WEED-CONTROL USING REDUCED RATES OF POSTEMERGENCE HERBICIDES IN NARROW AND WIDE ROW SOYBEAN

Field studies were conducted in 1994 and 1995 to examine the effects o f soybean [Glycine mw (L.) Merr.] row spacing and application rate and timing of four postemergence herbicide tank mixtures on weed control and soybean yield. Weed control and soybean yield were greater in narr ow rows (7.5 in.) than wide rows (30 in.). Herbicide tank mixtures app lied at 25% of the full recommended rate at an early postemergence tim ing followed by a second 25% application at a standard postemergence t iming (1/4x E Post + 1/4x Post) resulted in weed control and soybean y ield equal to that of herbicide tank mixtures applied at the full reco mmended rate at a standard postemergence timing (Ix Post). Three of fo ur tank mixtures in 1994 and two of four in 1995, applied at 50% of th e full rate applied at a standard postemergence timing (1/2x Post) res ulted in weed control and soybean yield equal to that of lx Post appli cations. All tank mixtures applied at 50% of the full rate at an early postemergence timing (1/2x E Post) resulted in poor weed control and low soybean yield. In most cases it was more profitable to plant soybe an in narrow rows than wide rows regardless of application rate or tim ing, based on economic gross margin calculations. Gross margins of tan k mixtures applied at 1/4x E Post + 1/4x Post were similar to or great er than the gross margin of the same tank mixture applied at the full rate in 13 of 16 cases. Gross margins of tank mixtures applied at 1/2x Post were similar to or greater than the gross margin of the same tan k mixture applied at the full rate in eight of 16 cases.