The southern United States produces 90% of the nation’s cotton, and the Texas High Plains is the largest contiguous cotton producing region. Since 2011, glyphosate-resistant Palmer amaranth has complicated cotton production, and alternatives to glyphosate are needed. Integrating soil residual herbicides into a weed management program is a crucial step to control glyphosate resistant weeds before emergence. The recent development of p-hydroxyphenylpyruvate dioxygenase (HPPD)-resistant cotton by BASF Corporation may allow growers to use isoxaflutole in future weed management programs. In 2019 and 2020, field experiments were conducted in New Deal, Lubbock, and Halfway, Texas, to evaluate HPPD-resistant cotton response to isoxaflutole applied preemergence (PRE) or early postemergence (EPOST) and to determine the efficacy of isoxaflutole when used as part of a season-long weed management program. At the New Deal location, cotton response was observed following the EPOST application, but it never exceeded 10%. Cotton response was greatest following the PRE application in Lubbock in 2019 but did not exceed 14%. In 2020 in Lubbock, cotton was replanted due to severe weather. There was <1% cotton response following the PRE application, and maximum cotton response observed was 9% following EPOST and mid-postemergence (MPOST) applications. Cotton lint yields were not different from those of the nontreated, weed-free control at either location. In non-crop weed control studies in Halfway, all treatments controlled Palmer amaranth ≥94% 21 d after the EPOST application. Twenty-one days after the MPOST treatment, systems with isoxaflutole applied EPOST controlled Palmer amaranth by 88% to 93%, while systems with isoxaflutole PRE controlled Palmer amaranth by 94% to 98%. End-of-season Palmer amaranth control was lowest in the system without isoxaflutole (88%) and when isoxaflutole was used EPOST (88% to 91%). These studies suggest that the use of isoxaflutole in cotton weed management systems may improve season-long control of several troublesome weeds with no adverse effects on cotton yield and quality.

Failure of glyphosate to control Palmer amaranth was first reported in Arkansas in Mississippi County in June, 2005. The objectives of this research were to (a) confirm glyphosate-resistant Palmer amaranth in Arkansas, and (b) determine the effectiveness of 15 postemergence- (POST) applied herbicides comprising eight modes of action in controlling the glyphosate-resistant biotype compared to glyphosate-susceptible accessions. The LD50 values were similar among three susceptible Palmer amaranth accessions, ranging from 24.4 to 35.5 g ae/ha glyphosate. The resistant biotype had an LD50 of 2,820 g/ha glyphosate, which was 79- to 115-fold greater than that of the susceptible biotypes and 3.4 times a normal glyphosate-use rate of 840 g/ha. The glyphosate-resistant biotype was effectively controlled with most of the evaluated herbicides, but the use of acetolactate synthase-inhibiting herbicides such as pyrithiobac, trifloxysulfuron, and imazethapyr is not a viable option for control of this Palmer amaranth population.

Methyl bromide (MeBr), classified as a Class I ozone-depleting substance, has been banned for ordinary agricultural uses. Weed control in commercial bell pepper production is complicated by the ban on MeBr and the lack of other available and effective soil fumigants. A field study was conducted to evaluate the effectiveness of allyl isothiocyanate (ITC) and metam sodium (methyl ITC generator) as MeBr alternatives for control of Palmer amaranth, large crabgrass, and yellow nutsedge; and for increasing marketable yields in low-density polyethylene (LDPE) –mulched bell pepper. Allyl ITC was applied at 450, 600, and 750 kg ha−1; metam sodium was applied at 180, 270, and 360 kg ha−1; and MeBr plus chloropicrin (67% and 33%, respectively) was applied at 390 kg ha−1. Allyl ITC and metam sodium did not injure bell pepper. Allyl ITC at 750 kg ha−1 or metam sodium at 360 kg ha−1 controlled Palmer amaranth (≥ 83%), large crabgrass (≥ 78%), and yellow nutsedge (≥ 80%) comparably to MeBr. Yellow nutsedge tuber density was ≤ 84 tubers m−2 in plots treated with the highest rate of allyl ITC and metam sodium and was comparable to the tuber density in MeBr-treated plots. Although allyl ITC at 750 kg ha−1 controlled weeds comparable to MeBr, total marketable bell pepper yield with allyl ITC was lower than the yield with MeBr. Conversely, total marketable bell pepper yield with the highest rate of metam sodium (53.5 ton ha−1) was equivalent to the yield (62.5 ton ha−1) in plots treated with MeBr. In conclusion, metam sodium at 360 kg ha−1 is an effective MeBr alternative for weed control in LDPE–mulched bell pepper.

In little over 20 yr, Palmer amaranth has risen from relative obscurity to its current status as one of the most widespread, troublesome, and economically damaging agronomic weeds in the southeastern U.S. Numerous factors have enabled Palmer amaranth to become such a dominant and difficult-to-control weed, including its rapid growth rate, high fecundity, genetic diversity, ability to tolerate adverse conditions, and its facility for evolving herbicide resistance. It is both a serious threat to several U.S. cropping systems and a fascinating model weed. In this paper, we review the growing body of literature on Palmer amaranth to summarize the current state of knowledge on the biology, agricultural impacts, and management of this weed, and we suggest future directions for research.