Date of Award

8-2014

Document Type

Thesis

Degree Name

Master of Science (MS)

Legacy Department

Plant and Environmental Science

Advisor

Marshall, Michael W

Committee Member

Tharayil , Nishanth

Committee Member

Monfort , Scott

Abstract

Weeds are the most limiting factor on soybean yields in South Carolina. With their early emergence and rapid growth, weeds compete with crops for resources. The recent evolution of herbicide resistant weeds has made it increasingly difficult for growers to actively control weeds in fields. Glyphosate and ALS-resistant Palmer amaranth (Amaranthus palmeri S. Watson) biotypes have spread rapidly throughout South Carolina, especially in areas where resistance management isn't practiced. In the near future, soybean varieties will be introduced with tolerance to 2,4-D (Dow AgroSciences) and dicamba (Monsanto Company). Field and greenhouse experiments were conducted at Edisto Research and Education Center located near Blackville, SC in 2012 and 2013 to evaluate 2,4-D and dicamba-based herbicide programs for weed management in soybean. Overall, the 2,4-D based herbicide treatments were effective in controlling weeds 2 weeks after second post emergence (WAP2). 2,4-D plus glyphosate premixture provided excellent Palmer amaranth, pitted morningglory [Ipomoea lacunosa (L.)] and large crabgrass [Digitaria sanguinalis (L.) Scop.] with > 95% control at 2 WAP2. A lack of soil moisture in 2012 caused a decrease in pitted morningglory control because s-metolachlor plus fomesafen was not activated due to lack of adequate soil moisture. In general, dicamba alone preemergence (PRE) application wasn't as effective as flumioxazin alone PRE. Dicamba PRE followed by glyphosate plus dicamba premixture POST1 gave excellent control (> 97%) 2 WAP1. In the greenhouse, glufosinate alone treatments provided the best control of Palmer amaranth, pitted morningglory and sicklepod [Senna obtusifolia (L.) Irwin and Barneby]. Synergism was observed when sicklepod was treated with glufosinate plus dicamba resulting in greater control than if either compound was applied alone. Glyphosate alone treatments provided the least control of all 3 weeds at all heights. Results from these studies demonstrated that Palmer amaranth was effectively controlled with auxinic herbicide mixtures. Also, flumioxazin PRE followed by glyphosate plus dicamba premixture POST1 and POST2 provided excellent Palmer amaranth and large crabgrass control ( > 99%) 2 WAP2. Glyphosate plus 2,4-D premixture provided excellent control of all 3 weed species evaluated. Based on the herbicide programs evaluated in these studies, herbicide resistant weeds, such as Palmer amaranth, can be effectively controlled when treated at the correct growth stage.

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