The ability of weed populations to evolve resistance to herbicides impact management strategies and profitability of crop production. The objective of this research was to screen Palmer amaranth accessions from Arkansas for glufosinate resistance. Additional efforts focused on the effectiveness of various herbicides, across multiple sites of action (SOA), on each putative-resistant accession. The three putative accessions were selected from 60 Palmer amaranth accessions collected in 2019 and 2020 and screened with to 0.5x and 1x rates of glufosinate. A dose-response experiment was conducted for glufosinate on accession A2019, A2020, and B2020. The effectiveness of various preemergence- and postemergence-applied herbicides were evaluated on each accession. Resistance ratios of A2019, A2020, and B2020 to glufosinate ranged from 5.1 to 27.4 when comparing LD50 values to two susceptible accessions, thus all three accessions were resistant to glufosinate. All three accessions (A2019, A2020, and B2020) were found to have a reduction equal to or greater than 20-percentage points in mortality to at least one herbicide from five different sites of action equal to or greater than 5 sites of action. Herbicides from nine different sites of action controlled A2019 at least 20 percentage points less than the susceptible accessions, which points to a need for additional research to characterize the response of this accession.

Winter annual weeds begin to germinate as zoysiagrass enters winter dormancy in autumn. These weeds can suppress zoysiagrass shoot development the following spring through competition for sunlight, moisture, and nutrients. Previous research involving winter annual weed control in dormant turfgrass has been conducted primarily on bermudagrass, but less is known about how various herbicides used for this purpose will influence zoysiagrass post-dormancy transition. Two field studies were conducted at 7 site yr between 2016 and 2020 to evaluate 17 herbicide treatments that are typically marketed for broadleaf weed control in spring and 18 herbicide treatments that are typically marketed for annual bluegrass control during winter for effects on a variety of weeds and semi-dormant ‘Meyer’ and dormant ‘Zeon’ zoysiagrass, respectively. Glufosinate, glyphosate + simazine, and indaziflam + simazine controlled Persian speedwell over 90% and significantly higher than auxin-type and other herbicide combinations evaluated. The combination of simazine and glyphosate had better dandelion and Persian speedwell control than glyphosate applied alone. Glufosinate controlled dandelion, hairy bittercress, and Persian speedwell more effectively than glyphosate. In ‘Meyer’ zoysiagrass, glyphosate and glufosinate controlled annual bluegrass equivalently while in ‘Zeon’ zoysiagrass, glyphosate controlled annual bluegrass more than glufosinate. Foramsulfuron or simazine containing treatments controlled annual bluegrass >90%. Flumioxazin admixture with diquat, glufosinate, or glyphosate improved annual bluegrass control. Herbicide treatments containing diquat, glufosinate, glyphosate, and metsulfuron alone or in a tank-mix should not be applied to ‘Meyer’ zoysiagrass with 5% visual green turf cover due to high injury potential. In both studies, glufosinate was more injurious to ‘Meyer’ and ‘Zeon’ zoysiagrass than glyphosate. Overall, several herbicides that control annual bluegrass or broadleaf weeds can be safely applied to ‘Zeon’ zoysiagrass during dormancy or ‘Meyer’ zoysiagrass during post-dormancy transition.

In the transition zone, turfgrass managers generally utilize dormancy period of warm-season turfgrass to apply herbicides for managing winter annual weeds. Although this weed control strategy is common in bermudagrass, it has been less adopted in zoysiagrass (Zoysia spp.) due to variable turfgrass injury during post-dormancy transition. Previous research reported that air temperature could affect weed control and crop safety from herbicides. Growth chamber studies were conducted to evaluate zoysiagrass response to glyphosate and glufosinate as influenced by three different temperature regimes during and after treatment. A field research study was conducted at four-site years to assess the influence of variable heat unit accumulation on zoysiagrass response to seven herbicides. In the growth chamber study, glufosinate injured zoysiagrass more than glyphosate and reduced time to reach 50% green cover reduction, regardless of the rate when incubated for 7 d under different temperature levels. When green zoysiagrass sprigs were incubated for 7 d at 10 C, the rate of green cover reduction was slowed for both herbicides; however, green cover was rapidly reduced under 27 C. After incubating treated zoysiagrass plugs having 5% green cover at 10 C for 14 d, glyphosate-treated plugs reached 50% green cover in 22 d similarly to nontreated plugs but less than the 70 d required for glufosinate-treated plugs. Zoysiagrass response to glyphosate was temperature dependent, but glufosinate injured zoysiagrass unacceptably regardless of temperature regime. Diquat, flumioxazin, glufosinate, and metsulfuron+rimsulfuron injured zoysiagrass at 200 or 300 growing-degree-day at base 5 C (GDD5C) application timings, but foramsulfuron and oxadiazon did not injure zoysiagrass regardless of GDD5C. The relationship of leaf density to green turf cover is dependent on zoysiagrass mowing height and both metrics are reduced by injurious herbicides. Research indicates that glufosinate injures zoysiagrass more than glyphosate, and speed and magnitude of herbicide injury generally increases with temperature.

Quizalofop-resistant rice (Oryza sativa L.) allows for postemergence applications of quizalofop. Previous research reported that soil moisture availability strongly influences the response of grass species to aryloxyphenoxypropionate herbicides.Experiments were conducted at Stuttgart, AR, and at Colt, AR, in 2021 to investigate the influence of early season soil moisture and nitrogen (N) applications on the tolerance of quizalofop-resistant cultivars to sequential quizalofop applications. The experiment was implemented as a two-factor, randomized complete block design. The factors evaluated were cultivar (PVL02 and RTv7231 MA) and herbicide treatment (nontreated control; nontreated control fertilized with ammonium sulfate [AMS] at 100 lb acre −1 and surface irrigated at the two-leaf stage; sequential quizalofop applications at 1× and 2× rates; sequential quizalofop at 1× and 2× rates surface irrigated at the two-leaf stage; and sequential quizalofop applications at 1× and 2× rates fertilized with AMS at 100 lb acre −1 then surface irrigated at the two-leaf stage). Sequential quizalofop applications were applied at the two-leaf and five-leaf stages, with 1× and 2× rates of quizalofop being 0.11 and 0.22 lb a.i. acre −1 , respectively. RTv7231 MA injury was 2-to6-percentage points higher than PVL02 at all visual injury ratings averaged across treatment and location. Sequential quizalofop applications caused 1-9% visible injury to quizalofop-resistant cultivars pooled over locations and rating timings. Growers could consider environmental conditions before herbicide applications to mitigate the risk of injury to quizalofop-resistant cultivars from quizalofop, and drier soil conditions could be used for quizalofop applications onto rice compared with saturated soil conditions.

Quizalofop-resistant rice allows for over-the-top applications of quizalofop, an acetyl-coenzyme A carboxylase-inhibiting herbicide. However, previous reports have indicated that quizalofop applied postemergence has been shown to cause significant injury to quizalofop-resistant rice. Therefore, field experiments were conducted to evaluate the response of quizalofop-resistant rice cultivars to quizalofop applications across different planting dates. Under controlled conditions, the effects of soil moisture content, air temperature, and light intensity on quizalofop-resistant rice sensitivity to quizalofop were investigated. In the planting date experiment, 11 percentage points higher injury was observed on early-planted rice compared to late-planted rice at the 5-leaf stage, with higher injury observed under saturated soil conditions. However, quizalofop applications at labeled rate caused ≤16% reduction in yield regardless of planting environment. Quizalofop-resistant cultivars had at least 25-percentage points more injury when soil was maintained at 90% or 100% of field capacity as PVL01, PVL02, and RTv7231 MA had ≥42%, 30%, and ≥54% injury, respectively, compared to ≤10%, ≤5%, and ≤22% injury, respectively, at 40% or 50% of field capacity, pooled over rating timing. Greater injury ranging from 18% to 31% observed on quizalofop-resistant rice grown under low light intensity (600µmol m-2s-1) compared to 5% to 14% injury under high light intensity (1150µmol m-2s-1). The injury persisted from 7 to 28 days after 5-leaf stage application (DAFT), averaged over quizalofop-resistant cultivars and air temperatures (20/15 C and 30/25 C day/night, respectively). At 7 DAFT, greater injury (5- to 21-percentage points) was observed on quizalofop-resistant cultivars; PVL01, PVL02, and RTv7231 MA had 33%, 9%, and 58% injury, respectively, under 20/15 C compared to 13%, 4%, and 37% injury, respectively, under 30/25 C day/night conditions averaged over light intensities. Overall, quizalofop applications are likely to cause a greater risk for injury to quizalofop-resistant rice if applied under cool, cloudy, and moist soil conditions.