Weed Management in Wide- and Narrow-Row Glyphosate Resistant Soybean

Abstract

Field experiments were conducted in 1996 and 1997 at East Lansing, MI, to evaluate weed management programs in soybean [Glycine max (L.) Merr.] planted in 30 in. and 7.5 in. rows. Efficacy of glyphosate applied alone, with ammonium sulfate (AMS), flumiclorac, or imazethapyr was compared with preemergence and postemergence weed management programs used in nontransgenic soybean. Glyphosate provided consistent weed control in wide- and narrow-row soybean in both years. Common ragweed (Ambrosia artemisiifolia L.) and velvetleaf (Abutilon theophrasti Medicus) control decreased when flumiclorac was tank mixed with glyphosate compared with glyphosate alone in wide-row soybean. Weed control was usually greater in narrow-than in wide-row soybean for treatments without glyphosate. Soybean yield with most herbicide treatments equaled the weed-free control in narrow-rows, and gross margins for soybean planted in narrow-rows were greater than wide-rows. Yield in wide-row soybeans was reduced compared with the weed-free control with all treatments except glyphosate, glyphosate + AMS, or glyphosate + imazethapyr. Glyphosate and glyphosate + AMS had the greatest gross margins for both years. Research Question Increasing weed control costs and low soybean prices have created the need to develop weed control programs that are not only efficacious but also cost-effective. The objectives of this study were to compare soybean response, weed control, soybean yield, and gross margins of postemergence glyphosate treatments,. reduced sequential application and manufacturer's recommended use rates of postemergence herbicide tank mixtures, and preemergence followed by postemergence weed management strategies in both narrow- and wide-row soybean. Literature Summary Producers are concerned about production costs and economic return from weed management strategies. Research suggests that higher soybean yields are often associated with soybean production in narrow rows rather than wide rows. Additionally, herbicides applied at reduced rates in narrow-row soybeans can be effective. In other research, preemergence followed by postemergence weed control systems improved the consistency of weed control. The introduction of herbicide resistant crops has provided producers with new weed management options. Research suggests that glyphosate applied to glyphosate-resistant soybean can be an effective alternative to standard weed management programs. Study Description ‘Asgrow 2701’ Roundup Ready soybean was planted in 30 in. (wide) and 7.5 in. (narrow) rows in 1996 and 1997. The experimental design was a randomized complete block split-plot with row spacing as the main plot and subplot herbicide treatments. Herbicide treatments included glyphosate, glyphosate + ammonium sulfate (AMS), glyphosate + flumiclorac, glyphosate + imazethapyr, reduced and the manufacturer's recommended use rates of fomesafen + thifensulfuron + fluazifop/fenoxaprop and quizalofop + thifensulfuron + chlorimuron, pendimethalin followed by imazethapyr + acifluorfen, and the commerical package mixture of chlorimuron and metribuzin followed by quizalofop. Soybeans were harvested for yield and gross margin was calculated as the difference between the gross receipts and weed management costs. Applied Questions Did the addition of AMS, flumiclorac, or imazethapyr to glyphosate influence weed control or soybean yield? Control of common ragweed and velvetleaf was reduced when flumiclorac was added to glyphosate in wide-row soybean. The addition of AMS, flumiclorac, or imazethapyr to glyphosate did not increase weed control or soybean yield. A single glyphosate application provided excellent weed control in narrow- and wide-row soybeans in both years. How effective were reduced rates of postemergence herbicides compared with the manufacturer's recommended use rate? Fomesafen + thifensulfuron + fluazifop/fenoxaprop at reduced rates controlled broadleaf weeds, but giant foxtail control was acceptable in 1 of 2 yr. Reduced rates of quizalofop + thifensulfuron + chlorimuron applied sequentially controlled common ragweed more effectively than a single application of the manufacturer's recommended use rate in 1996. Soybean yield was similar to the weed-free control for the reduced application rates in narrow-row soybean only. Did preemergence followed by postemergence weed management programs provide consistent weed control and soybean yields? The preemergence followed by postemergence weed management programs provided more consistent weed control compared with all of the total postemergence programs except those containing glyphosate. Soybean yield with preemergence followed by postemergence weed management programs was similar to the weed-free control in narrow-rows only. Was it more economical to produce soybean in wide- or narrow-row soybean? Narrow-row soybean had a $26 and $18/acre greater gross margin than wide-row soybean in 1996 and 1997, respectively, when averaged over all weed management programs. Recommendation Soybean planted in wide-rows and treated with glyphosate provided consistent weed control and high gross margins in both years. In narrow-rows, glyphosate, reduced rates, manufacturer's recommended use rate, and the commercial package mixture of chlorimuron and metribuzin followed by quizalofop provide soybean yields equal to the weed-free control. Reduced rates are not supported by the manufacturer's label and require two timely applications. Cost of the herbicide and soybean seed, time available for herbicide applications, and the risk a grower is willing to assume in making sequential applications of herbicides at reduced rates should be considered when choosing a weed management program.