Michael G. Patterson

Weed Management Programs for Glyphosate-Tolerant Cotton (Gossypium hirsutum)1

Abstract: Several herbicide-based weed management programs for glyphosate-tolerant cotton were compared in eight field studies across Alabama during 1996 and 1997. Weed management programs ranged from traditional, soil-applied residual herbicide programs to more recently developed total postemergence (POST) herbicide programs. Pitted morningglory and sicklepod control was best achieved with fluometuron applied preemergence (PRE) followed by (fb) a single POST over-the-top (POT) application of glyphosate fb a POST-directed application of glyphosate. Annual grass control was better with the preplant incorporated (PPI) programs at two of three locations in both years. Treatments that included at least one glyphosate POT application gave increased grass control over no glyphosate or pyrithiobac POT. Velvetleaf control was improved with the addition of glyphosate POT. A herbicide program using no POST herbicides yielded significantly less seed cotton than any program using POST herbicides at one location. PRE- and POST-only weed management programs at another location produced more seed cotton and gave greater net returns than PPI programs. Similarly, net returns at that same location were equivalent for both PRE- and POST-only programs, and less for PPI programs. POST-only programs yielded highest amounts of seed cotton and netted greater returns. Nomenclature: Fluometuron; glyphosate; pyrithiobac; cotton, Gossypium hirsutum L. ‘Deltapine 5415RR’, ‘Deltapine 5690RR’; pitted morningglory, Ipomoea lacunosa L. #3 IPOLA; sicklepod, Senna obtusifolia (L.) Irwin and Barneby # CASOB; velvetleaf, Abutilon theophrasti Medik. # ABUTH. Additional index words: Cotton yields, herbicide costs, herbicide-resistant crops, transgenic cotton. Abbreviations: ANOVA, analysis of variance; EPDS, early postemergence-directed spray; EPOT, early postemergence over-the-top; fb, followed by; GCS, Gulf Coast Research and Education Center; LPDS, late postemergence-directed spray; LPOT, late postemergence over-the-top; LSD, least significant difference; PDS, postemergence-directed spray; PEF, Prattville Experimental Field; POST, postemergence; POT, postemergence over-the-top; PPI, preplant incorporated; PRE, preemergence; TVS, Tennessee Valley Research and Education Center; WGS, Wiregrass Research and Education Center.

Weed Management with Fomesafen Preemergence in Glyphosate-Resistant Cotton1

Field studies were conducted in Alabama in 1998 and 1999 to evaluate fomesafen preemergence (PRE) in glyphosate-resistant cotton. Fomesafen (0.3 and 0.4 kg ai/ha), fluometuron (1.4 kg ai/ha), and pyrithiobac (0.05 kg ai/ha) were applied alone or in tank mixtures. Glyphosate (0.4 kg ae/ha) was applied postemergence over-the-top (POT) and postemergence directed (PD). Regardless of PRE treatment, POT followed by (fb) PD applications of glyphosate were necessary for greater than 82% sicklepod control at midseason. In the absence of glyphosate, fomesafen, and fomesafen-containing tank mixtures controlled common cocklebur and Ipomoea species 77 and 72%, respectively, 14 d after PD application. However, two applications of glyphosate were needed for >94% season-long control of common cocklebur and Ipomoea species because of continued germination throughout the growing season. Postemergence applications of glyphosate added a 1,000 kg/ha seed cotton yield increase to all PRE treatments at both locations. Common cocklebur, Ipomoea species, and sicklepod control was not significantly increased by the addition of fomesafen PRE tank mixtures fb glyphosate postemergence as compared with glyphosate postemergence only. Nomenclature: Fluometuron; fomesafen; glyphosate; pyrithiobac; common cocklebur, Xanthium strumarium L. #3 XANST; Ipomoea species # IPOSS; sicklepod, Senna obtusifolia (L.) Irwin and Barneby # CASOB; cotton, Gossypium hirsutum L. ‘Deltapine 655 BG/RR’, ‘Paymaster 1220 BG/ RR’. Additional index words: CASOB, fluometuron, fomesafen, glyphosate, glyphosate timing, pyrithiobac, residual herbicide, Senna obtusifolia, soil-applied herbicide, transgenic cotton, XANST, Xanthium strumarium. Abbreviations: fb, followed by; PD, postemergence directed; POT, postemergence over-the-top; PRE, preemergence; SRS, Syngenta Research Site; WGS, Wiregrass Research and Education Center.

Cotton and Weed Response to Glyphosate Applied with Sulfur-Containing Additives1

Field studies were conducted to assess two sulfur-containing additives for use with glyphosate applied postemergence to glyphosate-resistant cotton for the control of sicklepod and yellow nutsedge. Neither diammonium sulfate (AMS) nor ammonium thiosulfate (ATS), both applied at 2.24 kg/ha, increased control of either species. Effective control of both species was dependent on glyphosate (isopropylamine salt) rate alone, with optimum control at 1.26 kg ae/ha. Plant-mapping data further indicated that sulfur-containing additives generally had no effect on either cotton fruiting patterns or yield. However, applying glyphosate at any rate did increase seed cotton yield in 2 of 3 yr vs. no glyphosate. In addition, applying glyphosate at any rate resulted in an increase in the number of bolls vs. no glyphosate in the following plant-mapping responses: total number of bolls per plant, number of abcised bolls per plant, bolls at the top five sympodial nodes, and bolls at positions 1 and 2 on the sympodia. Glyphosate absorption and subsequent translocation, as influenced by the addition of the sulfur-containing additives, was evaluated using radiotracer techniques. Glyphosate absorption after 48 h was 86, 63, and 37% of amount applied in cotton, sicklepod, and yellow nutsedge, respectively. Absorption by sicklepod and yellow nutsedge was not affected by the addition of either of the additives. Absorption by cotton was reduced by ATS but was not affected by AMS. In yellow nutsedge and cotton, glyphosate concentration in the treated area and adjacent tissue was not affected by either additive. A greater portion of glyphosate was translocated away from the treated area in sicklepod with glyphosate plus AMS (32%) than with glyphosate plus ATS (21%). AMS and ATS may be used in glyphosate-resistant cotton without the risk of either crop injury or yield reduction. However, their use for increased control of annual weed species, such as sicklepod and yellow nutsedge, may not be warranted. Nomenclature: Glyphosate; ammonium thiosulfate; diammonium sulfate; sicklepod, Senna obtusifolia (L.) Irwin & Barneby #3 CASOB; yellow nutsedge, Cyperus esculentus L. # CYPES; cotton, Gossypium hirsutum L. ‘Paymaster 1218 BR’. Additional index words: Ammonium thiosulfate, CASOB, CYPES, diammonium sulfate, glyphosate absorption, glyphosate additives, glyphosate-resistant cotton. Abbreviations: AMS, diammonium sulfate; ATS, ammonium thiosulfate; DAT, days after treatment.

Effectiveness of Ammonium Thiosulfate to Enhance Weed Control and Reduce Cotton (Gossypium hirsutum) Injury1

Abstract: Field experiments were conducted in 1997 and 1998 at the Prattville Experiment Field in Prattville, AL and the Wiregrass Substation in Headland, AL to determine if ammonium thiosulfate (ATS) additions to monosodium methanearsonate (MSMA) affects weed control, reduces MSMA-induced cotton injury, lessens the fruiting and maturity delay on cotton caused by MSMA, and lessens MSMA-induced yield reductions in cotton. Treatments were applied before cotton was at the pinhead square stage and the weeds were 5 cm tall. Weeds evaluated were sicklepod, morningglory species, yellow nutsedge, and Texas panicum. ATS additions to MSMA occasionally enhanced control of all weeds 5 to 20%. However, the addition of ATS did not reduce crop injury caused by MSMA, the effects of MSMA on cotton maturity, or yield reductions caused by MSMA. Nomenclature: MSMA, monosodium methanearsonate; morningglory species, Ipomoea spp. #3 IPOZZ; sicklepod, Senna obtusifolia (L.) Irwin and Barneby # CASOB; Texas panicum, Panicum texanum Buckl. # PANTE; yellow nutsedge, Cyperus esculentus L. # CYPES; cotton, Gossypium hirsutum L. Abbreviations: ATS, ammonium thiosulfate; DAT, days after treatment; POST, postemergence.

Field Performance of Glyphosate as Influenced by Selected Adjuvants and a Low-Volume, Air-Assisted Sprayer1

Field studies were conducted at the Alabama Agricultural Experiment Station near Shorter, AL, from 1998 to 2000 to evaluate six commercial spray adjuvants and their effects on glyphosate applied to cotton with an air-assisted sprayer. Each adjuvant was evaluated with a conventional sprayer calibrated to deliver 94 L/ha solution and a low-volume, air-assisted sprayer calibrated to deliver 19 L/ha solution. Glyphosate was applied to two-leaf pitted and entireleaf morningglory growing in glyphosate-resistant cotton. Visual control of pitted and entireleaf morningglory was more dependent on the rate of herbicide application than on a particular adjuvant or sprayer, with glyphosate at 0.42 kg ae/ha providing 70% control 21 d after treatment. Pitted and entireleaf morningglory biomass measurements generally reflected visual control data. Ammonium sulfate and formulated glyphosate consistently gave the highest visual control and the greatest biomass reduction. Glyphosate application rate was more important than adjuvant addition or sprayer type, with the higher rates of application providing greater control. No differences in weed control were observed between spray systems; therefore, air-assisted sprayers may be used on a field-scale basis with consistent and adequate results. Nomenclature: Glyphosate; entireleaf morningglory, Ipomoea hederacea var. integriuscula Gray #3 IPOHE; pitted morningglory, Ipomoea lacunosa L. # IPOLA; cotton, Gossypium hirsutum L. ‘Paymaster 1220BG/RR’. Additional index words: Glyphosate-resistant cotton, IPOHE, IPOLA, surfactant. Abbreviations: AMS, ammonium sulfate; COC, crop oil concentrate; DAT, days after treatment; GLY+, formulated glyphosate; NIS-1, nonionic surfactant; NIS-2, nonionic wetter–spreader–penetrant; NIS-3, nonionic surfactant–penetrant–acidifier; POST, postemergence; PPI, preplant incorporated; WCA, water conditioning agent.