Date of Award


Degree Name

Master of Science


Plant and Soil Science

First Advisor

Young, Bryan


The expression of glyphosate resistance can vary within single field populations of common waterhemp and Palmer amaranth. This variability in expression can translate into control ranging from 20 to 80%, which could be the difference in a minor versus a major failure in weed management. Certain application parameters that have been previously associated with glyphosate efficacy, such as glyphosate application time of day and plant stress may exacerbate this variability and lead to failed control of plants on the lower end of the resistance spectrum. Greenhouse research was conducted in 2011 to determine the influence of glyphosate application time of day on the expression of resistance in common waterhemp and Palmer amaranth. Control of both glyphosate-susceptible (GS) and -resistant (GR) weed species showed similar trends in response to glyphosate with respect time of application. Decreased sensitivity of all Amaranthus biotypes was greatest at 9:00 pm and may be attributed to an observed shift in leaf orientation from horizontal to vertical at the time of glyphosate application in response to low-light conditions. The altered leaf orientation most likely reduced herbicide spray coverage. The magnitude of resistance, the difference in the sensitivity of the resistant versus susceptible biotypes, was unaffected by glyphosate application time of day; however, these results indicate that even in resistant populations glyphosate applied at suboptimal times of day such as the evening can cause a further increase in weed escapes from glyphosate. Greenhouse and field experiments were conducted in 2011 and 2012 to determine the influence the soil nutrient amendments on glyphosate sensitivity and growth rate and of GS and GR common waterhemp and Palmer amaranth. In both the GR and GS biotypes of common waterhemp the sensitivity to glyphosate was increased as fertilizer was introduced. However, only the sensitivity of the susceptible biotype of Palmer amaranth was increased with the addition of fertilizer. The lack of response in the GR Palmer amaranth population to fertilizer can be associated with the fact that due to carrier volume limitations enough glyphosate could not be applied to achieve 50% control. The magnitude of resistance was decreased numerically with the addition of fertilizer in both weed species; however, only in common waterhemp was the magnitude of resistance significantly different with the use of high rates fertilizer. The use of fertilizer also had an influence on the growth rate and dormancy of axillary buds. Lateral branching (broken dormancy in axillary buds) was increased in both common waterhemp and Palmer amaranth with the addition of fertilizer. Converting dormant buds to active meristems favors glyphosate translocation and could be responsible for increased glyphosate efficacy. In the field, glyphosate efficacy in GR common waterhemp and Palmer amaranth was also increased with addition of fertilizer; however, this effect was variable. Optimizing the efficacy of glyphosate when applied to even mixed populations of GS and GR Palmer amaranth and common waterhemp can reduce surviving weeds that can produce seed and perpetuate the frequency of glyphosate resistance in the field. Furthermore, greater efficacy of glyphosate may translate into relatively less significant failures in glyphosate applications allowing for successful rescue herbicide treatments and minimal impact on crop yield compared with a complete glyphosate failure with dramatic implications on reduced crop yield and increased weed seed production.




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