Field experiments were conducted in 2013, 2014, and 2015 in Columbia and Moberly, Missouri to determine the effects of cereal rye, Italian ryegrass, winter wheat, winter oat, crimson clover, Austrian winterpea, hairy vetch, oilseed radish, and cereal rye plus hairy vetch on winter and summer annual weed emergence in soybean. For comparison purposes, each experiment in each year included a Fall PRE, Spring PRE without residual, and Spring PRE residual herbicide programs. Cereal rye and cereal rye plus hairy vetch reduced winter annual weed emergence by 72 and 68%, respectively, but were not comparable to the Fall PRE which reduced winter annual weed emergence by 99%. The following spring, early-season waterhemp emergence was similar among treatments of cereal rye, cereal rye plus hairy vetch, and the Spring PRE residual herbicide program. In contrast, all cover crop species other than Italian ryegrass reduced late season waterhemp emergence between 21 and 40%, but were not comparable to the Spring PRE residual herbicide program, which reduced late season waterhemp emergence by 97%. All other summer annual weeds excluding waterhemp showed a similar response among cover crop and herbicide treatments. Overall, results from this experiment indicate that certain cover crops are able to suppress winter and summer annual weed emergence, but not to the extent of soil-applied residual herbicides.

Protoporphyrinogen IX oxidase (PPO)–inhibiting herbicides (WSSA Group 14) have been used in agronomic row crops for over 50 yr. Broadleaf weeds, including glyphosate-resistant Palmer amaranth, have been controlled by this herbicide site of action PRE and POST. Recently, Palmer amaranth populations were reported resistant to PPO inhibitors in 2011 in Arkansas, in 2015 in Tennessee, and in 2016 in Illinois. Historically, the mechanism for this resistance involves the deletion of a glycine at position 210 (ΔG210) in a PPO enzyme encoded by the PPX2 gene; however, the ΔG210 deletion did not explain all PPO inhibitor–resistant Palmer amaranth in Tennessee populations. Recently, two new mutations within PPX2 (R128G, R128M) that confer resistance to PPO inhibitors were identified in Palmer amaranth. Therefore, research is needed to document the presence and distribution of the three known mutations that confer PPO inhibitor resistance in Tennessee. In 2017, a survey was conducted in 18 fields with Palmer amaranth to determine whether resistance existed and the prevalence of each known mutation in each field. Fomesafen was applied at 265 g ai ha–1 to Palmer amaranth infestations within each field to select for resistant weeds for later analysis. Where resistance was described (70% of surviving plants), the ΔG210 mutation was detected in 47% of resistant plants. The R128G mutation accounted for 42% of resistance, similar to the frequency of the ΔG210 mutation. The R128M mutation was less frequent than the other two mutations, accounting for only 10% of the resistance. All mutations detected in this study were heterozygous. Additionally, no more than one of the three PPX2 mutations were detected in an individual surviving plant. Similar to previous research, about 70% of PPO resistance was accounted for by these three known mutations, leaving about 30% of resistance not characterized in Tennessee populations. Survivors not showing the three known PPO mutations suggest that other resistance mechanisms are present.

Stakeholders were surveyed across Nebraska to identify the problem weeds and assess common weed management practices. A total of 425 responses were returned across four Nebraska extension districts (Northeast, Panhandle, Southeast, and West Central). Collectively, 61.2% of total farmed or scouted areas in Nebraska were under no-till production, and corn and soybean were the major crops (82.3% of total farmed or scouted area). Common waterhemp, horseweed, and kochia were the most problematic weeds statewide. Widespread occurrence of glyphosate-resistant (GR) weeds such as common waterhemp, horseweed, kochia, and Palmer amaranth were a serious problem in GR crop production. Additionally, 60% of growers in Nebraska reported the presence of at least one GR weed species on their farms. The most commonly used preplant burndown herbicides were 2,4-D and glyphosate, followed by saflufenacil and dicamba. In Nebraska, 74% and 59% of corn and soybean growers, respectively, were using PRE herbicides; however, more than 80% of growers were using POST herbicides for in-crop weed management. Atrazine alone or in premix or tank mix with mesotrione, S-metolachlor, or acetochlor were the most widely applied PRE herbicides in corn and grain sorghum, whereas the most commonly used PRE herbicides in soybean were the inhibitors of acetolactate synthase (ALS) and protoporphyrinogen oxidase (PPO). Glyphosate was the most frequent choice of the survey respondents as a POST herbicide in GR corn and soybean; 2,4-D was the most commonly used POST herbicide in grain sorghum and wheat. In Nebraska, only 5.2% of total crop area was planted with glufosinate-resistant crops. Most of the respondents (89%) were aware of the new multiple herbicide–resistant crops, and 80% of them listed physical drift and volatility of the auxinic herbicides as their primary concern. Forty-eight percent of survey respondents identified herbicide-resistant weed management as their primary research and extension priority.

Bell pepper producers are faced with the challenge of controlling weeds following the phase-out of methyl bromide (MBr). Numerous attempts have been made to find a single fumigant or herbicide to control a broad spectrum of weeds. Adequate weed control in bell pepper will likely require weed management systems utilizing both fumigant and herbicide options. A weed management system including the fumigant dimethyl disulfide (DMDS) plus chloropicrin (Pic) plus the herbicide napropamide prior to transplant followed by S-metolachlor POST may be necessary to replace MBr. Field experiments were conducted during 2010 and 2011 near Ty Ty, Georgia to determine bell pepper and weed response to DMDS plus Pic or in systems with napropamide and/or S-metolachlor. Bell pepper were not significantly injured by DMDS plus Pic or napropamide. Injury caused by S-metolachlor was transient and plants fully recovered by 4 weeks after treatment (WAT). Yellow nutsedge control 6 WAT using DMDS plus Pic applied at 468 or 560 L ha−1 controlled yellow nutsedge 91 to 95%. Large crabgrass control 6 WAT was 92 to 100% when DMDS plus Pic was applied at 468 or 560 L ha−1 with or without a(n) herbicide (S-metolachlor or napropamide). Palmer Amaranth control prior to harvest was 21, 64, and 85% using DMDS plus Pic at 374, 468, or 560 L ha−1, respectively. DMDS plus Pic applied at 468 or 560 L ha-1 with napropamide followed by S-metolachlor POST gave 95 to 99% control of Palmer amaranth 6 WAT. Consistent weed control and optimum yields were obtained when DMDS plus Pic was used at 468 L ha−1 plus napropamide beneath plastic mulch followed by S-metolachlor POST.

With the release of dicamba-resistant crops, it is necessary to understand how technical and environmental conditions affect the application of dicamba. This study sought to evaluate drift from dicamba applications through flat-fan nozzles, under several wind speeds in a wind tunnel. Dicamba applications were performed through two standard (XR and TT) and two air induction (AIXR and TTI) 110015 nozzles at 0.9, 2.2, 3.6 and 4.9 ms−1 wind speeds. The applications were made at 276 kPa pressure and the dicamba rate was 561 g ae ha-1. The droplet spectrum was measured using a laser diffraction system. Artificial targets were used as drift collectors, positioned in a wind tunnel from 2 to 12 m downwind from the nozzles. Drift potential was determined using a fluorescent tracer added to solutions, quantified by fluorimetry. The air induction TTI nozzle produced the lowest percentage of dicamba drift at 2.2, 3.6 and 4.9 ms−1 wind speeds at all distances. Dicamba spray drift from XR, TT and AIXR nozzles increased exponentially as wind speed increased, whereas from TTI nozzle drift increased linearly as wind speed increased. Drift did not increase linearly as the volume percentage of droplets smaller than 100 µm and wind speed increased.

Broadleaves, grasses, and nutsedge species are persistent problems with limited management options for strawberry growers in Florida. Field experiments were conducted in 2015-2016 (year 1) and 2016-2017 (year 2) at the Gulf Coast Research and Education Center in Balm, FL, to evaluate weed control and strawberry tolerance to herbicides applied through the drip irrigation. 2940 g ai ha-1 EPTC, 105 g ai ha-1 flumioxazin, 570 g ai ha-1 fomesafen, 52 g ai ha-1 halosulfuron, 3585 g ai ha-1 napropamide, oxyfluorfen 560 g ai ha-1, and 1070 g ai ha-1S-metolachlor were applied through a single drip tape at 7 or 14 d prior to transplanting. Halosulfuron was the most injurious herbicide, causing 18 and 46% injury at 35 d after transplanting (DATP) in year 1 when the herbicide was applied 7 and 14 d prior to transplanting, respectively. However, strawberry plants recovered from the initial injury and there was no reduction in total berry yield. None of the other herbicides evaluated elicited significant crop injury nor reduced berry yield. Averaged over application timings, EPTC, fomesafen, and napropamide suppressed yellow nutsedge emergence to 49, 64, and 41% of the nontreated control, respectively. Flumioxazin, fomesafen, and halosulfuron suppressed black medic emergence to 55, 52, and 55% of the nontreated control, respectively. None of the herbicides evaluated adequately suppressed Carolina geranium. Overall, results suggest that the evaluated herbicides with the exception of halosulfuron are safe for use on strawberry and would give growers an alternative management option. Drip-applied herbicides permit application closer to the transplant date and would be helpful as part of a weed control program for weed suppression.