Glyphosate- and Trifloxysulfuron-Based Weed Control Programs in Roundup Ready Cotton

Transcription

1 Glyphosate- and Trifloxysulfuron-Based Weed Control Programs in Roundup Ready Cotton Oscar C. Sparks, Jim L. Barrentine, and Marilyn R. McClelland 1 RESEARCH PROBLEM The advent of herbicide-resistant cotton (Gossypium hirsutum) cultivars has allowed foliar postemergence control of many problematic weed species in cotton. Situations still remain in which a residual herbicide program may be beneficial in Roundup Ready cotton. When the first glyphosate application has to be made early in the season, residual herbicides may provide better control of larger weeds. Situations that do not promote rapid canopy development such as wider row spacings, dryland production, early-season insects, and unfavorable growing conditions may justify the use of residual postemergence (POST) herbicides. The objective of this experiment was to evaluate POST applications of metolachlor plus glyphosate followed by trifloxysulfuron for weed control in Roundup Ready cotton. BACKGROUND INFORMATION Roundup Ready cotton has been commercially available for many years. Glyphosate provides broad-spectrum control of many weed species; however, the lack of residual weed control fuels the on going question about the use of preemergence (PRE) herbicides. In using a PRE herbicide, growers have to deal with the potential of crop injury under cool wet conditions, and also crop failure options are limited once a PRE herbicide is applied. It was found that residual herbicides offered flexibility in timing later herbicide applications (McClelland and Barrentine, 1999). Residuals also tend to reduce germinating weed populations; however, additional postemergence herbicides were needed for season-long control (Welch, 1997). Some of the postemergence non-salvage options for broadleaf weed control in cotton are glyphosate, metolachlor, and trifloxysulfuron. It has been well documented that glyphosate provides adequate control of most weed species initially. Metolachlor provides residual 1 Graduate research assistant, professor/head, and senior research associate, respectively, Department of Crop, Soil, and Environmental Sciences, Fayetteville. 55

2 AAES Research Series 507 control of grass, sedge, and small-seeded broadleaf weeds. Trifloxysulfuron provides control of morningglories that are too large to control with glyphosate (Wells et al., 2001). There may be an advantage with respect to weed control in adding metolachlor to postemergence applications of glyphosate followed by postemergence applications of trifloxysulfuron in Roundup Ready cotton. RESEARCH DESCRIPTION An experiment was conducted in 2002 at the Cotton Branch Experiment Station, Marianna, AR, on a silt loam soil. The study design was a randomized complete block with four replications. Experimental units were by 40-ft plots that were overseeded with seed of Palmer amaranth (Amaranthus palmeri), pitted morningglory (Ipomoea lacunosa), prickly sida (Sida spinosa), and large crabgrass (Digitaria sanguinalis). Following incorporation of weed seed, cotton cultivar Paymaster 1218BR was planted at a rate of 60,000 seed/acre at a depth of 1.5 inches accompanied by an in-furrow application of the insecticide Temik at 4 lb/acre. Herbicide treatments consisted of a broadcast over-the-top application of glyphosate at 0.75 lb acid equivalent (ae)/acre to one- to two-leaf cotton followed by (fb) glyphosate at 0.75 lb ae/acre to four- or eight-leaf cotton or trifloxysulfuron at or lb active ingredient (ai)/acre. Glyphosate was also applied at 0.75 lb ae/ acre plus metolachlor at 0.95 lb ai/acre fb trifloxysulfuron at or lb ai/acre to five- to six-leaf cotton or seven- to eight-leaf cotton. Treatments were applied using a tractor-mounted sprayer calibrated to deliver 20 gal/acre. Data collected consisted of crop injury and weed control ratings by species on a percentage scale of 0 to 100 with 0 being no weed control or crop injury and 100 being complete weed control or total crop destruction. Data were subjected to analysis of variance, and treatment means were separated using Fisher s Protected Least Significant Difference (LSD) at the 0.05 level of significance. RESULTS AND DISCUSSION General observations in this study were that early timings of glyphosate to oneto two-leaf fb 4-leaf cotton did not control later germinating weeds; later glyphosate applications were more effective with respect to control of Palmer amaranth (Table 1). Although inadequate, a single application of glyphosate + metolachlor was better than or equal to early application timings of glyphosate (Table 2). The addition of metolachlor to POST applications of glyphosate provided enough suppression of Palmer amaranth and pitted morningglory to allow for later applications of trifloxysulfuron without sacrificing weed control. With respect to morningglory control, the addition of metolachlor to early POST applications of glyphosate improved control of pitted morningglory when trifloxysulfuron applications were delayed to the eight-leaf stage of cotton. This 56

3 Summaries of Arkansas Cotton Research 2002 improvement in control was evident early in the season (Table 1) and even more late in the season (Table 2). Trifloxysulfuron when applied alone was weak on prickly sida. The addition of metolachlor to POST applications of glyphosate improved prickly sida control from subsequent applications of trifloxysulfuron. The addition of metolachlor increased both early- and late-season control of large crabgrass (Tables 1 and 2) and goosegrass (data not presented). There appears to be a trend for higher cotton yields with the addition of metolachlor to POST applications of glyphosate. Later applications of glyphosate were more beneficial than early applications for controlling later germinating weeds and attaining season-long weed control. PRACTICAL APPLICATION It appears that the addition of metolachlor to POST applications of glyphosate did allow for some flexibility in timing later application of trifloxysulfuron. The addition of metolachlor to glyphosate without a follow-up treatment was comparable to many treatments that received two herbicide applications. The use of this herbicide treatment could allow the grower to avoid some of the early-season injury due to cold, wet conditions and also shift the usefulness of the metolachlor to the more rapid growth stages of cotton, possibly allowing for increased metabolism and decreased injury from metolachlor. Application timings of glyphosate + metolachlor should be further evaluated with trifloxysulfuron and in combination with Roundup Ready Flex cotton to evaluate the need for layby herbicide treatments. LITERATURE CITED McClelland, M.R. and J.L. Barrentine Postemergence weed control options in cotton. In: D.M. Oosterhuis (ed.). Proc Cotton Research Meeting and Summaries of Cotton Research in Progress. University of Arkansas Agricultural Experiment Station Special Report 193: Welch, A.K., P.R. Rahn, R.D. Voth, J.A. Mills, and C.R. Shumway Evaluation of preplant and preemergence herbicides in Roundup Ready cotton. Proc. Beltwide Cotton Conf., National Cotton Council, Memphis, TN. Vol. 1: Wells, J.W., J.C. Holloway, Jr., P.C. Forster, E.K. Rawls, and C.L. Dunne CGA for weed control in cotton. Proc. Beltwide Cotton Conf., National Cotton Council, Memphis, TN. Vol. 2:

4 AAES Research Series 507 Table 1. Weed control after a broadcast over-the-top application of glyphosate (glypho) or glypho plus metolachlor (meto) at one- to two-leaf cotton followed by (fb)glyphosate or trifloxysulfuron, Marianna, AR, Weed control 6 weeks after planting Palmer Prickly Pitted Large Treatment Rate z Timing Method y amaranth sida morningglory crabgrass (lb/acre) (%) Glypho x POST fb Glyphosate lf cotton POST Glyphosate lf cotton PDIR Trifloxysulfuron lf cotton POST Trifloxsulfuron lf cotton POST Trifloxysulfuron lf cotton PDIR Glypho plus meto z POST fb None POST Triufloxysulfuron lf cotton POST Trifloxysulfuron lf cotton POST Trifloxysulfuron lf cotton PDIR LSD (0.05) z Rates for glyphosate expressed in lb acid equivalent (ae) acre -1 and all other rates are expressed in lb active ingredient (ai)/acre. y POST = postemergence over-the-top; PDIR = post-directed. x Glyphosate applied at 0.75 lb ae/acre; metolachlor applied 0.95 lb ai/acre; fb = followed by. 58

5 Summaries of Arkansas Cotton Research 2002 Table 2. Weed control and seed-cotton yields after a broadcast over-the-top application of glyphosate (glypho) or glypho plus metolachlor (meto) at one- to two-leaf cotton followed by (fb) glyphosate or trifloxysulfuron, Marianna, AR, Weed control 18 weeks after planting Seed- Palmer Prickly Pitted Large cotton Treatment Rate z Timing Method y amaranth sida morningglory crabgrass yield (lb/acre) (%) (lb/acre) Glypho x POST fb Glyphosate lf cotton POST Glyphosate lf cotton PDIR Trifloxysulfuron lf cotton POST Trifloxsulfuron lf cotton POST Trifloxysulfuron lf cotton PDIR Glypho plus meto x POST fb None POST Triufloxysulfuron lf cotton POST Trifloxysulfuron lf cotton POST Trifloxysulfuron lf cotton PDIR LSD (0.05) z Rates for glyphosate expressed in lb acid equivalent (ae)/acre and all other rates are expressed in lb active ingredient (ai)/acre. y POST = postemergence over-the-top; PDIR = post-directed. x Glyphosate applied at 0.75 lb acid equivalent (ae)/acre, metolachlor applied 0.95 lb active ingredient (ai)/acre; fb = followed by. 59