Daniel L. Colvin

Differential Response of Five Bahiagrass (Paspalum notatum) Cultivars to Metsulfuron1

Bahiagrass is used for roadsides, pastures, and lawns in the southeastern United States mainly because of drought and nematode tolerance. Metsulfuron is a sulfonylurea herbicide, which selectively controls bahiagrass in bermudagrass. Certain cultivars of bahiagrass were observed to be tolerant to recommended rates of metsulfuron. Therefore, research was conducted to investigate the susceptibility of five major bahiagrass cultivars to metsulfuron applied at increasing rates to 42 g ai/ha. Five bahiagrass cultivars were evaluated: ‘Pensacola’, ‘Tifton-9’, ‘Argentine’, ‘Common’, and ‘Paraguayan’. Argentine, Common, and Paraguayan cultivars showed a four- to fivefold increased tolerance to metsulfuron compared with Pensacola. Because of yearly inconsistencies, results for Tifton-9 were inconclusive. Nomenclature: Metsulfuron; Argentine bahiagrass, Paspalum notatum Fluegge var. notatum ‘Argentine’; bermudagrass, Cynodon dactylon (L.) Pers; Common bahiagrass, Paspalum notatum Fluegge var. notatum ‘Common’; Paraguayan bahiagrass, Paspalum notatum Fluegge var. notatum ‘Paraguayan’; Pensacola bahiagrass, Paspalum notatum Fluegge var. saurae Parodi ‘Pensacola’; Tifton-9 bahiagrass, Paspalum notatum Fluegge var. saurae Parodi ‘Tifton-9’. Abbreviations: GR50, metsulfuron rate required to reduce regrowth to 50% of untreated; OM, organic matter; WAT, weeks after treatment.

Purple Nutsedge (Cyperus rotundus) Control with Glyphosate in Soybean and Cotton1

Studies were conducted at the University of Florida, West Florida Research and Education Center to determine the effect of glyphosate on purple nutsedge control and nutsedge tuber production when glyphosate was applied to the same plots over 3 y in glyphosate-resistant soybean and cotton. Greater than 90% control of purple nutsedge foliage was achieved with a single POST application of glyphosate at 0.9 kg ai/ha in soybean or a sequential glyphosate application of 1.1 kg/ha POST followed by 0.6 kg/ha POST-directed in cotton. By the end of the third year of the study, these same treatments reduced purple nutsedge tuber density to less than 0.2% of the nontreated. In cotton, cultivation alone reduced tuber numbers by greater than 90%. Viability of tubers was also reduced by 80% in soybean and by 65% in cotton in the glyphosate-treated plots. Comparison treatments of imazaquin PRE followed by imazaquin POST in soybean or norflurazon PRE followed by cyanazine plus MSMA POST-directed in cotton also reduced purple nutsedge tuber density by ≥85% after three consecutive years of treatment. Nomenclature: Cyanazine; glyphosate; fluometuron; imazaquin; MSMA; norflurazon; pendimethalin; purple nutsedge, Cyperus rotundus L., #3 CYPRO; cotton, Gossypium hirsutum L. ‘Delta Pine 5415 RR’; soybean, Glycine max L. ‘Hartz 7555RR’. Additional index words: CYPRO, transgenic cotton, cultivation, purple nutsedge population dynamics. Abbreviations: DAP, days after planting; POST-directed, postemergence directed; Early-POST, early postemergence.

Effect of Glyphosate and MSMA Application Timing on Weed Control, Fruiting Patterns, and Yield in Glyphosate-Resistant Cotton

The limited window of opportunity for glyphosate postemergence (POST) over-the-top applications in glyphosate-resistant cotton poses a problem for growers where a midseason salvage weed control remedy is necessary. The objectives of these experiments were to compare glyphosate and MSMA for midseason weed control and their subsequent effect on cotton fruiting characteristics and yield. Glyphosate at 0.85 kg ai/ha was more effective than MSMA at 1.7 kg ai/ha for POST control of sicklepod, redweed, and pitted morningglory. Single glyphosate treatments applied at the 8-, 10-, or 12-leaf cotton stage resulted in less-effective weed control than when applied at the four-leaf cotton stage. Glyphosate applied at the four-leaf cotton stage followed by a sequential POST-directed application at 6-, 8-, 10-, or 12-leaf cotton stage increased season-long weed control and yield compared with a single application at the four-leaf stage. Both glyphosate and MSMA controlled Florida beggarweed, regardless of POST application timing. Generally, cotton was more tolerant to glyphosate than MSMA when applied over-the-top. Glyphosate applied POST over-the-top to weed-free 12-leaf cotton resulted in a 19 and 14% yield loss compared with the weed-free nontreated cotton in 1997 and 1999. MSMA reduced yield by 58 and 36% in 1997 and 1999, respectively. Glyphosate did not affect weed-free cotton fruit development or yield when applied over-the-top to four-leaf cotton or when a POST-directed application was followed at the 12-leaf stage. Nomenclature: Glyphosate; MSMA; Florida beggarweed, Desmodium tortuosum (Sw.) DC. #3, DEDTO; pitted morningglory, Ipomoea lacunosa L. # IPOLA; redweed, Melochia corchorifolia L. # MEOCO; sicklepod, Senna obtusifolia L. #3 SENOB; cotton, Gossypium hirsutum L. Additional index words: Application timing, cotton boll retention, salvage weed control. Abbreviations: POST, postemergence; POST-directed, postemergence-directed; PPI, preplant incorporated; PRE, preemergence.

Weed Management in Peanut (Arachis hypogaea) with Pyridate and SAN 582 Systems1

Abstract: Field studies were conducted near Archer, FL, and Vienna, GA, in 1995 and 1996 to investigate pyridate and SAN 582 for weed management in peanut. At Archer, pyridate plus 2,4-DB applied 3 wk after emergence resulted in 75, 72, 59, and 85% early-season control of yellow nutsedge, Florida beggarweed, hairy indigo, and sicklepod, respectively. In Vienna, pyridate plus 2,4-DB resulted in 87 and 55% early-season control of yellow nutsedge and Florida beggarweed, respectively. At both Archer and Vienna, SAN 582 applied preplant incorporated prior to pyridate postemergence (POST) increased control of yellow nutsedge, Florida beggarweed, and hairy indigo; however, peanut yield was not improved. In greenhouse studies, pyridate plus 2,4-DB controlled prickly sida, common cocklebur, and ivyleaf morningglory. Reduced weed control was observed in greenhouse studies when SAN 582 was added to the pyridate plus 2,4-DB POST tank mix. Nomenclature: Pyridate; SAN 582 (proposed common name dimethenamid), 2-chloro-N-[(1-methyl-2-methoxy)ethyl]-N-(2,4-dimethyl-thien-3-yl)-acetamide; 2,4-DB; common cocklebur, Xanthium strumarium L. #3 XANST; Florida beggarweed, Desmodium tortuosum (Sw.) DC. # DEDTO; hairy indigo, Indigofera hirsuta Harvey # INDHI; ivyleaf morningglory, Ipomoea hederaceae (L.) Jacq. # IPOHE; prickly sida, Sida spinosa L. # SIDSP; sicklepod, Senna obtusifolia (L.) Irwin et Barneby # CASOB; yellow nutsedge, Cyperus esculentus L. # CYPES; peanut, Arachis hypogaea L. ‘GK-7’, ‘Marc III’, ‘Sunoleic 95R’, ‘Sunrunner’. Additional index words: Application timing. Abbreviations: NIS, nonionic surfactant; PPI, preplant incorporated; PRE, preemergence; POST, postemergence; WAE, weeks after emergence; WAT, weeks after treatment.

Effect of Pyridate Formulation and Adjuvants on Broadleaf Weed Control in Peanut (Arachis hypogaea)1

Abstract: Field studies were conducted near Archer, FL, and Vienna, GA, in 1995 and 1996 to investigate the effects of pyridate formulation and adjuvants on broadleaf weed control in peanut (Arachis hypogaea). Pyridate formulations SAN 319H 450EC 361LZ, SAN 319H 450EC 216LZ, and SAN 319H 600EC 418LZ were evaluated at two rates, 1.07 and 2.14 kg/ha. Pyridate at 1.07 kg/ha plus 2,4-DB at 0.23 kg/ha were evaluated alone and with five adjuvants. The adjuvants included a crop oil concentrate, a nonionic surfactant, a nonionic surfactant with organosilicone blend, urea ammonium nitrate plus a nonionic surfactant, and chlorothalonil (a fungicide) plus a nonionic surfactant. No pyridate treatment injured peanut. Pyridate formulation did not affect broadleaf weed control. Increasing pyridate rate increased weed control and yield. Mixing 2,4-DB with pyridate generally enhanced sicklepod (Senna obtusifolia) and common cocklebur (Xanthium strumarium) control. Florida beggarweed (Desmodium tortuosum), smallflower morningglory (Jacquemontia tamnifolia), hairy indigo (Indigofera hirsuta), sicklepod, and common cocklebur control with pyridate was not enhanced by adjuvants. Adding chlorothalonil to pyridate plus 2,4-DB did not affect weed control or peanut injury. Nomenclature: 2,4-DB; chlorothalonil; tetrachloroisophthalonitrile; pyridate; common cocklebur, Xanthium strumarium #3 XANST; Florida beggarweed, Desmodium tortuosum # DEDTO; hairy indigo, Indigofera hirsuta # INDHI; peanut, Arachis hypogaea ‘Sunrunner’ and ‘GK-7’; sicklepod, Senna obtusifolia # CASOB; smallflower morningglory, Jacquemontia tamnifolia # IAQTA. Additional index words: SAN 319H 450EC 361LZ, SAN 319H 450EC 216LZ, SAN 319H 600EC 418LZ, crop oil concentrate, nonionic surfactant, organosilicone-nonionic surfactant blend, urea ammonium nitrate. Abbreviations: COC, crop oil concentrate; NIS, nonionic surfactant; NIS/OS, nonionic surfactant with organosilicone; POST, postemergence; WAT, weeks after treatment.