Sicklepod is one of the most difficult to control weeds in peanut production in the southeastern United States due to its extended emergence pattern and limited effective herbicides for control. Growers rely on preemergence herbicides as the foundation of their weed control programs; however, postemergence herbicides are often needed for season-long weed control. The objectives of this study were to evaluate the effect of planting pattern and herbicide combinations for sicklepod control in peanut crops. Due to rapid canopy closure, twin-row planting improved late-season sicklepod control by 13% and peanut yield by 5% compared with a single-row pattern. A preemergence application of fluridone, flumioxazin, or fluridone + flumioxazin provided 76% to 89% control of sicklepod 28 d after preemergence. Regardless of the herbicide applied preemergence, paraquat + bentazon + S-metolachlor applied early postemergence was required to achieve ≥90% sicklepod control 28 d after early postemergence. All preemergence herbicide treatments followed by (fb) S-metolachlor or diclosulam + S-metolachlor applied early postemergence provided <90% control 28 d after early postemergence. A mid-postemergence application of imazapic + dimethenamid-P + 2,4-DB controlled sicklepod by 67% to 79% prior to peanut harvest, and biomass reduction was unacceptable (<80%), resulting in difficulty in peanut digging. The highest peanut yield was observed when paraquat + bentazon + S-metolachlor was applied early postemergence fb imazapic + dimethenamid-P + 2,4-DB applied mid-postemergence. Based on the results of this study, a herbicide combination of paraquat + bentazon + S-metolachlor is an important early-season tool for controlling sicklepod in peanut crops. The results also showed that a twin-row planting pattern improved late-season sicklepod control but did not reduce herbicide input to protect peanut yield.

Carrier water quality is an important consideration for herbicide efficacy. Field and greenhouse studies were conducted from 2021 to 2023 to evaluate the effect of carrier water pH and hardness on imazapic efficacy for sicklepod control in peanut crops. In separate field experiments imazapic was applied postemergence at 0.071 kg ai ha−1 with carrier water pH levels of 5, 6, 7, 8, or 9; and hardness levels of 0 (deionized water), 100, 200, 400, or 500 mg L−1 of CaCO3 equivalent. In greenhouse experiments, imazapic was applied to sicklepod that was either 10 cm, 15 cm, or 20 cm tall at similar carrier water pH levels and hardness levels of 0, 100, 200, 400, or 800 mg L−1 of CaCO3. In the field study, sicklepod control, density, and biomass reductions were lower with carrier water pH 5 or 9 compared with pH 7. In the greenhouse study, control was not different among carrier water pH levels when imazapic was applied to 10-cm-tall sicklepod; however, when applied to 15- or 20-cm-tall sicklepod, control was at least 25% greater with acidic (pH 5) compared to alkaline (pH 9) carrier water. Results from the field study showed that carrier water hardness ≤500 ppm did not reduce the efficacy of imazapic to control sicklepod. In the greenhouse study, regardless of sicklepod height, carrier water hardness of 800 mg L−1 reduced sicklepod control by 15% and biomass reduction by 17% compared with deionized water (pH 7). The effects of carrier water pH and hardness on imazapic efficacy did not compromise peanut yield in the field study. However, this study indicates that both acidic and alkaline carrier water pH and hardness (800 mg L−1 CaCO3 L−1) have the potential to reduce imazapic efficacy on sicklepod, and appropriate spray solution amendments maybe be needed to maintain optimum efficacy.

Herbicides are the primary tool for controlling weeds in peanut (Arachis hypogaea L.) and are crucial to sustainable peanut production in the United States. The literature on chemical weed management in peanut in the past 53 yr (1970 to 2022) in the United States was systematically reviewed to highlight the strengths and weaknesses of different herbicides and identify current research gaps in chemical weed management. Residual weed control in peanut is achieved mainly with dimethenamid-P, ethalfluralin, pendimethalin, and S-metolachlor. More recently, the use of the protoporphyrinogen oxidase inhibitor flumioxazin and acetolactate synthase inhibitors, such as diclosulam, for residual weed control in peanut has increased considerably. Postemergence broadleaf weed control in peanut is achieved mainly with acifluorfen, bentazon, diclosulam, imazapic, lactofen, paraquat, and 2,4-DB, while the graminicides clethodim and sethoxydim are the major postemergence grass weed control herbicides in peanut. Although several herbicides are available for weed control in peanut, no single herbicide can provide season-long weed control due to limited application timing, lack of extended residual activity, variability in weed control spectrum, and rotational restrictions. Therefore, effective weed management in peanut often requires herbicide mixtures and/or sequential application of preplant-incorporated, preemergence, and/or postemergence herbicides. However, the available literature showed a substantive range in herbicide efficacy due to variations in environmental conditions and flushes of weed germination across years and locations. Despite the relatively high efficacy of herbicides, the selection of herbicide-resistant weeds is another area of increasing concern. Future research should focus on developing new strategies for preventing or delaying the development of resistance and improving herbicide efficacy within the context of climate change and emerging constraints such as water shortages, rising temperatures, and increasing CO2 concentration.

The increased incidence of glyphosate-resistant weeds has led to an exponential increase in the use of glufosinate on glufosinate-resistant corn, cotton, and soybean crops. Field experiments were conducted in 2021 and 2022 to evaluate peanut response to glufosinate at 25 and 60 d after planting, corresponding to vegetative (V3) and reproductive (R4) growth stages, at 1.2, 4.7, 18.9, 75.5, and 302 g ai ha−1 representing 1/514 to 1/2 of the labeled rate of 604 g ha−1. Peanut injury and canopy and yield reductions from glufosinate were <10% when applied at 1.2, 4.7, and 18.9 g ha−1. However, at 75.5 and 302 g ha−1 peanut injury ranged from 24% to 72% at the V3 exposure timing and 33% to 54% at the R4 exposure timing. Similarly, glufosinate applied at 75.5 and 302 g ha−1 reduced peanut canopy width by 10% to 23% at the V3 exposure timing and by 43% to 57% at the R4 exposure timing. Averaged across exposure timing, peanut yield was reduced by 15% and 61% when glufosinate was applied at 75.5 and 302 g ha−1, respectively. Averaged across rates, peanut yield reduction was 18% at the V3 exposure timing, with glufosinate at 298 g ha−1 required to cause an estimated 50% reduction in yield. Peanut yield was reduced by 20% when glufosinate was applied at the R3 peanut growth stage, whereas glufosinate applied at 243 g ha-1 caused an estimated 50% reduction in yield. There was no difference in normalized difference vegetative index (NDVI) values between untreated plants and peanut exposed to glufosinate at 1.2, 4.7, and 18.9 g ha−1. However, peanut exposed to glufosinate at 75.5 and 302 g ha−1 was distinguished from untreated plants with lower NDVI values. Based on the Pearson correlation coefficient, the best timing for assessing potential yield reduction based on injury was between 2 and 4 wk after treatment.

Weed interference is a major factor that reduces peanut (Arachis hypogaea L.) yield in the United States. Peanut growers rely heavily on herbicides for weed control. Although effective, herbicides are not a complete solution to the complex challenge that weeds present. Therefore, the use of nonchemical weed management options is essential. The literature on weed research in peanut in the past 53 yr in the United States was reviewed to assess the achievements and identify current research gaps and prospects for nonchemical weed management for future research. More than half (79%) of the published studies were from the southeastern United States. Most studies (88%) focused on weed management, while fewer studies (12%) addressed weed distribution, ecology, and competitive mechanisms. Broadleaf weeds were the most frequently studied weed species (60%), whereas only 23% and 19% of the published studies were relevant to grasses and Cyperus spp., respectively. Seventy-two percent of the published studies focused on curative measures using herbicides. Nonchemical methods using mechanical (5%) and preventive (13%) measures that influence crop competition and reduce the buildup of the weed seedbank, seedling recruitment, and weed seed production have received less attention. In most studies, the preventive weed management measures provided weed suppression and reduced weed competition but were not effective enough to reduce the need for herbicides to protect peanut yield. Therefore, future research should focus on developing integrated weed management strategies based on multiple preventive measures rather than one preventive measure combined with one or more curative measures. We recommend that research on mechanical weed management should focus on the role of cultivation when integrated with currently available herbicides. For successful weed management with lasting outcomes, the dominant weed communities of specific target locations should be addressed within the context of climate change and emerging constraints rather than focusing on single problematic species.

We conducted an online survey of weed scientists in the United States and Canada to (1) identify research topics perceived to be important for advancing weed science in the next 5 to 10 years and (2) gain insight into potential gaps in current expertise and funding sources needed to address those priorities. Respondents were asked to prioritize nine broad research areas, as well as 5 to 10 subcategories within each of the broad areas. We received 475 responses, with the majority affiliated with academic institutions (55%) and working in cash crop (agronomic or horticultural) study systems (69%). Results from this survey provide valuable discussion points for policy makers, funding agencies, and academic institutions when allocating resources for weed science research. Notably, our survey reveals a strong prioritization of Cultural and Preventative Weed Management (CPWM) as well as the emerging area of Precision Weed Management and Robotics (PWMR). Although Herbicides remain a high-priority research area, continuing challenges necessitating integrated, nonchemical tactics (e.g., herbicide resistance) and emerging opportunities (e.g., robotics) are reflected in our survey results. Despite previous calls for greater understanding and application of weed biology and ecology in weed research, as well as recent calls for greater integration of social science perspectives to address weed management challenges, these areas were ranked considerably lower than those focused more directly on weed management. Our survey also identified a potential mismatch between research priorities and expertise in several areas, including CPWM, PWMR, and Weed Genomics, suggesting that these topics should be prime targets for expanded training and collaboration. Finally, our survey suggests an increasing reliance on private sector funding for research, raising concerns about our discipline’s capacity to address important research priority areas that lack clear private sector incentives for investment.

Weed management in cantaloupe and other melon crops is important to maximize fruit yield; however, there are few registered herbicides available in California. Several independent herbicide trials were conducted at University of California field stations in Davis (Yolo County), Five Points (Fresno County), and Holtville (Imperial County) from 2013 to 2019 to evaluate both registered and unregistered herbicides and incorporation methods (sprinklers, cultivation, or none) for crop safety and weed control in melons. Although specific treatments varied among locations depending on local practice and research objectives, ethalfluralin and halosulfuron were used in all experiments, and bensulide and S-metolachlor were evaluated in 4 of 6 site-years. Additional herbicides included clethodim, clomazone, DCPA, napropamide, pendimethalin, sethoxydim, and sulfentrazone. Among registered herbicides, halosulfuron, halosulfuron + ethalfluralin, and ethalfluralin + bensulide combinations provided consistently beneficial weed control across all site-years compared to the nontreated control. S-metolachlor performed as well as the best of the registered herbicides tested at each site-year; although moderate injury was noted at the Davis location, this did not reduce melon yield. The method used to incorporate preplant herbicides had a significant impact on weed control efficacy but varied by location. Mechanical incorporation of preplant herbicides resulted in improved weed control and yield compared to sprinklers. Early-season weed control, whether by herbicides or hand weeding, resulted in significant yield increase in most site-years.

The commercialization of crops that are resistant to 2,4-D plus glyphosate provided an opportunity to growers to apply the herbicide postemergence. However, the potential drift injury of these herbicides to peanuts grown near crops that are resistant to 2,4-D plus glyphosate is concerning. Field experiments were conducted in 2019 and 2020 to evaluate peanut response when exposed at 25, 50, and 75 d after planting (DAP) corresponding to vegetative, flowering, and pod development stages, respectively, to reduced rates of 1/512×, 1/128×, 1/32×, and 1/8× of the labeled rate of 2,4-D plus glyphosate (i.e., 1,077 + 1,132 g ae ha−1, respectively). 2,4-D plus glyphosate was more injurious to peanuts when exposed at 25 DAP compared with 50 and 75 DAP. Similarly, greater canopy height (12%) and canopy width (16%) reductions were observed at 25 DAP compared with 50 and 75 DAP exposure timings (3% to 9%). This result indicates that peanut is more sensitive to 2,4-D plus glyphosate exposure at the vegetative growth stage than at the flowering and pod development stages. However, yield reductions (13% to 16%) were not different between 25, 50, or 75 DAT exposure timings. Regression analysis indicated a linear response for peanut injury, canopy height, width, and yield reduction with an increasing rate of 2,4-D plus glyphosate. The highest rate of 2,4-D plus glyphosate (1/8× of the label rate) resulted in 38%, 22%, and 23% peanut injury, canopy height, and width reduction at 4 wk after treatment, and 33% yield reduction. There was a correlation between peanut injury and yield reduction, with Pearson’s rho values ranging from 0.70 to 0.73. The findings suggest that peanut injury rating data after 2,4-D plus glyphosate drift can be useful for estimating potential yield losses.

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.

Field experiments were conducted from 2017 to 2019 to determine the tolerance of carinata to several preemergence and postemergence herbicides. Preliminary screenings identified herbicides that caused large variation on carinata injury, indicating the potential for selectivity. Dose-response field studies were conducted to quantify the tolerance of carinata to select herbicides. Diuron applied preemergence at rates of 280 g ai ha−1 or higher reduced carinata population density 54% to 84% compared to the nontreated control. In certain locations, clomazone applied preemergence caused minor injury with an acceptable level of carinata tolerance and only doses above 105 g ai ha−1 caused yield reductions. Napropamide doses of 2,856 g ai ha−1 or higher applied preemergence caused at least 25% injury to carinata; however, the damage was not severe enough to reduce yields. Simazine applied postemergence at rates above 1,594 g ai ha−1 caused 50% or more injury, resulting in yield losses ranging from 0% to 95% depending on location. Clopyralid applied postemergence at 2,512 g ai ha−1 caused 25% injury with relative yield reductions, which varied across locations. The present study identified clomazone and napropamide applied preemergence, and clopyralid applied postemergence as potential herbicides for weed control in carinata. In contrast, diuron, simazine, metribuzin, imazethapyr, and chlorimuron caused high levels of carinata mortality and can be used to control volunteer carinata plants in rotational crops.

Ethiopian mustard (Brassica carinata A. Braun) is a biofuel crop recently introduced in the southeastern United States. For this crop to be successful, integrated weed management strategies that complement its rotation with summer cash crops must be developed. The objectives of this research were to evaluate the effect of previous season summer crops on winter weed emergence patterns during Ethiopian mustard growing season and to assess the impact of planting Ethiopian mustard on the emergence patterns of summer weed species. Gompertz models were fit to winter and summer weed emergence patterns. All models represented more than 80% of the variation, with root mean-square error values less than 0.20. The emergence pattern for winter weed species was best described using growing degree-day accumulation, and this model can be utilized for implementing weed control strategies at the critical Ethiopian mustard growth stages. The results also showed that summer weeds can emerge during the winter in northern Florida but do not survive frost damage, which might create off-season seedbank reductions before the summer crop growing season.

Rhizoma perennial peanut (RPP) is well adapted to the Gulf Coast region of the United States, but its varietal tolerance to glyphosate and triclopyr is not well defined. The research was conducted to determine the effect of various rates of glyphosate and triclopyr on established RPP, and the response of common RPP varieties to these herbicides. The RPP sward was approximately 7 yr younger at Zolfo Springs than at the Ona location. RPP showed moderate tolerance to glyphosate and triclopyr application, and injury level did not differ with the age of RPP sward. However, biomass production was negatively influenced by the age of the RPP sward. Overall, injury from glyphosate applications did not exceed 40% at either site. The glyphosate rate for 20% biomass reduction was predicted to be 0.53 and 2.17 kg ae ha−1 at Zolfo Springs and Ona, respectively. RPP injury from triclopyr was greater at the Zolfo Springs location than at Ona, and the triclopyr rate predicted to result in a 20% biomass reduction was 0.45 and 0.99 kg ae ha−1 at the Zolfo Springs and Ona locations, respectively. There was a difference on RPP varieties response to glyphosate and triclopyr application. ‘Florigraze’ and ‘Ona 33’ were less tolerant to glyphosate compared to ‘UF-Tito’ and ‘Ecoturf’ at 30 d after treatment. Likewise, UF-Tito and Florigraze were less tolerant to triclopyr compared to Ona 33 and Ecoturf. Overall, Florigraze showed highest injury and at least 2-fold reduction on biomass compared to the other three varieties from glyphosate or triclopyr application. Results from this research indicate that glyphosate and triclopyr appear to be safe to apply to long-established RPP stands, but herbicide rate and RPP varieties should be considered if stands are <5 yr old.

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.

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.

Spray water quality is an important consideration for optimizing herbicide efficacy. Hard water cations in the carrier water can reduce herbicide performance. Greenhouse studies were conducted to evaluate the influence of hard water cations and the use of ammonium sulfate (AMS) on the efficacy of 2,4-D choline and premixed 2,4-D choline plus glyphosate for giant ragweed, horseweed, and Palmer amaranth control. Carrier water hardness was established at 0, 200, 400, 600, 800, or 1,000 mg L−1 using CaCl2 and MgSO4, and each hardness level consisted of without or with AMS at 10.2 g L−1. One-third of the proposed use rates of 2,4-D choline at 280 g ae ha−1 and 2,4-D choline plus glyphosate at 266 plus 283 g ae ha−1, respectively, were applied in the study. An increase in carrier water hardness showed a linear trend for reducing 2,4-D choline and 2,4-D choline plus glyphosate efficacy on all weed species evaluated in both studies. The increase in water hardness level reduced giant ragweed control with 2,4-D choline and the premix formulation of 2,4-D choline plus glyphosate to a greater extent without AMS than it did with AMS in the spray solution. Increases in water hardness from 0 to 1,000 mg L−1 reduced weed control 20% or greater with 2,4-D choline. Likewise, the efficacy of the premixed 2,4-D choline plus glyphosate was reduced 21% or greater with increased water hardness from 0 to 1,000 mg L−1. The addition of AMS improved giant ragweed, horseweed, and Palmer amaranth control ≥ 17% and ≥ 10% for 2,4-D choline and 2,4-D choline plus glyphosate application, respectively. The biomass of all weed species was reduced by ≥ 8% and ≥ 5% with 2,4-D choline and 2,4-D choline plus glyphosate application, respectively, when AMS was added to hard water.

Carrier water quality is an important consideration for herbicide efficacy. Effect of carrier water pH (4, 6.5, or 9) and coapplied Zn or Mn foliar fertilizer was evaluated on glufosinate efficacy for horseweed and Palmer amaranth control in the field. Greenhouse studies were conducted to evaluate the effect of: (1) carrier water pH, foliar fertilizer (Zn, Mn, or without fertilizer), and ammonium sulfate (AMS) (at 0 or 2.5% v/v); and (2) carrier water hardness (0 to 1,000 mg L−1) and AMS (at 0 or 2.5% v/v) on glufosinate efficacy for giant ragweed, horseweed, and Palmer amaranth control. In a 2014 field study, control, plant density reduction, and biomass reduction were at least 8% greater for horseweed and at least 14% greater for Palmer amaranth when glufosinate was applied at carrier water pH 4 compared with pH 9. Glufosinate efficacy was at least 10 and 17% greater for giant ragweed and Palmer amaranth control, respectively, with carrier water pH 4 compared with pH 9 in the greenhouse. In the greenhouse studies, coapplied Zn or Mn fertilizer had no effect on glufosinate efficacy. Increased carrier water hardness from 0 to 1,000 mg L−1 negatively influenced glufosinate efficacy and resulted in 20 and 17% lesser control and biomass reduction, respectively, on giant ragweed or Palmer amaranth. Use of AMS enhanced glufosinate efficacy on giant ragweed control in both greenhouse studies, whereas only the Palmer amaranth control was enhanced in the water hardness study. Horseweed control with glufosinate as affected by carrier water pH, hardness, or AMS remained unaffected in both greenhouse studies. Carrier water at alkaline pH or hardness > 200 mg L−1 has potential to reduce glufosinate efficacy. Therefore, carrier water free of hardness cations and at acidic condition (pH = 4 to 6.5) should be considered for optimum glufosinate efficacy.

Water is the primary carrier for herbicide application, and carrier-water–related factors can influence herbicide performance. In a greenhouse study, premixed formulation of glyphosate plus dicamba was mixed in deionized (DI) water at 5, 18, 31, 44, or 57 C and applied immediately. In a companion study, glyphosate and dicamba formulation was mixed in DI water at temperatures of 5, 22, 39, or 56 C and sprayed after the herbicide solution was left at the respective temperatures for 0, 6, or 24 h. In both studies, glyphosate plus dicamba was applied at 0.275 plus 0.137 kg ae ha−1 (low rate), and 0.55 plus 0.275 kg ha−1 (high rate), respectively, to giant ragweed, horseweed, Palmer amaranth, and pitted morningglory. Glyphosate plus dicamba applied at a low rate with solution temperature of 31 C provided 14% and 26% greater control of giant ragweed and pitted morningglory, respectively, compared to application at solution temperature of 5 C. At both rates of glyphosate and dicamba formulation, giant ragweed and pitted morningglory control was 15% or greater at solution temperature of 44 C compared to 5 C. Weed control was not affected with premixture of glyphosate and dicamba applied ≤ 24 h after mixing herbicide. When considering solution temperature, glyphosate and dicamba applied at low rate provided 13 and 6% greater control of Palmer amaranth and pitted morningglory, respectively, with solution temperature of 22 C compared to 5 C. Similarly, giant ragweed control was 8% greater with solution temperature of 39 C compared to 5 C. Glyphosate and dicamba applied at high rate provided 8% greater control of giant ragweed at solution temperature of 22 or 39 C compared to 5 C. Therefore, activity of premixed glyphosate and dicamba could be reduced with spray solution at lower temperature; however, the result is dependent on weed species.

Isothiocyanates (ITCs) were evaluated as an alternative to methyl bromide (MeBr) for control of Palmer amaranth, large crabgrass, and yellow nutsedge; reduction of tuber density; and increase in marketable tomato yield in low density polyethylene (LDPE)-mulched tomato production. Allyl ITC was applied at 450, 600, and 750 kg ai ha−1; metham sodium (methyl ITC generator) was applied at 180, 270, and 360 kg ai ha−1; and MeBr plus chloropicrin (mixture of MeBr and chloropicrin at 67 : 33%, respectively) was applied at 390 kg ai ha−1. A nontreated weedy check was included for comparison. There was no injury to tomato plants following allyl ITC, metham sodium, or MeBr application. Allyl ITC at 750 kg ha−1 or metham sodium at 360 kg ha−1 controlled Palmer amaranth ≥ 79%, large crabgrass ≥ 76%, and yellow nutsedge ≥ 80% and was comparable to the weed control with MeBr. Highest rates of allyl ITC and metham sodium reduced yellow nutsedge tuber density (≤ 76 tubers m−2) comparable to the MeBr application. Total marketable tomato yield was ≥ 31.6 t ha−1 in plots treated with allyl ITC at 750 kg ha−1 or metham sodium at 360 kg ha−1. Marketable tomato yield from the highest rate of allyl ITC or metham sodium were similar to the yield (38.2 t ha−1) with MeBr treatment. Therefore, allyl ITC at 750 kg ha−1 and metham sodium at 360 kg ha−1 are effective alternatives to MeBr for Palmer amaranth, large crabgrass, and yellow nutsedge control in LDPE-mulched tomato.

Methyl bromide (MeBr), a widely used soil fumigant in tomato production, has been banned for ordinary agricultural uses. In the absence of MeBr, a viable alternative is imperative for weed control and prevention of economic loss in tomato production. A field study was conducted in the summers of 2010 and 2011 at Fayetteville, AR, to compare the efficacy and economics of herbicide programs consisting of pre-transplant followed by (fb) post-transplant herbicides in low-density polyethylene (LDPE) mulched tomato. Pre-transplant imazosulfuron at 0.112, 0.224, and 0.336 kg ai ha−1 and S-metolachlor at 1.6 kg ai ha−1 were fb a post-transplant mixture of trifloxysulfuron plus halosulfuron at 0.008 and 0.027 kg ai ha−1 at 4 wk after transplant (WATP). The standard MeBr treatment (2:1 mixture of MeBr plus chloropicrin at 390 kg ai ha−1), weed-free (hand weeding) control, and nontreated weedy check were used for comparison. Pre-transplant S-metolachlor fb post-transplant herbicides controlled Palmer amaranth ≥ 89%, large crabgrass ≥ 88%, and yellow nutsedge ≥ 90%, which was comparable to the control with MeBr. Tomato recovered the injury (≤ 19% at 6 WATP) from post-transplant herbicides in the later weeks. S-metolachlor–containing herbicide programs yielded marketable tomato fruit comparable to the yield with MeBr. Economic evaluation of the herbicide programs demonstrated a net return of $3,758.50 ha−1 from the S-metolachlor–containing herbicide program in LDPE-mulched tomato. Likewise, this herbicide program showed minimum loss of ≤ $671.61 ha−1 in net return relative to MeBr. In conclusion, a herbicide program consisting of pre-transplant S-metolachlor fb post-transplant trifloxysulfuron plus halosulfuron is a viable alternative to MeBr for weed control and marketable yield in LDPE-mulched tomato production.