Updated June 28, 2024:
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With the number of soybean acres increasing each year in southwest North Dakota, more information is needed on weed control methods for this important rotational crop. Weed control is vital for successful production of soybean in southwest North Dakota. In the no-till cropping system commonly used in southwest North Dakota, it is vitally important that weeds are controlled at planting to reduce competition for limited resources such as soil moisture and mineral nutrients. Many acres of soybean in southwest North Dakota will be with the glyphosate-resistant varieties developed through the NDSU Soybean Breeding Program. With glyphosate-resistant soybean, it will be very important to use preemergence herbicides at or before planting as postemergence options are limited and many weeds have already shown tolerance to glyphosate. This is especially true with weeds such as kochia, wild buckwheat, and common mallow, which are common in southwest North Dakota. Thus, it is vitally important that weeds be controlled prior to or at planting through application of herbicide combinations with postemergence and preemergence activity. Trials will be conducted to evaluate timing of preplant herbicides for weed control and herbicide tolerance with the following objectives: 1) Evaluate pre-plant herbicides for control of kochia, wild buckwheat, and other annual weeds in soybean, 2) Evaluate response of soybean to pre-plant herbicides, and 3) Evaluate soybean yield in response to herbicide applications for annual weed control in soybean.
A trial was conducted to evaluate timing of preemergence (PRE) herbicide application of herbicides commonly used in soybean in a no-till field at Hettinger, North Dakota. The previous crop at this site was spring wheat and there was a natural population of kochia (Bassia scoparia), common lambsquarters (Chnopodium album), wild buckwheat (Fallopia convolvulus), and green foxtail (Setaria veridis). Soil type at this location was a Wyola silty clay loam; 16% sand, 54% silt, and 30% clay, with a pH of 6.9 and organic matter at 2.9%. The location had been in continuous no-till production for more than 20 years.
PRE herbicide treatments to be compared for efficacy at different application timings included Valor (flumioxazin), Valor plus Zidua SC (pyroxasulfone), BroadAxe XC (sulfentrazone plus metolachlor), Authority Supreme (sulfentrazone plus pyroxasulfone), and Authority MTZ (sulfentrazone plus metribuzin) (Table 2). These treatments were tank-mixed with glyphosate (Roundup PowerMax 3) plus AMS to aid in the control of emerged weeds. These treatments were applied preplant (PPL) at 2 and 1 weeks prior to planting, and PRE, after soybean were planted. These treatments were compared with other standard treatments including: dicamba (Xtendimax) plus glyphosate applied PRE and as a POST treatment when soybean were at the V1 growth stage, with glyphosate applied PRE followed by a POST application at the V1 soybean growth stage, and with BroadAxe SC plus glyphosate applied PRE followed by glyphosate POST, and with an untreated control. Treatment dates for these different applications were May 10, May 19, May 30, and June 19, 2023. The entire trial was treated with glyphosate to R1 soybean following evaluation of weed control. Herbicide treatments were applied using a tractor-mounted research plot sprayer measuring 100 inches wide. With the exception of the dicamba treatments, herbicides were applied using TeeJet DG10002 flat fan nozzles and a spray volume of 10 gallons per acre. Dicamba treatments were applied at 15 gallons per acre using TeeJet TTI11003 air induction nozzles that meet requirements for Xtendimax application in Xtend soybean.
Soybean was seeded on May 26, 2023, using a John Deere row crop no-till planter with soybean seed plates. Soybean ‘XtendFlex 01072322’ was seeded at 110,000 seeds per acre at a depth of 1.5 inches in 30-inch rows; soybean seed inoculum was added to the seed box. Soybean emergence was on June 3. Soybean stand count was measured on June 20, 17 days after emergence (DAE) by counting the number of soybean plants in a meter row length of the two middle rows at two locations in each plot; counts were converted to plants per acre. Soybean height was measured on July 5, 32 days after soybean emergence by measuring the height of 10 random plants in each plot. Weed control was evaluated visually on a scale from 0 to 100, with 0% being no control and 100% being complete death of weed being evaluated; weed control was evaluated on June 27 (8 days after the POST application) and July 19 (30 days after the POST application).
Valor plus glyphosate provided only poor or fair control of kochia, but good to excellent control of common mallow, common lambsquarters, and wild buckwheat. Control of green foxtail and wild oat was best at the PRE timing as it had not full emerged at the earlier application dates and control was likely from glyphosate rather than flumioxazin. The addition of Zidua SC to the Valor plus glyphosate treatment improved control of kochia, green foxtail, and wild oat. Adequate rainfall was received for activation of Zidua; Four inches of rain fell in the week following the 2WPPL application, but only 0.1 inches of rain fell in the week after the 1WPPL application, while 0.69 inches of rain fell in the week following the PRE application. Rainfall is vitally important for weed control when using preemergence herbicides in no-till cropping systems. BroadAxe XC provided good control of kochia and excellent control of common mallow, common lambsquarters, and wild buckwheat, and good control of green foxtail at the 2WPPL and PRE applications, but poor green foxtail control for the 1WPPL. Poor control of green foxtail at this timing was likely due to low rainfall during the week following this application timing. Wild oat control was best at the PRE timing as it had not emerged at the two earlier timings. Authority Supreme and Authority MTZ both provided good to excellent control of kochia, common lambsquarters, and common mallow. At the earlier application timings, Authority Supreme controlled both green foxtail and wild oat better than Authority MTZ, but control was similar at the PRE application timing. Xtendimax plus glyphosate applied PRE provided good control of all broadleaf weeds, but only fair control of grass, likely because grasses continued to emerge after application; delaying this application to the V1 stage of soybean improved control of common mallow, green foxtail, and wild oat. Two applications of glyphosate, with and without BroadAxe provided good to excellent control of all weeds in this trial. Soybean stand count was not affected by any of the treatments in this trial (data not shown). Soybean height was generally greatest in soybean with poorer weed control as the soybean tried to outgrow neighboring weeds. Soybean yield was highest in treatments where weeds were controlled best, improving from 20 bu/A in the untreated soybean to 34 bu/A in the highest yielding treatments. Growing conditions were better than average for southwest North Dakota due to above normal rainfall (>20 inches) during the growing season. In years with rainfall that is closer to long-term averages, weed competition may have a larger impact of soybean yield than what occurred in 2023. Controlling weeds is vitally important when growing soybeans in order to achieve optimal yields. Weed control strategies that include both preemergence and postemergence herbicides will generally be most successful and reduce risk of weed control failures.