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Reduced Preplant-Incorporated Imazaquin Rates for Broadleaf Weed Control in Soybean

Dep. of Agronomy, Univ. of Kentucky, Lexington, KY 40546-0091

^* Corresponding author.

Public pressure and economic distress have together provided a strong incentive for reduced chemical use in U.S. agriculture. This field study was designed to determine if preplant-incorporated imazaquin {2-[4,5-dihydro-4-methyl-4-(1-methylethyl)-5-oxo-lH-imidazol-2-yl]-3-quinolinecarboxylic acid} rates could be reduced from the labeled rates when applied with alachlor [2-chloro-N-(2,6-diethylphenyl)-N-(methoxymethyl) acetamide] without incurring yield penalties due to broadleaf weed infestations. Imazaquin was applied at 0, 47, 93, and 141 g ha^–1 in earlyplanted soybean [Glycine max (L.) Merr.] studies in Fayette County in 1989, 1990, and 1991 and in Simpson County (1989), Hardin County (1990), and Hopkins County (1991), Kentucky. Preplant-incorporated alachlor was followed by postemergence fluazifop-P-butyl {butyl(R)-2-[4[[5-(triliuoromethyl)-2-pyridinyl]oxy]phenoxy]propanoate} (as needed) to minimize grass competition. Perennial broadieaf weeds were controlled with hand-wicked glyphosate [isopropylamine salt of N-(phosphonomethyl) glycine]. Acceptable broadleaf control was obtained at sub-label imazaquin rates, except for Fayette 1991, where ivyleaf morning-glory [Ipomoea hederacea (L.) Jacq.] and velvetleaf (Abutilon theophrasti Medic.) were planted. Two studies (Simpson 1989 and Hopkins 1991) showed minimal broadleaf weed pressure at any imazaquin rate; this may have been a result of the early planting dates used. The common lambsquarters (Chenopodium album L.) and Pennsylvania smartweed (Polygonum pennsylvanicum L.), which dominated in Fayette 1989 and 1990, were successfully controlled with 47 g ha^–1 imazaquin. While alachlor provides some control for these two species, their competition with soybean was severe without the addition of imazaquin. Producers who can accumulate detailed knowledge of the broadleaf weed species present in their soybean fields could potentially use such information to select herbicides likely to control their broadleaves at sub-label rates.

Received for publication March 29, 1993.