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The U.S. Department of Agriculture–Agricultural Research Service (USDA-ARS) has been a leader in weed science research covering topics ranging from the development and use of integrated weed management (IWM) tactics to basic mechanistic studies, including biotic resistance of desirable plant communities and herbicide resistance. ARS weed scientists have worked in agricultural and natural ecosystems, including agronomic and horticultural crops, pastures, forests, wild lands, aquatic habitats, wetlands, and riparian areas. Through strong partnerships with academia, state agencies, private industry, and numerous federal programs, ARS weed scientists have made contributions to discoveries in the newest fields of robotics and genetics, as well as the traditional and fundamental subjects of weed–crop competition and physiology and integration of weed control tactics and practices. Weed science at ARS is often overshadowed by other research topics; thus, few are aware of the long history of ARS weed science and its important contributions. This review is the result of a symposium held at the Weed Science Society of America’s 62nd Annual Meeting in 2022 that included 10 separate presentations in a virtual Weed Science Webinar Series. The overarching themes of management tactics (IWM, biological control, and automation), basic mechanisms (competition, invasive plant genetics, and herbicide resistance), and ecosystem impacts (invasive plant spread, climate change, conservation, and restoration) represent core ARS weed science research that is dynamic and efficacious and has been a significant component of the agency’s national and international efforts. This review highlights current studies and future directions that exemplify the science and collaborative relationships both within and outside ARS. Given the constraints of weeds and invasive plants on all aspects of food, feed, and fiber systems, there is an acknowledged need to face new challenges, including agriculture and natural resources sustainability, economic resilience and reliability, and societal health and well-being.
Green kyllinga is a relatively new weed species in well-irrigated turfs in the southwest US. To understand the germination requirements of green kyllinga, the effects of planting depth, temperature, pH, and osmotic potential were determined. Green kyllinga emergence was sensitive to planting depth. At 21 days after planting (DAP), emergence of green kyllinga was 60% when planted at the soil surface but only 20% at 0.5 cm and less than 5% at 1 cm. Germination occurred at temperatures between 17 and 30 C but was notably better between 20 and 24 C. Germination was uniform from pH 5.5 to 9.5. Germination decreased between the water potentials −0.1 to −0.6 MPa, although germination was high at water potentials found in irrigated turf. The environmental conditions found in nonstressed turf areas in Arizona will likely support the establishment of green kyllinga. The efficacy of herbicides for the control of green kyllinga was also determined. Control of kyllinga (95% or greater) was obtained with preemergence applications of oxadiazon, dithiopyr, and metolachlor, and with postemergence applications of DSMA, halosulfuron-methyl, and imazaquin.
A field study was conducted in 1994 and 1995 to determine the efficacy of MON 12037 on purple nutsedge in a bermudagrass turf. The percentage of bermudagrass infested with purple nutsedge following a single application of MON 12037 decreased from 60 to 30% at 18 g ai/ha and to approximately 20% at 36, 54, and 72 g/ha. Regrowth of purple nutsedge occurred between 28 and 55 d after treatment (DAT) and was greatest at 18 g/ha. A second application of MON 12037 reduced purple nutsedge percent infestation similarly to one application. Tuber weight and number were not affected by either one or two applications of MON 12037; however, tuber viability decreased after two applications. These results suggest that when using MON 12037, a program of several years duration may be required to successfully manage purple nutsedge in a bermudagrass turf because viable tubers persist following two applications of MON 12037 in a season.
Crops transformed to provide resistance to herbicides with two different
mechanisms of action provide new opportunities for control of
herbicide-resistant weeds. However, unexpected interactions may develop,
especially for herbicides not generally used in tank-mixtures. The
objectives of this study were to evaluate weed control and determine
herbicide interactions and fluorescence responses with combinations of
glyphosate and glufosinate on selected weeds prevalent in Michigan cropping
systems. Field studies to determine herbicide interactions resulted in
synergism only at 0.84 kg ae ha−1 of glyphosate and 0.47 kg ai
ha−1 glufosinate in 2008. Early synergism (7 d after treatment
[DAT]) was observed in the field at several combined rates for common
lambsquarters and velvetleaf in 2009, and in the greenhouse for giant
foxtail. Differences between years were perhaps due to the effect of
environmental conditions on herbicide absorption and translocation.
Antagonism was observed in the field in 2009 for velvetleaf, common
lambsquarters, and giant foxtail especially at 840 g ae ha−1
glyphosate and 118 g ai ha−1 glufosinate, 28 DAT. Antagonism was
also observed in the greenhouse for giant foxtail and Canada thistle, 28
DAT. Fluorescence measurements on Canada thistle in the greenhouse showed
that glufosinate and glufosinate plus glyphosate acted rapidly to quench
electron transport of photosystem II (PS II) system of photosynthesis, and
the fluorescence characteristics of the glyphosate and glufosinate
combinations were indistinguishable from glufosinate alone.
The effects of MT-101 and its herbicidally active form, NOP, on the germination and seedling growth of hemp sesbania and rice were investigated. MT-101 decreased the germination of hemp sesbania by 57 and 90% at 0.05 and 0.5 mM, respectively, 1 d after treatment (DAT) in petri dishes. The germination, however, recovered such that there was no significant difference between treatments 4 to 6 DAT. NOP completely inhibited the germination of hemp sesbania at both 0.05 and 0.5 mM 1 DAT. However, germination also similarly recovered, and there was no difference between treatments 4 to 6 DAT. Neither MT-101 nor NOP decreased the germination of rice 3 to 6 DAT. In greenhouse trials preemergence (PRE) application of MT-101 at 2.25 kg ai ha−1 decreased the density (number of plants pot−1), plant height, and dry weight of hemp sesbania by 85, 67, and 91%, respectively. When applied postemergence (POST), MT-101 at 2.25 kg ha−1 decreased the density, plant height, and dry weight by a maximum of 58, 61, and 82%, respectively, indicating that MT-101 may have greater activity when applied PRE. NOP had greater activity than MT-101 on hemp sesbania. NOP at 2.25 kg ai ha−1 decreased the density, plant height, and dry weight of hemp sesbania 99, 78, and 97%, respectively, with PRE application. A POST application of NOP at 2.25 kg ha−1 decreased the dry weight of hemp sesbania 91 to 94%. A PRE application of NOP at 2.25 kg ha−1 decreased the dry weight of rice by 58%. Rice was not affected by POST applications of MT-101 but was affected slightly by NOP. These results suggest that MT-101 is a possible weed control agent in rice.
Spurred anoda is a major competitor with cotton in the southern United States. Physiological and antioxidant responses of two species of cotton (Gossypium barbadense L. cv. ‘Pima S-7’ and Gossypium hirsutum L., Delta and Pine Land Company cv. ‘Delta Pine 5415’) and two accessions of spurred anoda [New Mexico (NM) and Mississippi (MS)] were investigated under nitrogen (N) -sufficient and -deficient conditions in the greenhouse. Pima S-7 had the highest leaf N content of all the plants regardless of treatment. Biomass decreased in all species when N was withheld, with Pima S-7 exhibiting the least reduction and MS the greatest. Plant height decreased in cotton but not spurred anoda under N stress. Height:node ratio increased 9% in MS, but decreased 8% in DP 5415 when they were deprived of N. Withholding N reduced photosynthesis 45% regardless of species. Comparable decreases were found in stomatal conductance and transpiration, suggesting strong stomatal regulation of gas exchange under N stress. The quantum efficiency of photosystem II (dark-adapted Fv/Fm) decreased 4% under N deficiency. Alpha-carotene decreased for all species when N was withheld, except for the NM accession, in which the levels increased. Total chlorophyll and lutein decreased under N stress regardless of species, but alpha-tocopherol and the xanthophyll cycle conversion state increased. Pima S-7 had the most chlorophyll and lutein, and both cotton species had more alpha-tocopherol, anthocyanins, and free-radical scavenging capacity than spurred anoda. These enhanced pigment and antioxidant profiles of cotton, particularly Pima S-7, may contribute to cotton's ability to compete for N with spurred anoda.
Transfer of herbicide resistance among closely related weed species is a
topic of growing concern. A spiny amaranth × Palmer amaranth hybrid was
confirmed resistant to several acetolactate synthase (ALS) inhibitors
including imazethapyr, nicosulfuron, pyrithiobac, and trifloxysulfuron.
Enzyme assays indicated that the ALS enzyme was insensitive to pyrithiobac
and sequencing revealed the presence of a known resistance conferring point
mutation, Trp574Leu. Alignment of the ALS gene for Palmer amaranth, spiny
amaranth, and putative hybrids revealed the presence of Palmer amaranth ALS
sequence in the hybrids rather than spiny amaranth ALS sequences. In
addition, sequence upstream of the ALS in the hybrids matched Palmer
amaranth and not spiny amaranth. The potential for transfer of ALS inhibitor
resistance by hybridization has been demonstrated in the greenhouse and in
field experiments. This is the first report of gene transfer for ALS
inhibitor resistance documented to occur in the field without
artificial/human intervention. These results highlight the need to control
related species in both field and surrounding noncrop areas to avoid
interspecific transfer of resistance genes.
A 4-yr field study was conducted during 1998 through 2001 at Stoneville, MS, to determine the effects of narrow-row transgenic cotton and soybean rotation on purple nutsedge populations and crop yield. Crop rotations over 4 yr included cotton and soybean sown in the following patterns: CCSS, CSCS, SCSC, SSCC, and continuous cotton (CCCC) and soybean (SSSS), where cotton is denoted as (C) and soybean as (S), all with herbicide programs that were glyphosate based, non–glyphosate based, or no purple-nutsedge control (NPNC). Purple nutsedge populations and shoot dry biomass were reduced in cotton and soybean rotation and continuous soybean by 72 and 92%, respectively, whereas in continuous cotton, purple nutsedge populations increased by 67% and shoot dry biomass was reduced by 32% after 4 yr. Reductions in purple nutsedge populations also occurred in soybean when cotton was rotated with soybean (CSCS and SCSC), compared with continuous cotton. Among herbicide programs, the glyphosate-based program was more effective in reducing purple nutsedge populations, compared with the non–glyphosate-based program. Seed cotton yield was greater with cotton following soybean (SCSC) than with cotton following cotton (CCCC, CCSS) in 1999. However, seed cotton yields were similar regardless of crop rotation systems in 2000 and 2001. Seed cotton yields were equivalent in the glyphosate-based and non–glyphosate-based programs in 1999 and 2001. During 1999 to 2001, seed cotton yields were reduced by 62 to 85% in NPNC compared with yields in glyphosate- and non–glyphosate-based programs. Soybean yields were unaffected by crop rotation systems in all the 4 yr. Among herbicide programs, non–glyphosate-based program in all 4 yr and glyphosate-based program in 1999 and 2000 gave higher soybean yield compared with NPNC. After 4 yr of rotation, purple nutsedge tubers and plant density were highest in continuous cotton and lowest in continuous soybean. Both herbicide programs reduced tubers per core and plant density compared with NPNC, and the glyphosate-based program was more effective than the non–glyphosate-based program. These results show that in cotton production, severe infestations of purple nutsedge can be managed by rotating cotton with soybean or by using glyphosate-based herbicide program in glyphosate-resistant cotton.
A field experiment was conducted in 2000, 2001, and 2002 at Stoneville, MS, to determine the effect of spurred anoda interference on yield loss of two cotton cultivars, ‘Delta Pine 5415’ and ‘Pima S-6’, grown under wide (1 m) (WR) and ultra narrow (0.25 m) row (UNR) spacings. The relationship between spurred anoda density and dry weight per plot was linear each year. At a spurred anoda density of 8 m−2, spurred anoda dry weight per plot was 507, 322, and 777 g m−2 in 2000, 2001, and 2002, respectively. However, spurred anoda did not interfere with seed cotton yield in 2001, which was probably attributable to the low branch development in that year. Yield losses exceeded 55% at a spurred anoda density of 8 m−2 compared with controls in both WR and UNR. The effect of spurred anoda density on boll numbers was nearly identical in 2000 and 2002, regardless of cotton cultivar and row spacing. Boll weights decreased in response to spurred anoda interference. Spurred anoda interference resulted in a decrease in cotton branch dry weight in WR but not in UNR. The yield decrease as a result of spurred anoda interference in WR was due to reduction in boll retention or fruiting sites (predicated on a decrease in branch weight). However, in UNR, the yield decrease was due to plant mortality; the plant density of both cotton cultivars decreased by one plant for each additional spurred anoda, but the yield per plant for surviving plants remained constant. Neither WR nor UNR cotton had significant advantage in response to spurred anoda interference. The decreased boll weight observed in UNR, and the failure to increase boll numbers m−2 to compensate for decreased boll weight in UNR compared with WR, may limit its appeal to cotton producers.
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