Date of Award
Master of Science
Plant and Soil Science
Field and greenhouse research was conducted in 2009 and 2010 on herbicide applications enabled by the integration of Optimum GAT crop traits providing for resistance to glyphosate and certain ALS-inhibiting herbicides. The herbicide concepts were evaluated for control of several winter and summer annual weed species, as well as the effect of the resulting weed control on grain yield of Optimum GAT soybean. The combination of chlorimuron + rimsulfuron did not provide sufficient efficacy on the winter annual grass species little barley and annual bluegrass. Factors contributing the sub-lethal activity include: 1) a relatively low inherent sensitivity of the species to these herbicides, 2) a significant reduction in herbicide efficacy with increases in weed plant height, and 3) a lack of herbicide enhancement with more aggressive foliar adjuvants. The tank-mixture of glyphosate with chlorimuron + rimsulfuron was frequently necessary to achieve a maximum herbicide activity above 90% on annual bluegrass and little barley. Optimum GAT herbicide treatments including chlorimuron + rimsulfuron + flumioxazin in field experiments provided the greatest control of horseweed and common waterhemp in glyphosate-susceptible and -resistant populations. The addition of chlorimuron + rimsulfuron to glyphosate and 2,4-D improved horseweed control above glyphosate and 2,4-D applied alone even as weed height increased with applications made closer to soybean planting. However, removal of competitive vegetation with herbicide combinations including chlorimuron + rimsulfuron selected for emergence of ALS-resistant common waterhemp. Inclusion of flumioxazin with chlorimuron + rimsulfuron was beneficial for control of common waterhemp when applied 7 days before planting. However, chlorimuron + rimsulfuron + flumioxazin provided only 80% control of common waterhemp in a glyphosate-resistant population which demonstrates opportunity for improvement in herbicide concepts enabled by Optimum GAT. Grain yield of Optimum GAT soybean was greatest for herbicide treatments which provided effective weed management throughout the growing season which were the herbicide treatments applied the closest to soybean planting (7 days before planting). Optimum GAT herbicide concepts for corn include chlorimuron + thifensulfruon + tribenuron, chlorimuron + rimsulfuron, and rimsulfuron + tribenuron + mesotrione. These herbicides provided similar to slightly increased control of annual morningglory (Ipomoea spp.) in comparison to glyphosate alone. The addition of atrazine increased the consistency of control of annual morningglory for any herbicide treatment with additional residual activity at 28 days after treatment. Optimum GAT enabled herbicide concepts can improve control of some problematic weed species, including some glyphosate-resistant weed populations, compared to current herbicide tactics that rely primarily on glyphosate for weed control in commercial glyphosate-resistant soybean and corn. However, the integration of postemergence soybean herbicides beyond the ALS chemistry is necessary to provide a broader spectrum of weed control when considering the challenges of managing both glyphosate- and ALS-resistant weed species that are becoming more frequent in commercial fields.
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