Experiments were conducted at Manhattan, KS in 2005 and 2006 to evaluate cotton response to simulated 2,4-D and dicamba drift rates at different stages of growth and multiple applications of 2,4-D. Cotton was treated with 2,4-D and dicamba at 0, 1/200, and 1/400 of the use rate (561 g ae/ha) when plants were at the three- to four-leaf, 8-, 14-, or 18-node growth stages. Injury symptoms after 2,4-D and dicamba application were more severe at the three- to four-leaf stage compared with other stages with greatest injury from 2,4-D. In general, plants partially recovered from 2,4-D and dicamba injury symptoms, and only 2,4-D applied at the 1/200 rate reduced fiber yield. In a separate study, cotton was treated with 2,4-D at 0, 1/400, 1/800, and 1/1,200 of the use rate for one, two, or three applications. Yield reduction increased as herbicide rate increased from 1/1,200 to 1/400 and the number of applications increased from one to three. In both studies, plants partially or fully recovered from injury symptoms and recovery was greater with dicamba than 2,4-D. Correlation coefficient analysis showed that visual injury ratings later in the growing season are a good predictor of yield reduction (R2 = 0.58).
Nomenclature: 2,4-D (2,4-dichlorophenoxy) acetic acid; dicamba, 3,6-dichloro-2-methoxybenzoic acid; ethephon, (2-chloroethyl) phosphonic acid; tribufos, S,S,S-tributyl phosphorotrithioate; cotton, Gossypium hirsutum L