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Weed Control Using Reduced Rates of Postemergence Herbicides in Narrow and Wide Row Soybean

1997; Wiley; Volume: 10; Issue: 3 Linguagem: Inglês

10.2134/jpa1997.0431

2689-4114

James A. Mickelson, Karen A. Renner,

Soybean genetics and cultivation

Field studies were conducted in 1994 and 1995 to examine the effects of soybean [Glycine max (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 narrow rows (7.5 in.) than wide rows (30 in.). Herbicide tank mixtures applied at 25% of the full recommended rate at an early postemergence timing followed by a second 25% application at a standard postemergence timing (1/4x E Post + 1/4x Post) resulted in weed control and soybean yield equal to that of herbicide tank mixtures applied at the full recommended rate at a standard postemergence timing (1x Post). Three of four tank mixtures in 1994 and two of four in 1995, applied at 50% of the full rate applied at a standard postemergence timing (1/2x Post) resulted in weed control and soybean yield equal to that of 1x Post applications. 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 soybean in narrow rows than wide rows regardless of application rate or timing, based on economic gross margin calculations. Gross margins of tank mixtures applied at 1/4x E Post + 1/4x Post were similar to or greater 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 tank mixture applied at the full rate in eight of 16 cases. Research Question Weed control programs that use reduced rates of postemergence herbicides offer the potential to lower producer input costs while maintaining weed control and soybean yield. Information on reduced rate herbicide programs for soybean growers in the northern production region of the USA is lacking. This research was initiated to determine the potential for using reduced rates of postemergence herbicides for weed control in this region. The objectives of this research were (i) to evaluate differences in weed control between narrow and wide row soybean using full and reduced postemergence herbicide rates and (ii) to determine whether planting soybean in narrow rows can increase the effectiveness of reduced rate postemergence herbicide programs. Literature Summary Weed control equal to using full rates has been achieved with 25 and 50% of the recommended rates of postemergence herbicides in conventionally tilled soybean. Reduced rate treatments have been applied at earlier-than-normal application timings when weeds are smaller and therefore more susceptible to postemergence herbicides. However, with early applications there is greater opportunity for weeds to emerge after the herbicide application and go uncontrolled due to a lack of residual soil activity with most postemergence herbicides. A sequential application of 25% of the full rate applied early followed by a second 25% application has provided the most consistent weed control when using reduced rates. Most of the research examining the efficacy of reduced herbicide rates has been conducted in wide row soybean, often in combination with cultivation to aid weed control. However, soybean planted in narrow rows can result in greater season long weed control than wide row soybean due to a faster closure of the soybean canopy, which results in greater shading and weed suppression. Also, planting soybean in narrow rows can result in higher soybean yield due to increased light interception by the soybean canopy. Therefore, planting soybean in narrow rows is an option that could increase weed control in a reduced rate postemergence herbicide program and maintain or increase soybean yield. Two aspects studied in this research have not been reported: applying reduced rates of herbicide tank mixtures containing a grass and two broadleaf herbicides in order to control a broad spectrum of weed species, and secondly, directly comparing the efficacy of reduced herbicide rate treatments in narrow and wide row soybean. Study Description A field experiment was conducted in 1994 and 1995 at East Lansing, MI. Four herbicide tank mixtures were each applied at four application rate/timing combinations to narrow (7.5 in.) and wide (30 in.) row soybean. Herbicide tank mixtures: clethodim + thifensulfuron + lactofen + nonionic surfactant fluazifop-P&fenoxaprop + thifensulfuron + fomesafen + nonionic surfactant + urea ammonium nitrate quizalofop + bentazon/acifluorfen + crop oil concentrate quizalofop + thifensulfuron + chlorimuron + nonionic surfactant + urea ammonium nitrate Rate/timing combinations: 25% applied at an early postemergence timing + 25% applied at a standard postemergence timing (1/4x E Post + 1/4x Post). 50% applied at an early postemergence timing (1/2x E Post). 50% applied at a standard postemergence timing (1/2x Post). 100% applied at a standard postemergence timing (1x Post). Application of early postemergence treatments was targeted to weeds 1 in. tall. Application of standard postemergence treatments was targeted to weeds 4 in. tall. Weed control and soybean yield data were collected and analyzed. An economic analysis compared the gross margin of reduced rate treatments to full rate treatments. Applied question Can applications of reduced rates of postemergence herbicide tank mixtures consistently provide effective weed control and maintain soybean yield? Herbicide tank mixtures applied at 1/4× E Post + 1/4× Post were the most effective reduced rate treatments, consistently resulting in weed control and soybean yield equal to that of 1x Post treatments. Herbicides applied at 1/2x E Post did not control annual grass, common ragweed, or velvetleaf due to weed emergence after herbicide application and resulted in low soybean yield. Herbicide tank mixtures applied at 1/2x Post resulted in weed control and soybean yield equal to that of 1x Post treatments in some cases, depending on herbicide tank mixture. Weed control and soybean yield were greater in narrow row soybean. There was 30% less weed biomass present in narrow row soybean than in wide row, which resulted in 14% higher soybean yield, when averaged across all postemergence herbicide treatments. It was more profitable to plant soybean in narrow rows than wide rows regardless of herbicide tank mixture, or application rate or timing, based on economic gross margin calculations. Gross margins of tank mixtures applied at 1/4x E Post + 1/4x Post were similar to or greater 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 tank mixture applied at the 111 rate in eight of 16 cases.