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Seedling Vigor Effects on Tolerance of Roundup Ready® Cotton Varieties to Roundup Ultra (TM) Herbicide

Kevin D. Howard, Thomas A. Kerby, Janet Burgess, Michael Casavechia and Alan Coskrey


 
ABSTRACT

The metabolism of glyphosate (N-phosphonomethylglycine), the active ingredient in Roundup Ultra, in Roundup Ready cotton is minimal, therefore, reproductive tolerance requires that glyphosate concentration be reduced below threshold levels primarily by growth dilution. A natural question is if seed lots vary in seed vigor, could this result in lower seed quality lots requiring more time to dilute glyphosate and perhaps result in decreased yield contribution from early fruiting positions. A two year study was conducted at Scott, MS and Hartsville, SC in 1998 and 1999 with high and low vigor index seed lots of DP 458 B/RR and DP 655 B/RR. These two varieties were selected based upon the a reputation of DP 458 B/RR a small seeded variety having a low natural vigor and DP 655 B/RR a larger seeded variety with a reputation for high vigor. The two qualities and two varieties were each treated with 0, or 0.75 LB AE/A (1.0 qt/A) Roundup Ultra applied sequentially over the top at the one and four node stage and sequentially 10 and 20 days after the four node application. The post-direct applications were not according to the label, but directed on the bottom one-third of the plant in a manner to create high levels of glyphosate in plants. Plant stand, late bloom height, nodes, NAWF, and retention of the bottom five FP-1 retention, end-of-season box map data to partition yield according to plant position, and machine harvested yield were taken for all plots. In 1998 five lots were blended to create a single lot for test purposes. In 1999, a single lot each with high and low seed vigor index was used. In 1998 high and low quality lots averaged 148 and 130 seed vigor index for DP 458 B/RR and 110 and 97 for DP 655 B/RR, respectively. In 1999 high and low quality lots averaged 150 and 98 seed vigor index for DP 458 B/RR and 167 and 106 for DP 655 B/RR, respectively. In 1998 the “high quality seed vigor index” lot for DP 655 B/RR would not have met minimal industry standards while the “low quality seed vigor index” lots for all varieties and years were well below industry standards. Due to favorable weather following all planting dates (minimum of 31 DD 60 for the 5 days following planting at the “coldest” location), plant stands were equivalent for all seed lots. There were no substantial differences between seed vigor treatments, glyphosate application, or varieties on plant development. Box map data indicated yield accumulation by node was affected by out-of-label application of glyphosate. This resulted in a yield decrease for the aggressive and out-of-label applications of glyphosate and a one node shift in maturity.





Reprinted from Proceedings of the 2001 Beltwide Cotton Conferences pp. 414 - 417
©National Cotton Council, Memphis TN

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Document last modified XXXXXX, XXX XX 2001