Comparison of Total Postemergence Weed Control Programs in Soybean

Abstract

Field studies were conducted in 1994 and 1995 in central and southern Illinois to compare several total postemergence weed control programs in soybean [ Glycine max (L.) Merr.]. Herbicide programs evaluated were imazethapyr (an acetolactate synthase (ALS] inhibiting herbicide) applied alone or in combination with lactofen and two non‐ALS herbicide programs consisting of combinations of bentazon, acifluorfen, and sethoxydim and combinations of fomesafen, fluazifop, and fenoxyprop. These treatments were applied early postemergence (EPOST, V‐1 soybean—first trifoliate) and postemergence (POST, V‐2 soybean—second trifoliate). Non‐ALS herbicide programs generally provide more effective weed control POST, while weed control with imazethapyr tended to be greater EPOST. Non‐ALS herbicide programs applied POST provided weed control levels that were equal to imazethapyr in three out of four experiments. In 1994 at Brownstown, broadleaf weed control was poor with non‐ALS herbicide programs when weed growth stages were larger and environmental conditions more extreme than other experiments. Adding lactofen to imazethapyr increased broadleaf weed control in some instances but decreased giant foxtail ( Setaria faberii L.) control. Imazethapyr plus lactofen tended to produce the greatest degree of soybean injury. Research Question Imaiethapyr herbicide is applied primarily postemergence for weed control in soybean and is effective in controlling a broad spectrum of grass and broadleaf weeds. The recent occurrence of weed biotypes in the Midwest resistant to acetolactate synthase (ALS) inhibiting herbicides may cause growers to use alternative, non‐ALS herbicides for weed control. The research reported in this paper was designed to compare two non‐ALS total postemergence herbicide programs to imazethapyr applied alone or in combination with lactofen applied EPOST (early post emergence) and POST (postemergence) for overall weed control and soybean tolerance. Literature Summary Total postemergence herbicide programs for weed control in soybean are widely used throughout the Midwest. Imazethapyr, which controls a broad spectrum of grass and broadleaf weeds, is used on approximately 40 to 50% of the soybean acres in the Midwest. Imazethapyr is generally the only herbicide treatment applied to the majority of these acres. Imazethapyr controls weeds by inhibiting the enzyme ALS. Recently, ALS resistant weed biotypes have been detected throughout the Midwest, which may force growers to switch to non‐ALS herbicide programs or to apply ALS herbicides in combination with non‐ALS herbicides. Decreasing reliance on ALS herbicides or applying herbicides with different modes of action may also help slow the development and spread of ALS resistant weeds. Recently, two non‐ALS total postemergence herbicide programs have been developed using a silicone adjuvant or an internal proprietary adjuvant system. However, information on the performance of these total postemergence systems compared with imazethapyr applied alone or in combination with lactofen (a non‐ALS herbicide) is limited. Study Description Two non‐ALS total postemergence herbicide programs, imazethapyr and imazethapyr plus lactofen were applied EPOST (V‐1 soybean) and POST (V‐2 soybean) to soybean planted in 30‐in. rows. Non‐ALS herbicide programs consisted of bentazon plus aciflourfen (Galaxy), applied at 0.92 lb/acre combined with sethoxydim plus DASH spray adjuvant (Poast Plus) applied at 0.19 lb/acre. A silicone spray adjuvant (Sylguard 309) was added to this treatment at 0.25% v/v. Fomesafen formulated with an internal proprietary adjuvant system (Flexstar), was applied at 0.31 lb/acre combined with fluazifop and fenoxyprop (Fusion) applied at 0.21 lb/acre. Urea ammonium nitrate (UAN, 28%) was added at 2.5% v/v. Imazethapyr was applied alone with nonionic surfactant (NIS) at 0.25% v/v and 2.5% v/v UAN or in combination with 0.13 lb/acre lactofen. NIS was replaced with crop oil concentrate at 0.5% v/v with the lactofen Combination. Soybean injury was visually estimated 10 d after treatment (DAT). Weed control was visually evaluated and soybean height determined 30 d after POST applications. Soybean grain yield was determined at maturity. Applied Questions What is the optimum application timing for the four herbicide treatments? The optimum application timing for the non‐ALS herbicide programs was POST targeting weeds ranging from 3 to 4 in. in height due to the lack of significant soil residual with these treatments. In contrast, the optimum application timing for imazethapyr was EPOST, targeting 1 to 3 in. weeds and most likely due to the significant soil residual with imazethapyr. In some cases, soybean injury with imazethapyr or imazethapyr plus lactofen was lower POST than EPOST. Did non‐ALS herbicide programs applied POST provide equal levels of weed control as compared with imazethapyr or imazethapyr plus lactofen applied EPOST? Non‐ALS herbicide programs applied POST provided weed control levels that were equal to imazethapyr applied EPOST in three out of four experiments. In 1994 at Brownstown, broadleaf weed control was poor with non‐ALS herbicide programs when weed growth stages were larger and environmental conditions more extreme than with other experiments. Adding lactofen to imazethapyr increased broadleaf weed control in some instances but decreased giant foxtail control. Imazethapyr plus lactofen tended to produce the greatest degree of soybean injury. Growers who choose to use imazethapyr and add lactofen to control ALS resistant weed biotypes may have to control giant foxtail with alternative methods or apply lactofen separately from imazethapyr.

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