Masters Theses

Date of Award

5-1998

Degree Type

Thesis

Degree Name

Master of Science

Major

Plant, Soil and Environmental Sciences

Major Professor

Robert M. Hayes

Committee Members

T. C. Mueller, G. N. Rhodes

Abstract

Few selective postemergence herbicides are available to control severely competitive weeds in cotton (Gossypium hirsutum). Common cocklebur (Xanthium strumarium L.) is a problem to producers due to its extremely competitive nature. Arsenical herbicides are available for postemergence control of cockleburs but cotton injury is a risk. Pyrithiobac (sodium 2-chloro-6-[(4,6-dimethoxy pyrumidin-2-yl)thio]benzoate) is effective but application timing is critical. Directed treatment is needed for season-long control. Cotton varieties were genetically engineered for tolerance to herbicides that provide broad-spectrum weed control. Roundup Ready' cotton varieties tolerate application of glyphosate as Roundup Ultra. The advantages of glyphosate use on glyphosate-tolerant cotton include control of both monocot and dicot weeds. Application of glyphosate is determined by the growth stage of the cotton plant. Experiments were conducted at the West Tennessee Experiment Station to determine the effect of glyphosate on cotton injury, fruiting, seed per boll, growth, lint yield and earliness. Of interest also was the effect of application timing of glyphosate on weed control with respect to weed competition. Two separate experiments were established. One was maintained weed free while the other was planted into a high density (> 10 plants/ m2) of common cocklebur. Treatments included a single over-the-top application of glyphosate to cotton at the 4-, 6-, 8-, 10-, and 12-leaf stage. Sequential treatments following an initial over-the-top treatment at the 4-leaf stage included applications at the 6", 8-, or 10-leaf stage over-the-top or post-directed. An untreated hoed and handweeded check was included for comparison. Insects were intensively managed, growth regulators were applied to control excessive vegetative gro\vth and irrigation was applied as needed. Weed control was visually estimated on a percentage basis with 100 being total control and zero being no control. Prior to defoliation cotton plants were mapped. Cotton was harvested twice with a John Deere 9930 spindle picker. Percent first harvest was calculated. Results were subjected to analysis of variance and means were separated by Fisher's protected LSD.

Cocklebur control was improved with two applications of glyphosate. Single applications were not effective in controlling cockleburs due to cocklebur resurgence after glyphosate treatments. In 1996 all sequential treatments controlled cocklebur >90 The 4-leaf followed by 10-leaf over-the-top application was the only application to control cocklebur >80% in 1997. No differences were detected between the post-directed treatments and the over-the-top treatments. The nodes at which glyphosate application occurred were the first to show increases in fruit retention. Cocklebur competition delayed cotton fruiting, earliness and lint yield.

In weed free plots, no differences in cotton plant height or the total number of bolls per meter occurred from glyphosate treatments. Fruit retention at position one was decreased both years by glyphosate. In 1996, fruit retention was increased at position two. More differences were evident in 1997 than in 1996. No effects in retention at position three were detected. Plants compensated for lost position one fruit and yield was not different in 1996. Single applications at the 10-leaf stage reduced yield in 1997. Maturity was delayed by applications later than the 6-leaf stage.

In conclusion, glyphosate controlled common cocklebur but more than one application was necessary to avoid common cocklebur competition with cotton. Earliness was delayed when glyphosate was applied after the 8-leaf stage. More reduction in yields occurs from common cocklebur interference than glyphosate application.

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