Johnsongrass is one of the most troublesome weeds of the world and is listed as a noxious weed in Arkansas. Reduced johnsongrass control with the recommended application rate of glyphosate (840 g ae ha−1) was reported in a continuous soybean field near West Memphis, AR, in the fall of 2007. A greenhouse study was conducted (1) to confirm and characterize glyphosate resistance in the johnsongrass biotype from West Memphis and (2) to determine whether resistant and susceptible biotypes have differential glyphosate absorption or translocation. Dose–response studies revealed that the resistant biotype was five- to seven-fold less sensitive to glyphosate than the susceptible biotype. Glyphosate absorption was similar in resistant and susceptible biotypes at 72 h after treatment (HAT). However, the treated leaf of the resistant biotype retained 28 percentage points more absorbed14C glyphosate compared to the susceptible biotype at 72 HAT. Additionally, the resistant biotype had less14C glyphosate translocated to the aboveground tissue below the treated leaf and to roots compared to the susceptible biotype at 24 and 72 HAT. Reduced translocation and increased retention of glyphosate in treated leaves is a probable mechanism of resistance in this glyphosate-resistant johnsongrass biotype.
Field experiments were conducted in 2009 and 2012 at Fayetteville, AR, to compare four herbicides across three application timings for johnsongrass control and to evaluate the effect of late-season herbicide applications on johnsongrass control, fecundity, seed viability, and progeny emergence. In the first experiment, glyphosate at 840 g ae ha−1and clethodim at 68 and 136 g ai ha−1provided 75 to 94% control of 15- to 60-cm tall johnsongrass and up to 98% stand reduction 4 wk after treatment (WAT). Glufosinate and nicosulfuron were generally effective on ≤30-cm tall johnsongrass; however, efficacy declined as johnsongrass size increased. In the second experiment, glyphosate at 840 g ha−1provided at least 89% johnsongrass control when applied at boot or after panicle emergence and up to 95% stand reduction 3 WAT. Applications of glyphosate at 420 and 840 g ha−1, clethodim at 68 and 136 g ha−1, and glufosinate at 740 g ai ha−1at the boot stage reduced viable seed production of treated plants 94 to 99%. Irrespective of rate, glyphosate and clethodim applied at boot stage reduced progeny emergence by 77 to 95% and 100-seed weight of treated plants 62 to 96% compared to the nontreated check. This research demonstrates the importance of herbicide selection, particularly for controlling johnsongrass plants larger than 30 cm. Additionally, the results demonstrate the benefits of a single application of glyphosate or clethodim at the boot stage of johnsongrass for decreasing the soil seedbank and reducing the success of johnsongrass progeny in future years.
Three field experiments were conducted in 2010 and 2012 in a soybean production field near West Memphis, AR, containing glyphosate-resistant johnsongrass. The goal of this research was to develop effective herbicide programs for glyphosate-resistant johnsongrass in glufosinate-resistant soybean. Control of the resistant johnsongrass was greater with glufosinate at 590 and 740 g ai ha−1than at 450 g ha−1. Sequential glufosinate applications were more effective than a single application, irrespective of rate. A PRE application of flumioxazin at 71 g ai ha−1immediately after planting provided no more than 26% johnsongrass control 6 wk after soybean emergence (WAE). The addition of clethodim at 136 g ai ha−1to sequential applications of glufosinate at 450 g ha−1improved control over sequentially applied glufosinate alone. Herbicide programs containing imazethapyr or imazamox in combination with glufosinate followed by clethodim plus glufosinate controlled johnsongrass at least 94% at 10 WAE and provided three distinct mechanisms of action, a highly effective resistance management strategy. Results from this research indicate that a high level of glyphosate-resistant johnsongrass control can be achieved through the use of several herbicide options in glufosinate-resistant soybean.
In 2008, a population of johnsongrass collected from a soybean field near West Memphis, AR, in Crittenden County was confirmed resistant to glyphosate. This was the first documented case of glyphosate-resistant johnsongrass in Arkansas. The purpose of this study was to determine the geographical distribution of glyphosate-resistant johnsongrass in Arkansas crops and screen for resistance to additional herbicides. A total of 141 johnsongrass accessions were collected from 14 counties in Arkansas in the fall of 2008, 2009, and 2010 and screened for resistance to four of the most commonly used POST herbicides for johnsongrass control—imazethapyr, glyphosate, clethodim, and fluazifop. One accession potentially resistant to glyphosate (J12) and another with apparent resistance to imazethapyr (J14) were further evaluated in a dose–response experiment. The lethal dose required to kill 50% of the plants from the putative glyphosate-resistant and imazethapyr-resistant accessions was higher than that of a biotype known to be susceptible to these herbicides. The J12 accession had an LD50of 1,741 g ae ha−1glyphosate, which was 8.5-fold greater than the susceptible biotype. The J14 accession had an LD50of 73 g ai ha−1imazethapyr, which was 3.7-fold greater than the LD50of the susceptible biotype. All other accessions were effectively controlled by the four evaluated herbicides. Widespread herbicide-resistant johnsongrass was not found in Arkansas, although accession J12 was resistant to glyphosate and J14 resistant to imazethapyr.
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