Residual herbicides are primarily degraded in the soil through microbial breakdown. Any practices that result in increased soil biological activity, such as cover cropping (between cash crop seasons), could lead to a reduced persistence of herbicides in the soil. Furthermore, cover crops can also interfere with herbicide fate by interception. Field trials were conducted between 2020 and 2023 in a corn (Zea mays L.)–soybean [Glycine max (L.) Merr.] rotation to investigate the influence of cover crop (cereal rye [Secale cereale L.] and crimson clover [Trifolium incarnatum L.]) use on soil enzyme activities (β-glucosidase [BG] and dehydrogenase [DHA]), its effect on the concentration of residual herbicides (sulfentrazone, S-metolachlor, cloransulam-methyl, atrazine, and mesotrione) in the soil, and the interception of herbicides by cover crop residue. The use of cover crops occasionally resulted in increased BG and DHA activities relative to the fallow treatment. However, even when there was an increase in the activity of these two enzymes, increased degradation of the residual herbicides was not observed. The initial concentrations of all residual herbicides in the soil were significantly reduced due to interception by cereal rye biomass. Nevertheless, significant reductions in early-season weed biomass were observed when residual herbicides were included in the tank mixture applied at cover crop termination relative to the application of glyphosate plus glufosinate. Results from this research suggest that the use of cereal rye or crimson clover as cover crops (between cash crop seasons) do not impact the persistence of residual herbicides in the soil or reduce their efficacy in controlling weeds early in the growing season.

Cereal rye cover crop (cereal rye) and preemergence (PRE) herbicides are becoming common practices for managing herbicide-resistant weeds in soybean production. Adopting these two practices in combination raises concerns regarding herbicide fate in soil, given that the cereal rye biomass can intercept the herbicide spray solution, preventing it from reaching the soil. Delaying cereal rye termination until soybean planting (planting green) optimizes biomass accumulation but might also increase PRE interception. To better understand the dynamics between cereal rye and PRE herbicides, a field experiment was conducted to evaluate two soil management practices (tillage and no-till) and two cereal rye termination practices in the planting-green system (glyphosate [1,260 g ae ha−1] and roller-crimper) on the spray deposition and fate of PRE herbicides and soybean yield. The spray deposition was assessed by placing water-sensitive paper cards on the soil surface before spraying the PRE herbicides (sulfentrazone [153 g ai ha−1] + S-metolachlor [1,379 g ai ha−1]). Herbicide concentration in soil (0 to 7.6 cm) was quantified 25 d after treatment (DAT). The presence of no-till stubble and cereal rye biomass reduced the spray coverage compared to tillage at PRE application, which reflected in a reduction in the concentration of both herbicides in soil 25 DAT. Soybean yield was reduced in all three years when the cereal rye was terminated with a roller-crimper but only reduced in one year when terminated with glyphosate. Our findings indicate that mainly cereal rye biomass reduced the concentration of PRE herbicides in the soil due to the interception of the spray solution during application. Although higher cereal rye biomass accumulation can provide better weed suppression according to the literature, farmers should be aware that the biomass can lower the concentration of PRE herbicides reaching the soil, thus intensifying field scouting to ensure that weed control is not being negatively affected.

Aminocyclopyrachlor (AMCP) is a synthetic auxin herbicide used for broadleafweed control in pasture and rangeland. The tolerance and fate of AMCP withinpertinent grass species is not well understood. Research was conducted toestablish the tolerance of four grass species to AMCP application andobserve their absorption, translocation, and metabolism. Results indicatethat tall fescue is the most tolerant of AMCP at rates required for weedcontrol. Bahiagrass and bermudagrass are marginally tolerant, and cogongrassis the most sensitive. Tall fescue and bahiagrass absorbed more AMCP thanbermudagrass and cogongrass, but cogongrass absorption is the most rapid andcomplete within 2 days after treatment (DAT). Cogongrass and bermudagrasstranslocated the least amount out of the target area, whereas bahiagrass andtall fescue translocated the most. Radioisotope imaging revealed that tallfescue may sequester absorbed AMCP in leaf tips. This sequestering may bethe basis of the greater tolerance to AMCP by tall fescue relative to theother species evaluated. No metabolism of AMCP was detected in any grassspecies out to 42 DAT.

Perennial ryegrass is overseeded in bermudagrass and Kentucky bluegrass toimprove turf quality, but selective control may be warranted for transitionback to monostand turfgrass. Flucarbazone–sodium controls perennial ryegrassin bermudagrass and Kentucky bluegrass, but the physiological basis ofselectivity has received limited investigation. Greenhouse and laboratoryexperiments were conducted to evaluate efficacy, absorption, translocation,and metabolism of flucarbazone–sodium in these grasses. Flucarbazone–sodiumreduced perennial ryegrass shoot mass from the nontreated an average ≈ 22times and 3 times more than bermudagrass and Kentucky bluegrass at 4 wkafter treatment, respectively. In laboratory experiments, foliar and rootabsorption of 14C–flucarbazone–sodium were similar among species.Bermudagrass distributed ≈ 25% more foliar-absorbed 14C tonontreated shoots than Kentucky bluegrass and perennial ryegrass. From rootapplications, all grasses averaged 84% distribution of 14C toshoots. Bermudagrass and Kentucky bluegrass metabolized 100% and 74% of 14C–flucarbazone–sodium at 1 d after treatment (DAT), whereasperennial ryegrass metabolism measured 44, 58, and 65% at 1, 3, and 7 DAT,respectively. Bermudagrass, Kentucky bluegrass, and perennial ryegrass had4, 4, and 2 metabolites after 7 d, respectively. Results suggestdifferential metabolism of flucarbazone–sodium is attributed to selectivityfor controlling perennial ryegrass in bermudagrass and Kentuckybluegrass.