Seed retention, and ultimately seed shatter, are extremely important for the efficacy of harvest weed seed control (HWSC) and are likely influenced by various agroecological and environmental factors. Field studies investigated seed-shattering phenology of 22 weed species across three soybean [Glycine max (L.) Merr.]-producing regions in the United States. We further evaluated the potential drivers of seed shatter in terms of weather conditions, growing degree days, and plant biomass. Based on the results, weather conditions had no consistent impact on weed seed shatter. However, there was a positive correlation between individual weed plant biomass and delayed weed seed–shattering rates during harvest. This work demonstrates that HWSC can potentially reduce weed seedbank inputs of plants that have escaped early-season management practices and retained seed through harvest. However, smaller individuals of plants within the same population that shatter seed before harvest pose a risk of escaping early-season management and HWSC.

Potential effectiveness of harvest weed seed control (HWSC) systems depends upon seed shatter of the target weed species at crop maturity, enabling its collection and processing at crop harvest. However, seed retention likely is influenced by agroecological and environmental factors. In 2016 and 2017, we assessed seed-shatter phenology in 13 economically important broadleaf weed species in soybean [Glycine max (L.) Merr.] from crop physiological maturity to 4 wk after physiological maturity at multiple sites spread across 14 states in the southern, northern, and mid-Atlantic United States. Greater proportions of seeds were retained by weeds in southern latitudes and shatter rate increased at northern latitudes. Amaranthus spp. seed shatter was low (0% to 2%), whereas shatter varied widely in common ragweed (Ambrosia artemisiifolia L.) (2% to 90%) over the weeks following soybean physiological maturity. Overall, the broadleaf species studied shattered less than 10% of their seeds by soybean harvest. Our results suggest that some of the broadleaf species with greater seed retention rates in the weeks following soybean physiological maturity may be good candidates for HWSC.

Seed shatter is an important weediness trait on which the efficacy of harvest weed seed control (HWSC) depends. The level of seed shatter in a species is likely influenced by agroecological and environmental factors. In 2016 and 2017, we assessed seed shatter of eight economically important grass weed species in soybean [Glycine max (L.) Merr.] from crop physiological maturity to 4 wk after maturity at multiple sites spread across 11 states in the southern, northern, and mid-Atlantic United States. From soybean maturity to 4 wk after maturity, cumulative percent seed shatter was lowest in the southern U.S. regions and increased moving north through the states. At soybean maturity, the percent of seed shatter ranged from 1% to 70%. That range had shifted to 5% to 100% (mean: 42%) by 25 d after soybean maturity. There were considerable differences in seed-shatter onset and rate of progression between sites and years in some species that could impact their susceptibility to HWSC. Our results suggest that many summer annual grass species are likely not ideal candidates for HWSC, although HWSC could substantially reduce their seed output during certain years.

Residual herbicides applied to summer cash crops have the potential to injure subsequent winter annual cover crops, yet little information is available to guide growers’ choices. Field studies were conducted in 2016 and 2017 in Blacksburg and Suffolk, Virginia, to determine carryover of 30 herbicides commonly used in corn, soybean, or cotton on wheat, barley, cereal rye, oats, annual ryegrass, forage radish, Austrian winter pea, crimson clover, hairy vetch, and rapeseed cover crops. Herbicides were applied to bare ground either 14 wk before cover crop planting for a PRE timing or 10 wk for a POST timing. Visible injury was recorded 3 and 6 wk after planting (WAP), and cover crop biomass was collected 6 WAP. There were no differences observed in cover crop biomass among herbicide treatments, despite visible injury that suggested some residual herbicides have the potential to effect cover crop establishment. Visible injury on grass cover crop species did not exceed 20% from any herbicide. Fomesafen resulted in the greatest injury recorded on forage radish, with greater than 50% injury in 1 site-year. Trifloxysulfuron and atrazine resulted in greater than 20% visible injury on forage radish. Trifloxysulfuron resulted in the greatest injury (30%) observed on crimson clover in 1 site-year. Prosulfuron and isoxaflutole significantly injured rapeseed (17% to 21%). Results indicate that commonly used residual herbicides applied in the previous cash crop growing season result in little injury on grass cover crop species, and only a few residual herbicides could potentially affect the establishment of a forage radish, crimson clover, or rapeseed cover crop.

Herbicide resistance is a major problem in United States and global agriculture, driving farmers to consider other methods of weed control. One of these methods is harvest weed seed control (HWSC), which has been demonstrated to be effective in Australia. HWSC studies were conducted across Virginia in 2017 and 2018, targeting Italian ryegrass in continuous winter wheat as well as common ragweed and Palmer amaranth in continuous soybean. These studies assessed the impact of HWSC (via weed seed removal) on weed populations in the next year’s crop compared with conventional harvest (weed seeds returned). HWSC reduced Italian ryegrass tillers compared with the conventional harvest at two locations in April (29% and 69%), but no difference was observed at a third location. At wheat harvest, HWSC at one location reduced Italian ryegrass seed heads (41 seed heads m−2) compared with conventional harvest (125 seed heads m−2). In soybean, before preplant herbicide applications and POST herbicide applications, HWSC reduced common ragweed densities by 22% and 26%, respectively, compared with the conventional harvest plots. By soybean harvest, no differences in common ragweed density, seed retention, or crop yield were observed, because of effectiveness of POST herbicides. No treatment differences were observed at any evaluation timing for Palmer amaranth, which is attributed to farmer weed management (i.e., effective herbicides) and low weed densities making any potential treatment differences difficult to detect. Across wheat and soybean, there were no differences observed in crop yield between treatments. Overall, HWSC was demonstrated to be a viable method to reduce Italian ryegrass and common ragweed populations.

Grape hyacinth is a perennial bulbous species in the Liliaceae. It is commonly grown as an ornamental plant, but it can spread into agricultural fields and become weedy, potentially interfering with harvest and fall-planted crops. There has been limited research on controlling grape hyacinth in cropping systems. Fall and spring applied field-research studies were conducted to determine grape hyacinth control with herbicides labeled for use in wheat or winter fallow before planting soybean. Among fall-applied herbicides, paraquat resulted in the greatest initial grape hyacinth control (90% to 100%). Grape hyacinth control, 16 months after application (MAA), was variable, but the top-performing treatments were glyphosate and metsulfuron plus paraquat, resulting in 65% and 50% control, respectively. After spring applications, grape hyacinth control in November (7 MAA) was variable, but top-performing treatments were glyphosate and metsulfuron, which resulted in at least 26% control. Spring-applied paraquat, carfentrazone, metsulfuron, and sulfosulfuron resulted in 73%, 68%, 69%, and 60% reductions in grape hyacinth bulb counts, compared with the nontreated control 7 MAA, and were the top-performing treatments. Despite product-label prohibitions on rotation to soybeans, no soybean yield reductions were observed from any treatment in either study. Single applications of certain herbicides in the fall or spring can result in good control (>80%) of grape hyacinth initially, but long-term control is poor, and additional research is required.

Studies were conducted to determine the tolerance of sweetpotato and Palmer amaranth control to a premix of flumioxazin and pyroxasulfone pretransplant (PREtr) followed by (fb) irrigation. Greenhouse studies were conducted in a factorial arrangement of four herbicide rates (flumioxazin/pyroxasulfone PREtr at 105/133 and 57/72 g ai ha–1, S-metolachlor PREtr 803 g ai ha–1, nontreated) by three irrigation timings [2, 5, and 14 d after transplanting (DAP)]. Field studies were conducted in a factorial arrangement of seven herbicide treatments (flumioxazin/pyroxasulfone PREtr at 40/51, 57/72, 63/80, and 105/133 g ha–1, 107 g ha–1 flumioxazin PREtr fb 803 g ha–1S-metolachlor 7 to 10 DAP, and season-long weedy and weed-free checks) by three 1.9-cm irrigation timings (0 to 2, 3 to 5, or 14 DAP). In greenhouse studies, flumioxazin/pyroxasulfone reduced sweetpotato vine length and shoot and storage root fresh biomass compared to the nontreated check and S-metolachlor. Irrigation timing had no influence on vine length and root fresh biomass. In field studies, Palmer amaranth control was≥91% season-long regardless of flumioxazin/pyroxasulfone rate or irrigation timing. At 38 DAP, sweetpotato injury was≤37 and≤9% at locations 1 and 2, respectively. Visual estimates of sweetpotato injury from flumioxazin/pyroxasulfone were greater when irrigation timing was delayed 3 to 5 or 14 DAP (22 and 20%, respectively) compared to 0 to 2 DAP (7%) at location 1 but similar at location 2. Irrigation timing did not influence no.1, jumbo, or marketable yields or root length-to-width ratio. With the exception of 105/133 g ha–1, all rates of flumioxazin/pyroxasulfone resulted in marketable sweetpotato yield and root length-to-width ratio similar to flumioxazin fb S-metolachlor or the weed-free checks. In conclusion, flumioxazin/pyroxasulfone PREtr at 40/51, 57/72, and 63/80 g ha–1 has potential for use in sweetpotato for Palmer amaranth control without causing significant crop injury and yield reduction.

Field studies were conducted in 2015 and 2016 in North Carolina to determine the response of ‘Covington’ and ‘Murasaki-29’ sweetpotato cultivars to four rates of linuron (420, 560, 840, and 1,120 g ai ha–1) alone or with S-metolachlor (803 g ai ha–1) applied 7 or 14 d after transplanting (DAP). Injury (chlorosis/necrosis and stunting) to both cultivars was greater when linuron was applied with S-metolachlor as compared to linuron applied alone. Herbicide application at 14 DAP caused greater injury (chlorosis/necrosis and stunting) to both cultivars than when applied at 7 DAP. At 4 wk after treatment (WAT), stunting of Covington and Murasaki-29 (hereafter Murasaki) from linuron at 420 to 1,120 g ha–1 increased from 27% to 50% and 25% to 53%, respectively. At 7 or 8 WAT, crop stunting of 8% or less and 0% was observed in Covington and Murasaki, respectively, regardless of application rate and timing. Murasaki root yields were similar in the linuron alone or with S-metolachlor treatments, and were lower than the nontreated check. In 2016, no. 1 and marketable sweetpotato yields of Covington were similar for the nontreated check, linuron alone, or linuron plus S-metolachlor treatments, but not in 2015. Decreases in no. 1 and marketable root yields were observed when herbicides were applied 14 DAP compared to 7 DAP for Covington in 2015 and for Murasaki in both years. No. 1 and marketable yields of Covington were similar for 420 to 1,120 g ha–1 linuron and nontreated check except marketable root yields in 2015. No. 1 and marketable sweetpotato yields of Murasaki decreased as application rates increased.

Diphenyl ether herbicides are commonly applied POST in soybean to control weeds late in the growing season that have not been controlled by other previous weed management tactics. These “rescue” applications can occur during reproductive soybean growth. The effect of these herbicides on the developing flowers and pods is not known. Field research studies were conducted over 3 yr to determine how soybean flowers and developing pods respond to fomesafen, acifluorfen, and lactofen when applied at R1, R3, and R5 growth stages. Flower and pod counts in the nontreated check showed an increase (17.1, 5.8, and 2.21 at R1, R3, and R5 stage, respectively) and were statistically the same as the herbicide treatments 1 wk after treatment. Fomesafen, acifluorfen, and lactofen applied at 395, 420, and 219 g ai ha–1 at R1, R3, and R5 stage had no negative impact on soybean flowers and developing pods when compared to the nontreated check. No significant differences were observed in soybean yield between any treatments in all site-years of the study.

Field studies were conducted to determine the influence of herbicides on the development of internal necrosis (IN) in sweetpotato storage roots. In a slip propagation study, herbicide treatments included PRE application (immediately after covering seed roots with soil) of clomazone (0.42, 0.84 kg ai ha-1), flumioxazin (0.11, 0.21 kg ai ha-1), fomesafen (0.28, 0.56 kg ai ha-1), linuron (0.56, 1.12 kg ai ha-1), S-metolachlor (0.8, 1.6 kg ai ha-1), flumioxazin plus S-metolachlor (0.11 + 0.8 or 1.6 kg ha-1), and napropamide (1.12, 2.24 kg ai ha-1), and POST application (2 to 4 wk prior to cutting slips) of ethephon (0.84, 1.26 kg ai ha-1) and paraquat (0.14, 0.28 kg ai ha-1). In a field production study, flumioxazin, fomesafen, linuron, and paraquat were applied PREPLANT (one d prior to sweetpotato transplanting), clomazone, S-metolachlor, and napropamide were applied PRE [4 d after transplanting (DAP)], flumioxazin PREPLANT followed by (fb) S-metolachlor PRE, and ethephon applied POST (2 wk prior to harvest). Herbicide rates were similar to those used in the slip propagation study. Yield of sweetpotato in both studies was not affected by herbicide treatment. In both studies, IN incidence and severity increased with time and was greatest at 60 d after curing. No difference was observed between herbicide treatments for IN incidence and severity in the slip production study which indicates herbicide application at time of slip propagation does not impact the development of IN. In the field production study, the only treatment that increased IN incidence compared to the nontreated was ethephon with 53% and 2.3 incidence and severity, respectively. The presence of IN affected roots in nontreated plots indicates that some other pre- or post-curing factors other than herbicides are responsible for the development of IN. However, the ethephon application prior to sweetpotato root harvest escalates the development of IN.