Submitted to: Agronomy Journal
Publication Type: Peer reviewed journal
Publication Acceptance Date: 8/21/2006
Publication Date: 11/15/2006
Citation: Nuti, R.C., Viator, R.P., Casteel, S., Edmisten, K., Well, R. 2006. EFFECT OF PLANTING DATE, MEPIQUAT CHLORIDE, AND GLYPHOSATE APPLICATION TO GLYPHOSATE RESISTANT COTTON IN NORTH CAROLINA. Agronomy Journal, v98:1627-1633. Interpretive Summary: About 95% of the cotton planted in North Carolina is genetically modified for resistance to the non-selective herbicide glyphosate. Glyphosate is an economical herbicide that controls many weeds that are harmful in cotton production. Leaves and stems of glyphosate resistant cotton are more resistant to glyphosate compared to their cotton fruit, where the fiber is made. The recommendations for use of glyphosate on glyphosate resistant cotton prohibit broadcast applications of glyphosate after the four-leaf stage, and allow post-directed applications until first bloom. After the four-leaf stage, cotton fruit are continually being produced, so glyphosate spray contacting the plant would cause damage to fruit and potentially hurt crop yield. Since the four-leaf stage is early in the season, and farmers usually have lots of acres to spray, they may end up going against the recommendations and spraying broadcast glyphosate on some fields after the four-leaf stage. Most post-direct applicators do not avoid all contact of spray to crop plants. Other research has shown that cotton will compensate for early season cotton fruit loss as long as the rest of the season is favorable for continued growth. North Carolina has a short growing season compared to the rest of the US cotton production areas. If cotton is planted too early or too late, it may either be stunted or not have enough warm days to produce an acceptable crop. Mepiquat chloride is a plant growth regulator that is commonly used to reduce stem growth and increase early maturity in cotton. Experiments were done in North Carolina in 2001, 2002, and 2003 to define the best planting date and mepiquat chloride recommendations for use with glyphosate resistant cotton. Five glyphosate treatments were tested including one with no glyphosate. All treatments with glyphosate were sprayed broadcast at the four-leaf stage. At the eight-leaf stage three of these four were sprayed again either broadcast, non-precision post-direct, or precision post-direct. Non-precision post-direct allowed glyphosate contact to the bottom third of plants and precision post-direct prevented glyphosate contact to plants. Cotton was planted in early May and early June of each year for the two planting dates. Rate and timing of mepiquat chloride was according to North Carolina Extension recommendations. Cotton planted in May yielded better than June planted cotton every year. Broadcast glyphosate at the eight-leaf stage reduced yield 50% of the time. Yield was improved by MC in one of 2 years. Glyphosate did not affect the total number of bolls produced per plant, but did reduce the amount of bolls produced on the first position of fruiting branches. Cotton planted in May retained a higher portion of bolls on the lower part of the plant than cotton planted in June. Cotton that had either non-precision post direct or broadcast glyphosate at the eight-leaf stage in 2001 and 2002 had more bolls produced on the top of the plant. Mepiquat chloride caused a greater portion of bolls to be set lower on the plant and improved cotton fiber quality characteristics of cotton planted in June. Results from this study show that minimizing glyphosate contact to glyphosate resistant cotton plants after the four-leaf stage will positively impact yield about 50% of the time. May planted cotton has a better chance for making up for glyphosate damage than June planted cotton in North Carolina. The recommendations for mepiquat chloride use gave positive benefit to the cotton grown in this study.
Technical Abstract: Field studies were conducted near Rocky Mount, North Carolina in 2001, 2002, and 2003. Objectives were to determine if planting date affected the ability of glyphosate resistant (GR) cotton to compensate for fruit loss from glyphosate and evaluate mepiquat chloride’s (MC) contribution to fruiting compensation. Cotton was planted at optimum and late dates each year. Five glyphosate treatments were evaluated, including one no-herbicide treatment. All glyphosate treatments were sprayed over-the-top (OT) at the four-leaf stage (4OT), and three received additional glyphosate at the eight-leaf stage consisting of an OT (8OT), non-precision post-direct (8 non-Prec. PD), and a precision post-direct (8 Prec. PD) at 0.84 kg ae ha-1 glyphosate. The 10 planting date and glyphosate combinations were factored across programs using MC and no MC as needed according to North Carolina extension recommendations. Plants were mapped prior to harvest. Optimal-planted cotton yielded more than late-planted cotton in each year. Glyphosate 8OT reduced optimal-planted cotton yield in 2001and late-planted cotton yield in 2001 and 2002. Yield was improved by MC in 1 of 2 years. Glyphosate did not affect total bolls per plant but decreased first-position sympodial bolls and increased monopodial bolls. Optimal-planted cotton retained a higher portion of sympodial bolls below node 10 than late-planted cotton. Plants with glyphosate contact after the 4-leaf stage in 2001 and 2002 shifted boll load above node 10. Mepiquat chloride caused a greater portion of bolls to be set lower in the profile and positively affected micronaire of late-planted cotton.