Specialty crops, like flowers, herbs, and vegetables, generally do not have an adequate spectrum of herbicide chemistries to control weeds and have been dependent on hand weeding to achieve commercially acceptable weed control. However, labor shortages have led to higher costs for hand weeding. There is a need to develop labor-saving technologies for weed control in specialty crops if production costs are to be contained. Machine vision technology, together with data processors, have been developed to enable commercial machines to recognize crop row patterns and control automated devices that perform tasks such as removal of intrarow weeds, as well as to thin crops to desired stands. The commercial machine vision systems depend upon a size difference between the crops and weeds and/or the regular crop row pattern to enable the system to recognize crop plants and control surrounding weeds. However, where weeds are large or the weed population is very dense, then current machine vision systems cannot effectively differentiate weeds from crops. Commercially available automated weeders and thinners today depend upon cultivators or directed sprayers to control weeds. Weed control actuators on future models may use abrasion with sand blown in an air stream or heating with flaming devices to kill weeds. Future weed control strategies will likely require adaptation of the crops to automated weed removal equipment. One example would be changes in crop row patterns and spacing to facilitate cultivation in two directions. Chemical company consolidation continues to reduce the number of companies searching for new herbicides; increasing costs to develop new herbicides and price competition from existing products suggest that the downward trend in new herbicide development will continue. In contrast, automated weed removal equipment continues to improve and become more effective.

Flumioxazin and fomesafen are commonly used to control glyphosate-resistant Palmer amaranth in cotton and other crops, thus increasing risk to select for Palmer amaranth biotypes resistant to protoporphyrinogen oxidase (PPO) inhibitors. A field experiment was conducted to determine the potential for fluridone and acetochlor to substitute for soil-applied PPO inhibitors in a Palmer amaranth management system with glufosinate applied twice POST and diuron plus MSMA POST-directed in conservation tillage cotton. Fluridone and flumioxazin applied preplant 23 to 34 d prior to planting were similarly effective. Fluridone and acetochlor plus diuron applied PRE controlled Palmer amaranth as well as fomesafen plus diuron PRE. All systems with preplant and PRE herbicides followed by glufosinate POST and diuron plus MSMA layby controlled Palmer amaranth well. Cotton yield did not differ among herbicide treatments. This research demonstrates that fluridone and acetochlor can substitute for soil-applied PPO-inhibiting herbicides in management systems for Palmer amaranth.

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.

Feral populations of cultivated crops have the potential to function as bridges and reservoirs that contribute to the unwanted movement of novel genetically engineered (GE) traits. Recognizing that feral alfalfa has the potential to lower genetic purity in alfalfa seed production fields when it is growing in the vicinity of foraging pollinators in alfalfa seed fields, industry has established production standards to control feral plants. However, with the commercialization of GE glyphosate-resistant (GR) alfalfa and the need to support the coexistence of both GE and conventional production, effective methods to control transgenic feral alfalfa need to be developed. Therefore, a study was conducted in 2012, 2013, and 2014 to determine the effect of several synthetic auxin herbicides on seed development in GR alfalfa. GR alfalfa, var. Genuity (R44BD16), was treated with dicamba, 2,4-D, triclopyr, and aminopyralid when alfalfa plants contained green seed pods. Two weeks after herbicide application, plants were harvested, air dried, and seed yield, seed germination, and seedling emergence from the soil were determined. In 2013, dicamba, triclopyr, and 2,4-D decreased alfalfa seed yield per plant compared wih nontreated plants, whereas in 2014, all four herbicides decreased alfalfa seed yield per plant 24 to 49% (by weight) compared with nontreated plants. The same trend was evident in 2012, but seed yield was variable and was not significantly different among treatments. Seed germination averaged 43, 50, and 72% in 2012, 2013, and 2014, respectively, and was not affected by the four herbicides applied at early pod-fill stage. However, seeds harvested from plants treated with dicamba, 2,4-D, and triclopyr often produced deformed and abnormal seedlings, and when planted in soil, frequently failed to emerge. The combined effects of dicamba, 2,4-D, and triclopyr in reducing seed yield, seedling emergence, and seedling growth could contribute to managing feral alfalfa populations.

Crop safety is one of the many considerations when deciding which POST herbicide to use. This research examined relative corn injury as a result of POST herbicides and the effect of including the safener isoxadifen, the choice of a sensitive or tolerant hybrid, or both. The herbicides included commercial combinations of dicamba, diflufenzopyr, nicosulfuron, rimsulfuron, and thifensulfuron, all at twice the labeled rate. Isoxadifen reduced twisting from dicamba plus diflufenzopyr but not with dicamba plus rimsulfuron. Isoxadifen had negligible effect on chlorosis. In general, rimsulfuron plus thifensulfuron caused the most corn stunting, whereas including isoxadifen or using a tolerant hybrid often reduced corn injury. In two of the four years, treatments with rimsulfuron plus thifensulfuron resulted in yield reductions. Although using products with isoxadifen or selecting tolerant hybrids may influence injury, herbicide selection will have the greatest effect on corn injury.

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.

The objective of this research was to explore the influence that weed decision threshold (DT; expressed as plants m−2), weed spatial distribution patterns, and spatial resolution of sampling have on potential reduction in herbicide use under site-specific weed management. As a case study, a small plot located in a typical corn field in central Spain was used, constructing very precise distribution maps of the major weeds present. These initial maps were used to generate herbicide prescription maps for each weed species based on different DTs and sampling resolutions. The simulation of herbicide prescription maps consisted of on/off spraying decisions based on information from two different approaches for weed detection: ground-based vs. aerial sensors. In general, simulations based on ground sensors resulted in higher herbicide savings than those based on aerial sensors. The extent of herbicide reductions derived from patch spraying was directly related to the density and the spatial distribution of each weed species. Herbicide savings were potentially high (up to 66%) with relatively sparse patchy weed species (e.g., johnsongrass) but were only moderate (10 to 20%) with abundant and regularly distributed weed species (e.g., velvetleaf). However, DT has proven to be a key factor, with higher DTs resulting in reductions in herbicide use for all the weed species and all sampling procedures and resolutions. Moreover, increasing DT from 6 to 12 plants m−2 resulted in additional herbicide savings of up to 50% in the simulations for johnsongrass and up to 28% savings in the simulations for common cocklebur. Nonetheless, since DT determines the accuracy of patch spraying, the consequences of using higher DTs could be leaving areas unsprayed, which could adversely affect crop yields and future weed infestations, including herbicide-resistant weeds. Considering that the relationship between DT and accuracy of herbicide application depends on weed spatial pattern, this work has demonstrated the possibility of using higher DT values in weeds with a clear patchy distribution compared with weeds distributed regularly.

Red beet growers have expressed interest in adopting the microrate herbicide approach originally implemented in sugarbeet to achieve season-long weed management. Several red beet herbicides were first labeled for use in sugarbeet and lack substantial residual weed control. In response, red beet herbicide programs were evaluated that included a PRE application followed by up to three POST applications of various herbicide combinations. This research, however, indicated that herbicide programs that included PRE herbicides followed by as few as one or two POST applications that involve multiple active ingredients can provide season-long weed control. This observation was consistent across a broad spectrum of weeds, between two study locations that varied in soil type, and during two growing seasons. Herbicide programs that included only a PRE and six-leaf red beet growth stage application were successful at two locations in maintaining weed control and crop yield relative to hand-weeded red beet. Furthermore, these herbicide programs reduced the number of applications by 50% compared with the full programs, reducing crop injury risk and grower cost.

Methyl bromide (MBr) was a widely used fumigant in plasticulture because of its effectiveness against soil-borne pests and weeds in high-value crops; however, it was found to be a class 1 ozone-depleting substance and is no longer available for use in most of the United States. A mixture of 1,3-dichloropropene and chloropicrin (Pic-Clor 60) is an alternative that has been used to control soil-borne pathogens and nematodes, and has marginal control of weeds. Virtually impermeable film (VIF) and totally impermeable film (TIF) retain fumigants in the soil longer than the low- and high-density polyethylene films typically used in plasticulture production systems. The increased retention by these films may result in greater control of recalcitrant weeds such as nutsedge. Four rates of Pic-Clor 60 (112, 168, 224, and 280 kg ai ha−1) used with TIF, 280 kg ai ha−1 of Pic-Clor 60 used with VIF, and one nontreated control with VIF were evaluated for 2 yr to assess control of yellow and purple nutsedge and to determine an efficacious Pic-Clor 60 rate used with TIF. TIF with a lower rate of 224 kg ai ha−1 of Pic-Clor 60 significantly controlled nutsedge populations compared to a high rate of 280 kg ai ha−1 with VIF.

The herbicide pinoxaden is a phenylpyrazoline inhibitor of acetyl coenzyme A carboxylase. Research was conducted to determine the effects of pinoxaden (90 g ai ha−1) alone and in combination with herbicide safeners on creeping bentgrass injury as well as perennial ryegrass and roughstalk bluegrass control. Greenhouse experiments determined that herbicide safeners cloquintocet-mexyl, fenchlorazole-ethyl, and mefenpyr-diethyl were more effective in reducing creeping bentgrass injury from pinoxaden than benoxacor, isoxadifen-ethyl, and naphthalic-anhydride. Other experiments determined that creeping bentgrass injury from pinoxaden decreased as rates (0, 23, 45, 68, 90, 225, or 450 g ha−1) of cloquintocet-mexyl, fenchlorazole-ethyl, and mefenpyr-diethyl increased. On the basis of creeping bentgrass responses to various safener rates, safeners were applied at 68 and 450 g ha−1 in additional experiments to evaluate their effects on pinoxaden (90 g ha−1) injury to creeping bentgrass and efficacy against perennial ryegrass and roughstalk bluegrass. In these experiments, safeners mefenpyr-diethyl and cloquintocet-mexyl reduced pinoxaden-induced creeping bentgrass injury (from 25 to ≤ 5%) more than fenchlorazole-ethyl at 2 wk after treatment. Safeners reduced pinoxaden efficacy against roughstalk bluegrass. Perennial ryegrass was controlled > 80% by pinoxaden and herbicide safeners did not reduce control. Field experiments should evaluate pinoxaden in combination with cloquintocet-mexyl and mefenpyr-diethyl to optimize safener : herbicide ratios and rates for creeping bentgrass safety as well as perennial ryegrass and roughtstalk bluegrass control in different climates and seasons.

A major hindrance to establishment of successful complementary forage systems that include warm-season perennial grasses and clovers is tolerance of the latter to herbicides available for weed control. Field experiments were conducted in 2013 at two locations in northeast Louisiana to evaluate simulated residual rate effects of fluroxypyr plus triclopyr and 2,4-D plus picloram applied at 0, 0.25, 0.38, and 0.5× use rates immediately after fall planting of ball, white, crimson, and red clover. For all clovers, when averaged across herbicide rates, plant population 161/171 d after planting (DAP), ground cover, and height 184/196 DAP were equivalent for fluroxypyr plus triclopyr and the nontreated control and greater than 2,4-D plus picloram. Averaged across clovers, plant height after all rates of fluroxypyr plus triclopyr was equivalent to the nontreated control (14.2 to 14.3 vs. 15.3 cm) and greater than 2,4-D plus picloram. Compared with the nontreated control, 2,4-D plus picloram at 25, 38, and 50% of the normal use rates reduced height 58, 76, and 85%, respectively. When averaged across clover species, yield for fluroxypyr plus triclopyr at all rates was equivalent to the nontreated control (2,624 to 2,840 vs. 2,812 kg ha−1). Compared with the nontreated control, 2,4-D plus picloram at the 0.25, 0.38, and 0.50× use rates reduced yield 65, 89, and 99%, respectively.

Volunteer corn can affect dry bean by reducing yields; expanding the life cycle of insects, mites, and pathogens; interfering with harvest; and contaminating bean seed. Field studies were conducted at Lingle, WY, and Scottsbluff, NE, to determine the relationship between volunteer corn density and dry bean yield, establish the proper time of volunteer corn removal, and determine whether dry bean yield was affected by the method used to remove volunteer corn. Volunteer corn reduced dry bean yields, as recorded in other crops. Growing conditions for each location were different, as indicated by the accumulated growing degree days (GDD): Lingle 2008 (990), Lingle 2009 (780), and Scottsbluff 2009 (957). No difference in dry bean yields was observed between hand removal of volunteer corn and herbicide application. Dry bean yield loss increased with longer periods of volunteer corn competition and ranged from 1.2 to 1.8% yield loss for every 100 GDD that control was delayed. Control measures should be implemented 15 to 20 d after planting when volunteer corn densities are close to 1 plant m−2. Dry bean yield losses also increased as volunteer corn densities increased, with losses from 6.5 to 19.3% for 1 volunteer corn plant m−2. Based on 2015 prices, the cost of controlling volunteer corn would be the equivalent of 102 kg ha−1 of dry bean, and potential losses above 4% would justify control and should not be delayed beyond 15 to 20 d after planting.

Two varieties of bahiagrass were evaluated under Florida conditions for forage tolerance to the new herbicide, aminocyclopyrachlor (ACP), which is essential for product development decisions. Herbicide treatments included ACP alone at 70 and 140 g ai ha−1, ACP + chlorsulfuron at 69 + 27 and 138 + 54 g ai ha−1, ACP + 2,4-D amine at 70 + 532 g ai ha−1 and 140 + 1,064 g ai ha−1, ACP + triclopyr-amine at 70 + 140 g ai ha−1 and 140 + 280 g ai ha−1, and ACP + metsulfuron at 46 + 7, 78 + 12, and 168 + 26 g ai ha−1, and also included a nontreated check. ‘Argentine’ bahiagrass was the most tolerant forage species, and ‘Pensacola’ bahiagrass was sensitive to ACP + metsulfuron and initially to ACP + chlorsulfuron. Herbicide applications using ACP, when labeled, will likely provide good to excellent control of several weed species, with little long-term impact on bahiagrass forage production when the cultivar is known.

In plasticulture vegetable production systems, broadleaf weeds and grasses emerge in the crop planting holes and between the raised beds. Weeds emerging on the bare ground between the raised beds can be the most difficult to control. Experiments were conducted in the spring and fall of 2014 at the Gulf Coast Research and Education Center in Balm, FL, to evaluate a range of herbicides for use in row middles in vegetable crops. Weed counts and control ratings did not differ over time and as a result are averaged across 2, 4, and 8 wk after treatment. In the absence of PRE herbicides, carfentrazone did not differ from the nontreated control, whereas paraquat reduced total weed density by 67 and 61% in the spring and fall, respectively. In the fall, carfentrazone tank-mixed with flumioxazin, S-metolachlor + flumioxazin, or paraquat tank-mixed with flumioxazin, S-metolachlor, metribuzin, or S-metolachlor + flumioxazin had 81 to 90% fewer broadleaf weeds than the nontreated control. Total weed density was 70 to 90% lower than the nontreated control when carfentrazone was tank-mixed with flumioxazin or S-metolachlor + flumioxazin or paraquat was tank-mixed with flumioxazin, S-metolachlor, oxyfluorfen, metribuzin or S-metolachlor + flumioxazin. Similar trends occurred in the spring. A tank mix of a soil residual herbicide and POST burn-down herbicides tended to have a greater reduction in weed numbers compared to the POST treatment alone. The most consistent row middle tank-mixes were paraquat tank-mixed with flumioxazin, S-metolachlor + flumioxazin, or S-metolachlor + oxyfluorfen.

Two experiments were conducted in 2015 at multiple locations in Florida to evaluate the effects of planting depth and application timing on S-metolachlor injury in sesame. In both studies, sesame responded negatively to increases in S-metolachlor rate. Altering sesame planting depth did not provide increased safety to PRE S-metolachlor applications. Sesame establishment declined with increased planting depth, likely because of the physical inability of the small seed to emerge from the 3.8-cm depth. Delaying applications of S-metolachlor by 3 or 6 d after planting (DAP) consistently improved sesame establishment. Applications 3 and 6 DAP resulted in 89 to 92% seedling emergence at 2 wk after planting (WAP), relative to 55 to 63% emergence when S-metolachlor was applied the day of planting (0 DAP) or 3 days before (−3 DAP), respectively. Applications 3 DAP resulted in 21 and 2% plant stunting when evaluated 3 and 6 WAP, respectively, whereas all other timings caused 25 to 51% stunting. Yield was reduced 22 and 33% by the −3 DAP and 0 DAP application timings, respectively, whereas no reduction in yield was observed by the delayed application timings. Therefore, delaying applications of S-metolachlor by 3 to 6 days will likely result in improved sesame seedling establishment and total seed yield.

Methiozolin is a new isoxazoline herbicide that has scarcely been tested in Kentucky bluegrass turf. A field trial was conducted in Blacksburg, VA, to determine response of 110 Kentucky bluegrass varieties and winter annual weeds to sequential fall applications of methiozolin. At 1.5 and 6 mo after initial treatment (MAIT), Kentucky bluegrass injury I30 values (predicted methiozolin rate that causes 30% Kentucky bluegrass injury) ranged between 3.4 to more than 10 times the recommended methiozolin rate for annual bluegrass control. Methiozolin at all rates reduced cover of annual bluegrass, common chickweed, corn speedwell, hairy bittercress, mouseear chickweed, and Persian speedwell but increased cover of parsley-piert. For all varieties, methiozolin at 2 kg ai ha−1 increased Kentucky bluegrass cover, turf quality, and turf normalized difference vegetation index (NDVI) relative to the nontreated check at 6 MAIT. Kentucky bluegrass relative cover change (RCC) was attributed primarily to weed control but was inversely correlated with methiozolin rates because of increased weed control and reduced Kentucky bluegrass growth. Despite the decline in RCC with increasing methiozolin rates, most Kentucky bluegrass varieties treated with the highest methiozolin rate (6 kg ha−1) still had greater Kentucky bluegrass cover than the nontreated check at 6 MAIT. Results from this study indicate that two fall applications of methiozolin at rates beyond that previously reported for annual bluegrass control can safely be applied to a broad range of Kentucky bluegrass varieties spanning most of the known genetic classifications.

Crop losses from weed interference have a significant effect on net returns for producers. Herein, potential corn yield loss because of weed interference across the primary corn-producing regions of the United States and Canada are documented. Yield-loss estimates were determined from comparative, quantitative observations of corn yields between nontreated and treatments providing greater than 95% weed control in studies conducted from 2007 to 2013. Researchers from each state and province provided data from replicated, small-plot studies from at least 3 and up to 10 individual comparisons per year, which were then averaged within a year, and then averaged over the seven years. The resulting percent yield-loss values were used to determine potential total corn yield loss in t ha−1 and bu acre−1 based on average corn yield for each state or province, as well as corn commodity price for each year as summarized by USDA-NASS (2014) and Statistics Canada (2015). Averaged across the seven years, weed interference in corn in the United States and Canada caused an average of 50% yield loss, which equates to a loss of 148 million tonnes of corn valued at over U.S.$26.7 billion annually.

The tripling of glyphosate use in the Canadian prairies during the past decade has raised concerns over the possible accumulation of glyphosate and its main metabolite AMPA in soil over time and whether there could be any detrimental effects on crop production. A controlled environment study was conducted at two locations in Alberta, Canada, to determine glyphosate and AMPA soil concentrations that would injure wheat, field pea, and canola. Treatments included glyphosate acid or AMPA applied at 0, 10, 25, 100, 250, and 500 mg kg−1 soil. Shoot and root biomass determinations 8 wk after emergence (WAE) indicated that shoot and root biomass of all crops progressively declined with increasing soil concentrations of glyphosate at both locations. In contrast, AMPA reduced crop shoot and root biomass at only one of two sites. Estimated soil concentrations of glyphosate causing 20% reductions in shoot and root biomass ranged from 80 to 190, 90 to 350, and 120 to 320 mg kg−1 for field pea, canola, and wheat, respectively. Soil concentrations of AMPA causing 20% crop biomass reductions ranged from 40 to 70, 20 to 30, and 80 to 120 mg kg−1 for field pea, canola, and wheat, respectively. Although substantial crop injury occurred in this study, it must be noted that these rates are very high in terms of field application rates that would be required to achieve these soil concentrations. Doses causing crop injury would convert to estimated glyphosate field rates ranging from 17.6 to 77 kg ha−1. Overall results indicate that even with frequent high-dose glyphosate applications over several years, the likelihood of wheat, field pea, and canola injury from soil residues is low. Nevertheless, there may be merit in greater monitoring of glyphosate and AMPA soil residues in cropping systems with high glyphosate utilization over a long time period.