effect of the emerging seedling, indicating this effect may exacerbate the issue of selection for PPO-R waterhemp. Overall, data presented in this dissertation provides important information on the under-studied interaction between PPO-inhibiting herbicides and PPO-R waterhemp to safeguard the sustained efficacy of herbicides within this site of action.

<p dir="ltr">Waterhemp (<i>Amaranthus tuberculatus</i>)<i> </i>is one of the most problematic weeds in soybean production in the United States. The ability of waterhemp to quickly evolve resistance threatens the utility of many herbicides. The introduction of Xtend^®and Xtendflex^®soybeans allow for the in-season application of dicamba and glufosinate. With an increase in dicamba use in soybeans plus its continued use in corn, there have been many reports of dicamba failure on waterhemp.</p><p dir="ltr">Greenhouse dose response experiments were conducted to screen six populations of waterhemp for resistance to dicamba. Each population was compared to a known sensitive and known resistant population, with 50% growth reduction (GR₅₀) values of 39 g ae ha^-1and 226 g ae ha^-1, respectively. Low-level dicamba resistance was identified in all populations, as they had GR₅₀values that were different from the known sensitive and R:S ratios that varied from 1.7 to 4.4. Additionally, all six populations exhibited at least 50% survival at a 1/2X rate of dicamba where the sensitive only had 30% survival. Therefore, we can conclude that dicamba resistance in waterhemp is present in multiple counties in Indiana.</p><p dir="ltr">In addition to characterizing populations from Indiana, a growth monitoring study was conducted to determine how emergence timing impacted waterhemp growth. In 2021, plants that emerged in the earliest cohort were taller, had more branches, and accumulated more biomass in comparison to later emerging plants at six weeks after flagging. In 2022, drought conditions throughout the month of June impacted growth of earlier emerging plants, and waterhemp that emerged in the latest cohort were taller, had more branches, and accumulated more biomass than earlier emerging cohorts at six weeks after flagging. Seed yields per plant were low in both years, but all cohorts were able to produce seed. This research concludes that in years when soil sufficient moisture is available, earlier emerging waterhemp plants are competitive with crops and later emerging plants can still produce seed.</p><p dir="ltr">Additionally, field trials were conducted to determine herbicide programs in the Enlist^® and Xtendflex^® soybean systems that would best control a waterhemp population with multiple herbicide resistance. At 21 days after the second postemergence application, waterhemp control was highest in two pass systems. The addition of pyroxasulfone to the second postemergence application increased control of waterhemp in the Xtendflex^® system. Waterhemp densities were the lowest and soybean yield was the highest in two pass herbicide programs for both systems. The results indicate that waterhemp resistant to chemistries in HRAC Groups #2, #4, #5, #9, #14, and #27 was most effectively controlled by programs with two herbicides applications, either a preemergence followed by postemergence or two pass postemergence, and included 2,4-D and glufosinate in the postemergence application(s).</p><p dir="ltr">Finally, a waterhemp population from Francesville, IN was characterized for herbicide resistance via a series of field, greenhouse, and laboratory experiments. Preliminary laboratory analysis confirmed resistance to herbicide actives in the HRAC Groups #2 and #14 via target site mutations and to Group #9 via gene amplification. Field research trials indicated inadequate waterhemp control with preemergence applications of pendimethalin and atrazine and postemergence applications of herbicide actives from Groups #2, #9, #14, and #27 as well as glufosinate and dicamba. Greenhouse dose response experiments revealed GR₅₀values for the Francesville population that were significantly higher for dicamba, mesotrione, and topramezone than the known sensitive. R:S ratios of 4.4, 3.3, and 1.8, were documented for dicamba, mesotrione, and topramezone, respectively. Data from all experiments demonstrated that the Francesville population is resistant to herbicide actives in Groups #2, #4, #5, #9, #14, and #27.</p>

Master of Science / Agronomy / Mithila Jugulam / Common waterhemp (Amaranthus rudis) is a problematic weed species of cropping systems throughout the Midwestern states, including Kansas. Recently, waterhemp populations from Kansas were found to have evolved resistance to the widely used herbicide glyphosate as a result of amplification of the 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS), the enzyme target of glyphosate. The objectives of this research were to 1) perform glyphosate dose-response study and determine the relationship between relative EPSPS genomic copies and EPSPS gene expression in glyphosate-resistant waterhemp, and 2) characterize the genomic configuration and distribution of EPSPS copies using florescence in situ hybridization (FISH) in three glyphosate-resistant waterhemp populations. Waterhemp populations from eastern Kansas were screened with 868 g ae haˉ¹ (field used rate) of glyphosate, and genomic DNA and total RNA was isolated from the survivors to determine the EPSPS genomic copies and EPSPS gene expression relative to the acetolactate synthase (ALS) gene using qPCR. Furthermore, waterhemp specific EPSPS probes were synthesized to perform florescence in situ hybridization (FISH) on these glyphosate-resistant plants. Results of these experiments indicate a positive correlation between level of glyphosate resistance, EPSPS copies, and their expression. As expected, a negative correlation was found between shikimate accumulation and EPSPS copies. Sequencing of the EPSPS gene showed no presence of the proline 106 mutation, which is known to be associated with glyphosate resistance suggesting that an insensitive EPSPS enzyme was not involved in the mechanism of glyphosate resistance. FISH analysis of resistant plants illustrated presence of amplified EPSPS copies on two homologous chromosomes, likely near the centromeric region. . This is the first report demonstrating a positive relationship between EPSPS copies and expressions, as well as chromosome configuration of EPSPS copies in glyphosate- resistant waterhemp from Kansas.

The continued spread of glyphosate-resistant common waterhemp [Amaranthus tuberculatus (Moq.) Sauer (syn. rudis)] and Palmer amaranth [Amaranthus palmeri (S. Wats.)] have complicated weed control efforts in soybean and corn production in Illinois. A thorough understanding of the weed biology of these species is fundamental in developing effective weed management strategies. The determination of emergence patterns as well as the influence of tillage practices on soil microclimate and soil seed bank will allow control strategies to be implemented at the most effective timing. Field experiments were conducted in southern Illinois throughout the growing season of 2013 and 2014 on two separate sites with populations of common waterhemp and Palmer amaranth. Three tillage treatments were evaluated: no-tillage; early tillage, preferably performed around a recommended soybean planting date of May 1st; and late tillage, preferably performed on June 1st to simulate a late soybean planting. Amaranthus seedlings were identified and enumerated in the center 1 m2 quadrat of each plot within a 7-day interval from April through November or first frost. All weed seedlings were removed from the plot area after each enumeration. Soil temperature and soil moisture were recorded hourly throughout the experiment using data loggers established in the plot area. First emergence of common waterhemp occurred earlier in the season than did Palmer amaranth. In 2013, initial emergence of common waterhemp and Palmer amaranth was observed at the first and second week of May, respectively. In 2014, initial common waterhemp emergence was observed in late April, while Palmer amaranth initial emergence was similar to previous year. Palmer amaranth emerged over a longer period compared to waterhemp. By the end of June, 90% of common waterhemp had emerged regardless of tillage or year. By the same measure, Palmer amaranth emergence was extended to the third week of July and second week of August in 2013 and 2014, respectively. Soil temperature did not differ across tillage treatments in both years. On the other hand, differences in soil moisture were observed, mostly over two weeks following each tillage operation. The single best predictor for common waterhemp emergence was soil temperature (weekly highs and lows) followed by soil moisture. For Palmer amaranth emergence the single best predictor was spikes in soil moisture (high for the week). In 2013, common waterhemp emergence was initially positively and later in the growing season negatively interacted with maximum temperature 13 days prior to counts, with temperatures above 30 C observed with decreased emergence (R2 = 0.35). In the same year spikes in soil moisture interacted with Palmer amaranth emergence were those observed 11 days before each seedling enumeration date (R2 = 0.30). In 2014, with first common waterhemp emergence in April, a positive interaction to high soil temperature was initially observed followed by a positive interaction to minimum temperatures later in the season (R2 = 0.55). Spikes in soil moisture observed 2 weeks prior to emergence and weekly high temperatures 8 days prior to emergence were the best predictors of Palmer amaranth emergence in 2014 (R2 =0.37). Soil seed bank depletion was also estimated by comparing field emergence with greenhouse experiment results of soil seed bank estimation. Greater emergence of common waterhemp from the soil seed bank was observed in early tillage in 2013 and no-tillage in 2014 than late tillage, respectively; for Palmer amaranth, the greatest emergence from the soil seed bank was observed in no-tillage and late tillage in 2013, and no-tillage, in 2014. The emergence patterns observed in this research suggest that although common waterhemp and Palmer amaranth exhibit discontinuous emergence throughout the growing season, greater attention should be placed on managing peaks of emergence from late April to late July, which is critical to provide a foundation for early-season weed management. Furthermore, knowledge regarding the emergence patterns of common waterhemp and Palmer amaranth combined with monitoring environmental factors such as soil moisture and soil temperature may assist efforts for scouting fields to determine the likely presence of these weed species. The timing of viable postemergence herbicide options for control of glyphosate-resistant waterhemp and Palmer amaranth is critical and monitoring weather patterns to direct scouting efforts may improve the timeliness of these postemergence applications.

The expression of glyphosate resistance can vary within single field populations of common waterhemp and Palmer amaranth. This variability in expression can translate into control ranging from 20 to 80%, which could be the difference in a minor versus a major failure in weed management. Certain application parameters that have been previously associated with glyphosate efficacy, such as glyphosate application time of day and plant stress may exacerbate this variability and lead to failed control of plants on the lower end of the resistance spectrum. Greenhouse research was conducted in 2011 to determine the influence of glyphosate application time of day on the expression of resistance in common waterhemp and Palmer amaranth. Control of both glyphosate-susceptible (GS) and -resistant (GR) weed species showed similar trends in response to glyphosate with respect time of application. Decreased sensitivity of all Amaranthus biotypes was greatest at 9:00 pm and may be attributed to an observed shift in leaf orientation from horizontal to vertical at the time of glyphosate application in response to low-light conditions. The altered leaf orientation most likely reduced herbicide spray coverage. The magnitude of resistance, the difference in the sensitivity of the resistant versus susceptible biotypes, was unaffected by glyphosate application time of day; however, these results indicate that even in resistant populations glyphosate applied at suboptimal times of day such as the evening can cause a further increase in weed escapes from glyphosate. Greenhouse and field experiments were conducted in 2011 and 2012 to determine the influence the soil nutrient amendments on glyphosate sensitivity and growth rate and of GS and GR common waterhemp and Palmer amaranth. In both the GR and GS biotypes of common waterhemp the sensitivity to glyphosate was increased as fertilizer was introduced. However, only the sensitivity of the susceptible biotype of Palmer amaranth was increased with the addition of fertilizer. The lack of response in the GR Palmer amaranth population to fertilizer can be associated with the fact that due to carrier volume limitations enough glyphosate could not be applied to achieve 50% control. The magnitude of resistance was decreased numerically with the addition of fertilizer in both weed species; however, only in common waterhemp was the magnitude of resistance significantly different with the use of high rates fertilizer. The use of fertilizer also had an influence on the growth rate and dormancy of axillary buds. Lateral branching (broken dormancy in axillary buds) was increased in both common waterhemp and Palmer amaranth with the addition of fertilizer. Converting dormant buds to active meristems favors glyphosate translocation and could be responsible for increased glyphosate efficacy. In the field, glyphosate efficacy in GR common waterhemp and Palmer amaranth was also increased with addition of fertilizer; however, this effect was variable. Optimizing the efficacy of glyphosate when applied to even mixed populations of GS and GR Palmer amaranth and common waterhemp can reduce surviving weeds that can produce seed and perpetuate the frequency of glyphosate resistance in the field. Furthermore, greater efficacy of glyphosate may translate into relatively less significant failures in glyphosate applications allowing for successful rescue herbicide treatments and minimal impact on crop yield compared with a complete glyphosate failure with dramatic implications on reduced crop yield and increased weed seed production.