Seed Production and Control of Sicklepod (Senna obtusifolia) and Pitted Morningglory (Ipomoea lacunosa) with 2,4-D, Dicamba, and Glyphosate Combinations

Abstract

Sicklepod and pitted morningglory are two of the most important weed species in row-crop production in the southeastern United States. The upcoming introduction of soybean and cotton varieties resistant to 2,4-D and dicamba will increase the reliance on these auxinic herbicides. However, it is not clear how these herbicides will affect sicklepod and pitted morningglory control. Field experiments were conducted in 2013 and 2014 in Jay, FL to determine whether 2,4-D (560 and 1,120 g ae ha−1), dicamba (420 and 840 g ae ha−1), and glyphosate (1,060 g ae ha−1) alone or in combination applied when weed shoots were 11 (early POST [EPOST]) and 22 (late POST [LPOST]) cm long effectively control and prevent seed production of sicklepod and pitted morningglory. LPOST provided more effective control of sicklepod than EPOST. This was attributed to emergence of sicklepod seedlings after the EPOST application. When glyphosate was tank mixed with 2,4-D or dicamba, sicklepod control was higher (78 to 89% and 87 to 98% in 2013 and 2014, respectively) than for single-herbicide treatments (45 to 77% and 38 to 80% in 2013 and 2014, respectively) 6 wk after treatment (WAT). Pitted morningglory control was not affected by application timing, and 2,4-D provided 91 to 100% 6 WAT, which was equivalent to treatments with tank mixtures containing glyphosate. Dicamba applied at 420 g ha−1had the lowest pitted morningglory control (44 to 70% and 82 to 86% in 2013 and 2014, respectively). Sicklepod and pitted morningglory plants that survived and recovered from herbicide treatments produced the same number of viable seeds as nontreated plants in most treatments. The results of the present study indicated that the use of 2,4-D and dicamba alone will not provide adequate extended control of sicklepod, and the use of tank mixtures that combine auxinic herbicides with glyphosate or other POST herbicides will be necessary to manage sicklepod adequately in 2,4-D- or dicamba-resistant soybean and cotton. Because sicklepod plants that survived a single herbicide application are capable of producing abundant viable seeds, integrated approaches that include PRE herbicides and sequential POST control options may be necessary to ensure weed seed bank reductions.