Effect of Pre-Emergent Herbicides on Environmentally Stressed Cotton

The use of pre-emergent herbicides declined as adoption of glyphosate resistant crops gained popularity in the late 1990s and early 2000s.  As weed populations with natural resistance to glyphosate increased and spread, the need for pre-emergent herbicides has again become evident.  Under periods of stress, such as may be observed under extreme early or late plantings, young cotton plants may be exposed to doses of herbicides deemed safe under ‘normal’ conditions.  Will McCarty, Mississippi State University Cotton Extension Specialist, wrote in 1994 that “all preemergence herbicides can damage young cotton and reduce or destroy a stand if a high concentration of the herbicide comes in contact with the germinating seed or young seedling. Tolerance depends upon soil type, organic matter content and amount and timeliness of rainfall the first two weeks after planting.” 

            Due to a warm dry spring in 2012, many Missouri cotton producers were able to plant early. Herbicides normally incorporated by rain, may have instead have been concentrated on the soil surface and contacted seedlings in high enough dosages to slow or even stop development.  In 2013, however, the spring was cool and damp and many acres of cotton were planted later than normal. Due to higher than normal precipitation, early pre-plant and pre-emergent herbicides may have dissipated enough to allow for weeds to begin germinating before or shortly after cotton was planted, reducing their effectiveness.