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Water is the primary carrier for herbicide applications. Spray water qualities such as pH, hardness, temperature, or turbidity can influence herbicide performance and may need to be amended for optimum weed control. Water quality factors can affect herbicide activity by reducing solubility, enhancing degradation in the spray tank, or forming herbicide-salt complexes with mineral cations, thereby reducing the absorption, translocation, and subsequent weed control. The available literature suggests that the effect of water quality varies with herbicide chemistry and weed species. The efficacy of weak-acid herbicides such as glyphosate, glufosinate, clethodim, sethoxydim, bentazon, and 2,4-D is improved with acidic water pH; however, the efficacy of sulfonylurea herbicides is negatively impacted. Hard-water antagonism is more prevalent with weak-acid herbicides, and trivalent cations are the most problematic. Spray solution temperature between 18 C and 44 C is optimum for some weak-acid herbicides; however, their efficacy can be reduced at relatively low (5 C) or high (56 C) water temperature. The effect of water turbidity is severe on cationic herbicides such as paraquat and diquat, and herbicides with low soil mobility such as glyphosate. Although adjuvants are recommended to overcome the negative effect of spray water hardness or pH, the response has been inconsistent with the herbicide chemistry and weed species. Moreover, information on the effect of spray water quality on various herbicide chemistries, weed species, and adjuvants is limited; therefore, it is difficult to develop guidelines for improving weed control efficacy. Further research is needed to determine the effect of spray water factors and develop specific recommendations for improving herbicide efficacy on problematic weed species.
Dose-response trials to determine the tolerance of summer squash and watermelon to fomesafen applied (over the top of black polyethylene mulch and respective row middles) pre-transplanting were performed between 2020 and 2021 at three Indiana locations: the Meigs Horticulture Research Farm (MEIGS), the Pinney Purdue Agricultural Center (PPAC), and the Southwest Purdue Agricultural Center (SWPAC). Summer squash trials were performed at the MEIGS and PPAC locations, and watermelon trials were performed at the MEIGS and SWPAC locations. The experiments for both summer squash and watermelon had a split-plot arrangement in which the main plot was herbicide rate, and the subplot was cultivar. Summer squash injury included necrotic leaf margin, chlorosis, brown and white spots, and stunting. Fomesafen rates from 262 to 1,048 g ai ha−1 in 2020 at both locations, and from 280 to 1,120 g ai ha−1 in 2021 at MEIGS did not affect summer squash yield. However, in 2021 at PPAC, fomesafen applied at rates from 280 to 1,120 g ha−1 delayed summer squash harvest and decreased marketable yield from 95% to 61% compared with the nontreated control. Watermelon injury included bronzing and stunting. Fomesafen rates from 210 to 840 g ai ha−1 did not affect marketable watermelon yield or fruit number. Crop safety was attributed to rain, which washed off most of the herbicide from the polyethylene mulch before plants were transplanted or little to no rain after transplant. Injury was observed only when there was no rain before transplant followed by excessive rain shortly after transplant. Overall, the 1× rate used for each trial was safe for use 1 d before transplanting summer squash and 6 to 7 d before transplanting watermelon.
Three dose-response trials were performed in 2020 and 2021 to determine the tolerance of two Jack O’Lantern pumpkin cultivars to fomesafen applied preemergence at two Indiana locations: the Southwest Purdue Agricultural Center (SWPAC) and the Pinney Purdue Agricultural Center (PPAC). The experiment was a split-plot arrangement in which the main plot was the fomesafen rate of application (0, 280, 560, 840, and 1,220 g ai ha–1), and the subplot was the pumpkin cultivar (‘Bayhorse Gold’ and ‘Carbonado Gold’). As the fomesafen rate increased from 280 to 1,120 g ha–1, the predicted pumpkin emergence decreased from 85% to 25% of the nontreated control at SWPAC-2020, but only from 99% to 74% at both locations in 2021. The severe impact on emergence at SWPAC-2020 was attributed to rainfall. Visible injury included bleaching and chlorosis due to the herbicide splashing from the soil surface onto the leaves and included stunting, but injury was transient. As the fomesafen rate increased from 280 to 1,120 g ha–1, the predicted marketable orange pumpkin yield decreased from 95% to 24% of the nontreated control at SWPAC-2020 and 98% to 74% at PPAC-2021. Similarly, the predicted marketable orange pumpkin fruit number decreased from 94% to 21% at SWPAC-2020 and 98% to 74% at PPAC-2021. Fomesafen rate did not affect marketable orange pumpkin yield and fruit number at SWPAC-2021 and marketable orange pumpkin fruit weight at any location-year. Overall, the fomesafen rate of 280 g ha–1 was safe for use preemergence in the pumpkin cultivars ‘Bayhorse Gold’ and ‘Carbonado Gold’ within one day after planting, but there is a risk of increased crop injury with increasing rainfall.
Morningglories (Ipomoea spp.) are among the most troublesome weeds in cucurbits in the United States; however, little is known about Ipomoea spp. interference with horticultural crops. Two additive design field studies were conducted in 2020 at two locations in Indiana to investigate the interference of ivyleaf morningglory (Ipomoea hederacea Jacq.), entireleaf morningglory (Ipomoea hederacea Jacq. var. integriuscula A. Gray.), and pitted morningglory (Ipomoea lacunosa L.) with triploid watermelon [Citrullus lanatus (Thunb.) Matsum. & Nakai]. Immediately after watermelon was transplanted, Ipomoea spp. seedlings were transplanted into the watermelon planting holes at densities of 0 (weed-free control), 3, 6, 12, 18, and 24 plants 27 m−2. Fruit was harvested once a week for 4 wk, and each fruit was classified as marketable (≥4 kg) or non-marketable (<4 kg). At 1 wk after the final harvest, aboveground biomass samples were collected from 1 m2 per plot and oven-dried to obtain watermelon and Ipomoea spp. dry weight. Seed capsules and the number of seeds in 15 capsules were counted from the biomass sample to estimate seed production. Ipomoea spp. densities increasing from 3 to 24 plants 27 m−2 increased marketable watermelon yield loss from 58% to 99%, reduced marketable watermelon fruit number 49% to 98%, reduced individual watermelon fruit weight 17% to 45%, and reduced watermelon aboveground biomass 83% to 94%. Ipomoea spp. seed production ranged from 549 to 7,746 seeds m−2, greatly increasing the weed seedbank. Ipomoea spp. hindered harvest due to their vines wrapping around watermelon fruits. The most likely reason for watermelon yield loss was interference with light and consequently less dry matter being partitioned into fruit development due to less photosynthesis. Yield loss was attributed to fewer fruits and the weight of each fruit.
Commercialization of 2,4-D-resistant soybean varieties allows for postemergence (POST) applications of 2,4-D in soybean. With the increase in POST applications of 2,4-D in soybean, shifts in weed populations may occur. A long-term field trial was conducted over 7 yr in a corn-soybean rotation. Weed populations were subjected to four herbicide strategies with variable levels of 2,4-D reliance. The strategies used included 1) diversified glyphosate strategy with six herbicide sites of action (SOAs); 2) 2,4-D reliant strategy with three SOAs; 3) diversified 2,4-D reliant strategy with seven SOAs; and 4) fully diversified strategy with eight SOAs. Soil residual herbicides were used for both corn and soybean years, except for the 2,4-D-reliant strategy, which used only a residual herbicide during the corn years. A 52% or greater reduction in weed densities for all herbicide strategies, except the 2,4-D-reliant strategy, was observed by the end of the study. However, the density of weeds tolerant to 2,4-D, such as monocots, increased after 3 yr of selection pressure, and more than doubled after 5 yr of selection pressure in the 2,4-D-reliant strategy. Additionally, in the 2,4-D-reliant strategy with three SOAs, species richness was 30% higher in the soil seedbank compared to herbicides strategies with six or more SOAs. In order to delay weed shifts, diversified herbicide strategies with more than three SOAs that include residual herbicides should be used in corn:soybean rotational systems that use 2,4-D-resistant soybean.
Cover crops can be utilized to suppress weeds via direct competition for sunlight, water, and soil nutrients. Research was conducted to determine if cover crops can be used in label-mandated buffer areas in 2,4-D-resistant soybean cropping systems. Delaying termination of cover crops containing cereal rye to at or after soybean planting resulted in a 25 to more than 200 percentage point increase in cover crop biomass compared to a control treatment. Cover crops generally improved horseweed control when 2,4-D was not used. Cover crops reduced grass densities up to 54% at four of six site-years when termination was delayed to after soybean planting. Cover crops did not reduce giant ragweed densities. Cover crops reduced waterhemp densities by up to 45%. Cover crops terminated at or after planting were beneficial within buffer areas for control of grasses and waterhemp, but not giant ragweed. Yield reductions of 14% to 41% occurred when cover crop termination was delayed to after soybean planting at three of six site-years. Terminating the cover crops at planting time provided suppression of grasses and waterhemp within buffer areas and had similar yield to the highest-yielding treatment in five out of six site-years.
Rapid vegetative growth and adverse application conditions are common factors leading to the failure of postemergence herbicides on Palmer amaranth. A sequential herbicide application, or respray, is often necessary to control weeds that have survived the initial herbicide application to protect crop yield and minimize weed seed production. The optimum timing after the initial application and the most effective herbicide for control of Palmer amaranth has not been characterized. The objectives of these experiments were to determine the optimum herbicide for treating Palmer amaranth regrowth, the optimum timing for each of those herbicides, and how the initial failed herbicide might affect efficacy of a second herbicide application. Bare ground field experiments were performed in 2017 and 2018 in which glufosinate or fomesafen herbicide failure was induced on Palmer amaranth plants that were 30 cm in height. Respray treatments of glufosinate, fomesafen, lactofen, 2,4-D, and dicamba were applied once at timings of 4 to 5 d, 7 d, or 11 d after the initial spray application. Nearly all herbicide treatment and timing combinations increased control by at least 13 percentage points compared to no respray herbicide treatment. Regardless of initial herbicide, glufosinate applied as a respray treatment was the most consistent and efficacious with up to 97% control. The specific herbicide used in the second application impacted final weed control more so than timing of the respray application. For instance, control by glufosinate respray treatments was 10 to 18 percentage points greater than control from lactofen respray treatments, whereas control decreased by 3 percentage points when respray applications of any herbicide were made 11 d after initial application of glufosinate compared to 4 to 5 and 7 d after initial application of glufosinate. In the event of failure to control Palmer amaranth with glufosinate or fomesafen, glufosinate should be applied in order to maximize control.
As herbicide-resistant weeds become more problematic, producers will consider the use of cover crops to suppress weeds. Weed suppression from cover crops may occur especially in the label-mandated buffer areas of dicamba-resistant soybean where dicamba use is not allowed. Three cover crops terminated at three timings with three herbicide strategies were evaluated for their effect on weed suppression in dicamba-resistant soybean. Delaying termination until soybean planting or after and using cereal rye or cereal rye + crimson clover increased cover-crop biomass by at least 40% compared to terminating early or using a crimson clover–only cover crop. Densities of problematic weed species were evaluated in early summer before a blanket POST application. Plots with cereal rye had 75% less horseweed compared to crimson clover at two of four site-years. Cereal rye or the mixed cover crop terminated at or after soybean planting reduced waterhemp densities by 87% compared to early termination timings of crimson clover and the earliest termination timing of the mix at one of two site-years. Cover crops were not as effective in reducing waterhemp densities as they were in reducing horseweed densities. This difference was due to a divergence in emergence patterns; waterhemp emergence generally peaks after termination of the cover crop, whereas horseweed emergence coincides with establishment and rapid vegetative growth of cereal rye. Cover crops alone were generally not as effective as was using a high-biomass cover crop combined with an herbicide strategy that contained dicamba and residual herbicides. However, within label-mandated buffer areas where dicamba cannot be used, a cover crop containing cereal rye with delayed termination until soybean planting combined with residual herbicides could be used to improve suppression of horseweed and waterhemp.
The addition of dicamba as a weed control option in soybean [Glycine max (L.) Merr.] is a valuable tool. However, this technology must be utilized with other herbicide sites of action (SOAs) to reduce selection pressure on weed communities and ensure its prolonged usefulness. A long-term trial was conducted for 7 yr in Indiana to evaluate weed community densities and species richness with four levels of dicamba selection pressure in a corn (Zea mays L.)–soybean rotation. Monocot densities and richness increased over time in the dicamba-reliant treatment. Dicot densities in the dicamba-reliant treatment declined over time, but dicot richness increased. The soil weed seedbank was affected by the varying herbicide strategies. The dicamba-reliant strategy had greater than 43% higher total weed density than all other treatments, primarily due to having a monocot density that was at least 71% higher than the other treatments. The fully diversified strategy with eight SOAs and residual herbicides used every year had the lowest total weed species richness in the soil seedbank, which supported the in-field observations.
Foliar herbicide applications to waterhemp can result in inadequate control, leading to subsequent regrowth that often necessitates a second herbicide application to prevent crop interference and seed production. The most effective herbicides and application timings are unknown in situations where waterhemp has regrown from previous injury, such as failed applications of glufosinate or fomesafen. The objective of this research was to determine the optimum combination of herbicide and time from the first failed herbicide application to a sequential herbicide application for control of waterhemp regrowth. Reduced rates of either glufosinate or fomesafen were applied to 30-cm waterhemp plants to mimic failure of the initial herbicide application in separate bare-ground experiments. Respray treatments of glufosinate, fomesafen, lactofen, 2,4-D, or dicamba were applied 3, 7, or 11 d after the initial application. Glufosinate and fomesafen as respray treatments resulted in 90% to 100% control of waterhemp regardless of application timing following a failed glufosinate application. After a failed application of fomesafen, applying glufosinate or 2,4-D resulted in 87% to 99% control of waterhemp. Waterhemp control with fomesafen and lactofen was 13% to 21% greater, respectively, when those treatments followed glufosinate compared with fomesafen as the initial herbicides. On the basis of these results, glufosinate and fomesafen should be used for respray situations after inadequate control from glufosinate; and 2,4-D or glufosinate should be used for respray situations following inadequate control from fomesafen where crop tolerance and herbicide product labels allow. Although glufosinate followed by glufosinate was very effective for controlling waterhemp regrowth, caution should be exercised to avoid sequential application of herbicide with the same site of action.
Field experiments were conducted in 2017 and 2018 at two locations in Indiana to evaluate the influence of cover crop species, termination timing, and herbicide treatment on winter and summer annual weed suppression and corn yield. Cereal rye and canola cover crops were terminated early or late (2 wk before or after corn planting) with a glyphosate- or glufosinate-based herbicide program. Canola and cereal rye reduced total weed biomass collected at termination by up to 74% and 91%, in comparison to fallow, respectively. Canola reduced horseweed density by up to 56% at termination and 57% at POST application compared to fallow. Cereal rye reduced horseweed density by up to 59% at termination and 87% at POST application compared to fallow. Canola did not reduce giant ragweed density at termination in comparison to fallow. Cereal rye reduced giant ragweed density by up to 66% at termination and 62% at POST application. Termination timing had little to no effect on weed biomass and density reduction in comparison to the effect of cover crop species. Cereal rye reduced corn grain yield at both locations in comparison to fallow, especially for the late-termination timing. Corn grain yield reduction up to 49% (4,770 kg ha–1) was recorded for cereal rye terminated late in comparison to fallow terminated late. Canola did not reduce corn grain yield in comparison to fallow within termination timing; however, late-terminated canola reduced corn grain yield by up to 21% (2,980 kg ha–1) in comparison to early-terminated fallow. Cereal rye can suppress giant ragweed emergence, whereas canola is not as effective at suppressing large-seeded broadleaves such as giant ragweed. These results also indicate that early-terminated cover crops can often result in higher corn grain yields than late-terminated cover crops in an integrated weed management program.
Synthetic-auxin herbicides are often applied for horseweed control before soybean planting. However, certain days of planting interval must be maintained before soybean planting, depending on the product and rate used, because of potential crop phytotoxicity. Halauxifen-methyl is a new synthetic-auxin herbicide for horseweed control in preplant applications in soybean. Field experiments were conducted in 2015 and 2016 in Indiana to evaluate soybean phytotoxicity in response to applications of halauxifen-methyl (5 g ae ha−1) at five preplant intervals (0, 1, 2, 3, and 4 weeks before planting [WBP]). In 2015, soybean phytotoxicity was not observed for any of the preplant intervals at any of the sites. In 2016, 0% to 15% phytotoxicity was observed at 14 d after planting (DAP) when halauxifen-methyl was applied at planting, 1 WBP, and 2 WBP at different sites. Soybean phytotoxicity was expressed in the unifoliate leaves only at 14 DAP. However, the first trifoliate did not show any injury symptoms at 21 DAP from any preplant application timing. Preplant application intervals for halauxifen-methyl did not affect soybean stand counts or grain yield in any site-year. Therefore, field results indicated that halauxifen-methyl applied alone can cause slight soybean phytotoxicity in preplant applications. In growth-chamber bioassays, reductions in soybean biomass, plant length, and emergence were accentuated at 30 C, compared with 20 or 15 C, when halauxifen-methyl was applied at 20 or 40 g ae ha−1. These results contradict the currently held paradigm in which lower temperatures generally increase crop phytotoxicity levels to herbicide soil residual.
Carrier water pH is an important factor for enhancing herbicide efficacy. Coapplying agrochemical products with the herbicide might save time and resources; however, the negative effect of foliar fertilizers on herbicide efficacy should be thoroughly evaluated. In greenhouse studies, the effect of carrier water pH (4, 6.5, and 9), foliar fertilizer (zinc [Zn], manganese [Mn], or without fertilizer), and ammonium sulfate (AMS) at 0% or 2.5% vol/vol was evaluated on 2,4-D and premixed 2,4-D plus glyphosate efficacy for giant ragweed, horseweed, and Palmer amaranth control. In addition, a field study was conducted to evaluate the effect of carrier water pH (4, 6.5, and 9); and Zn or Mn foliar fertilizer on premixed 2,4-D plus glyphosate efficacy for horseweed and Palmer amaranth control. In the greenhouse study, 2,4-D and premixed 2,4-D plus glyphosate provided 5% greater weed control at acidic compared with alkaline carrier water pH. Coapplied Mn foliar fertilizer reduced 2,4-D and premixed 2,4-D plus glyphosate efficacy at least 5% for weed control. Addition of AMS enhanced 2,4-D and premixed 2,4-D plus glyphosate efficacy at least 6% for giant ragweed, horseweed, and Palmer amaranth control. In the field study, few significant differences occurred between coapplied Zn or Mn foliar fertilizer for any treatment variables. Therefore, carrier water pH, coapplied foliar fertilizer, and water-conditioning adjuvants have potential to influence herbicide performance. However, weed species could play a role in the differential response of these factors on herbicide efficacy.
Herbicide carrier water hardness and pH can be variable depending on the source and geographic location. Herbicide efficacy can be affected by the pH and hardness of water used for spray solution. Field and greenhouse studies were conducted to evaluate the effect of carrier water pH and hardness on premixed dicamba and glyphosate efficacy. Treatments were combinations of water pH at 4, 6.5, or 9; and water hardness at 0 (deionized water), 400, or 800 mg L−1 of CaCO3 equivalent. In the field study, dicamba and glyphosate were applied at 0.55 and 1.11 kg ae ha−1, respectively, and half of these rates were applied in the greenhouse study. There was no interaction between carrier water pH and hardness on dicamba and glyphosate efficacy; however, the main effects of carrier water pH and hardness were significant. Herbicide efficacy was reduced with carrier water at pH 9 compared with pH 4. In the field study, common lambsquarters, common ragweed, horseweed, or Palmer amaranth control was improved 6% or more at carrier water at pH 4 compared with pH 9. Similar results were observed with water pH for giant ragweed, Palmer amaranth, or pitted morningglory control in the greenhouse study. Carrier water hardness at 400 or 800 mg L−1 reduced common ragweed, giant ragweed, or horseweed control compared with 0 mg L−1. Similarly, common lambsquarters, Palmer amaranth, or pitted morningglory control was reduced at least 10% with carrier water hardness at 800 mg L−1 compared with 0 mg L−1. These results indicate carrier water at acidic pH and of no hardness is critical for dicamba and glyphosate application, and spray solution needs to be amended appropriately for an optimum efficacy.
Evolution of glyphosate-resistant (GR) weeds, such as horseweed, presents major challenges in no-till soybean production systems. Effective GR horseweed control with preplant burndown applications is necessary to prevent potential soybean yield losses due to competition and to manage the soil weed seedbank. Halauxifen-methyl is a new synthetic auxin herbicide for broadleaf weed control in preplant burndown applications for soybean and other crops at low use rates (5 g ae ha–1). Experiments were conducted to evaluate the efficacy of herbicide treatments containing halauxifen-methyl for control of GR horseweed in comparison to existing herbicide treatments utilized in no-till GR soybean systems. Glyphosate alone controlled horseweed 33%. Herbicide treatments that included halauxifen-methyl, dicamba, or saflufenacil in combination with glyphosate controlled horseweed 87% to 96%, 89%, and 93%, respectively, 35 d after burndown application (DAB). Horseweed control, horseweed density reduction, and ground cover reduction by halauxifen-methyl plus glyphosate was similar to dicamba plus glyphosate. Horseweed control was greater for halauxifen-methyl plus glyphosate than for 2,4-D plus glyphosate. Cloransulam, cloransulam plus flumioxazin, and cloransulam plus sulfentrazone added to halauxifen-methyl plus glyphosate increased horseweed control and reduced horseweed density. No herbicide injury or soybean yield reduction was observed for treatments containing halauxifen-methyl.
Synthetic auxin herbicides such as 2,4-D and dicamba are often utilized to control broadleaf weeds in preplant burndown applications to soybean. Halauxifen-methyl is a new synthetic auxin herbicide for broadleaf weed control in preplant burndown applications to corn, cotton, and soybean at low use rates (5 g ae ha–1). Field experiments were conducted to evaluate efficacy and weed control spectrum of halauxifen-methyl applied alone and in mixtures with 2,4-D (560 g ae ha–1), dicamba (280 g ae ha–1), and glyphosate (560 g ae ha–1). Glyphosate-resistant (GR) horseweed was controlled with halauxifen-methyl applied alone (90% control) and in mixtures (87% to 97% control) 35 d after treatment (DAT). Common ragweed was controlled 93% with halauxifen-methyl applied alone and 91% to 97% in mixtures 35 DAT. Halauxifen-methyl applied alone resulted in poor giant ragweed control 21 DAT (73% control); however, mixtures of halauxifen-methyl with 2,4-D, dicamba, or glyphosate controlled giant ragweed (86% to 98% control). Halauxifen-methyl alone resulted in poor redroot pigweed control (62% control) 21 DAT; however, mixtures of halauxifen-methyl with dicamba, 2,4-D, or glyphosate controlled redroot pigweed (89% to 98% control). Halauxifen-methyl controls GR horseweed and common ragweed applied alone and in mixtures with other synthetic auxin herbicides and glyphosate. Furthermore, mixing 2,4-D or dicamba with halauxifen-methyl can increase the weed control spectrum in preplant burndown applications.
Halauxifen-methyl is a new synthetic auxin herbicide for control of broadleaf weeds, including preplant applications for corn (Zea mays L.) or soybean [Glycine max (L.) Merr.]. The objective of this study was to investigate the efficacy of halauxifen-methyl in comparison to the current auxin standards, 2,4-D and dicamba, on glyphosate-resistant (GR) horseweed (Erigeron canadensis L.) at different plant heights. In field experiments, a foliar application of halauxifen-methyl at the recommended use rate of 5 g ae ha−1 resulted in 81% control. Dicamba applied at 280 g ae ha−1 provided a comparable level of efficacy of 80%, while 2,4-D at 560 g ae ha−1 resulted in 49% control. The addition of glyphosate improved GR E. canadensis control with 2,4-D more than with halauxifen-methyl or dicamba, possibly due to the higher level of control observed with halauxifen-methyl or dicamba alone. Even though applied at 50 to 100 times lower application rates, the efficacy of halauxifen-methyl on E. canadensis was similar to dicamba and greater than 2,4-D. Thus, halauxifen-methyl should be an effective tool for management of GR E. canadensis before planting both conventional and herbicide-resistant soybean varieties, and it precludes the extended preplant application interval required for dicamba in some soybean management systems.
Knowledge of the effects of burial depth and burial duration on seed viability and, consequently, seedbank persistence of Palmer amaranth (Amaranthus palmeri S. Watson) and waterhemp [Amaranthus tuberculatus (Moq.) J. D. Sauer] ecotypes can be used for the development of efficient weed management programs. This is of particular interest, given the great fecundity of both species and, consequently, their high seedbank replenishment potential. Seeds of both species collected from five different locations across the United States were investigated in seven states (sites) with different soil and climatic conditions. Seeds were placed at two depths (0 and 15 cm) for 3 yr. Each year, seeds were retrieved, and seed damage (shrunken, malformed, or broken) plus losses (deteriorated and futile germination) and viability were evaluated. Greater seed damage plus loss averaged across seed origin, burial depth, and year was recorded for lots tested at Illinois (51.3% and 51.8%) followed by Tennessee (40.5% and 45.1%) and Missouri (39.2% and 42%) for A. palmeri and A. tuberculatus, respectively. The site differences for seed persistence were probably due to higher volumetric water content at these sites. Rates of seed demise were directly proportional to burial depth (α=0.001), whereas the percentage of viable seeds recovered after 36 mo on the soil surface ranged from 4.1% to 4.3% compared with 5% to 5.3% at the 15-cm depth for A. palmeri and A. tuberculatus, respectively. Seed viability loss was greater in the seeds placed on the soil surface compared with the buried seeds. The greatest influences on seed viability were burial conditions and time and site-specific soil conditions, more so than geographical location. Thus, management of these weed species should focus on reducing seed shattering, enhancing seed removal from the soil surface, or adjusting tillage systems.
Palmer amaranth (Amaranthus palmeri S. Watson) is a problematic weed encountered in U.S. cotton (Gossypium hirsutum L.) and soybean [Glycine max (L.) Merr.] production, with infestations spreading northward. This research investigated the influence of planting date (early, mid-, and late season) and population (AR, IN, MO, MS, NE, and TN) on A. palmeri growth and reproduction at two locations. All populations planted early or midseason at Throckmorton Purdue Agricultural Center (TPAC) and Arkansas Agriculture Research and Extension Center (AAREC) measured 196 and 141 cm or more, respectively. Amaranthus palmeri height did not exceed 168 and 134 cm when planted late season at TPAC and AAREC, respectively. Early season planted A. palmeri from NE grew to 50% of maximum height 8 to 13 d earlier than all other populations under TPAC conditions. In addition, the NE population planted early, mid-, and late season achieved 50% inflorescence emergence 5, 4, and 6 d earlier than all other populations, respectively. All populations established at TPAC produced fewer than 100,000 seeds plant−1. No population planted at TPAC and AAREC produced more than 740 and 1,520 g plant−1 of biomass at 17 and 19 wk after planting, respectively. Planting date influenced the distribution of male and female plants at TPAC, but not at AAREC. Amaranthus palmeri from IN and MS planted late season had male-to-female plant ratios of 1.3:1 and 1.7:1, respectively. Amaranthus palmeri introduced to TPAC from NE can produce up to 7,500 seeds plant−1 if emergence occurs in mid-July. An NE A. palmeri population exhibited biological characteristics allowing it to be highly competitive if introduced to TPAC due to a similar latitudinal range, but was least competitive when introduced to AAREC. Although A. palmeri originating from different locations can vary biologically, plants exhibited environmental plasticity and could complete their life cycle and contribute to spreading populations.
The introduction of 2,4-D–resistant soybean will provide an additional POST herbicide site of action for control of herbicide-resistant broadleaf weeds. The introduction of this technology also brings concern of off-site movement of 2,4-D onto susceptible crops such as sensitive soybean and tomato. The 2,4-D formulation approved for use in 2,4-D–resistant soybean restricts application of the herbicide to nozzles that produce very coarse to ultra-coarse droplet spectrums. The use of larger droplet spectrums for broadcast applications can reduce herbicide deposition onto target weeds and thus influence herbicide efficacy. Field experiments were conducted to evaluate the influence of nozzle design on herbicide deposition onto target plants and the resulting efficacy of a POST application of 280 g ha−1 glyphosate plus 280 g ha−1 2,4-D. The TTI11004 nozzle produced an ultra-coarse droplet spectrum and reduced coverage and deposition density on spray cards as compared with the XR11004 and TT11004 nozzles that produced medium droplet spectrums. The AIXR11004 nozzle also reduced deposition density on spray cards but did not reduce coverage. Herbicide solution deposition onto glyphosate-resistant Palmer amaranth, tall waterhemp, giant ragweed, and horseweed ranged from 0.28 to 0.72 µl cm−2 and was not influenced by nozzle design. Herbicide efficacy was reduced by the TTI11004 nozzle on Palmer amaranth and horseweed compared with the AIXR11004, TT11004, and XR11004 nozzles when applications were made to either high densities of plants or plants exceeding the labeled height. The use of the AIXR11004 and TTI11004 nozzles that are listed as approved nozzles for glyphosate plus 2,4-D applications on 2,4-D–resistant soybean did not reduce herbicide deposition onto four of the most troublesome broadleaves and did not reduce herbicide efficacy when applied in conjunction with lower weed densities and smaller weeds.