As growers look for more ways to maximize soybean yields and profits, many are considering a fall herbicide application to save time and control winter annuals. A reduction in fall tillage and a decrease in reliance on in-season, residual herbicides that had the efficacy to control many of the winter annuals in the fall have made fall herbicide applications more of a consideration than in the past. Milder winters of late have also contributed to the need for winter weed control, as this warmer weather increases the winter growing season of many difficult-to-control weeds.
The Benefits of a Fall Application
At first glance, this time management strategy has its advantages including the option for growers, especially those with a large number of acres, to start planting earlier in the spring. Fall-applied herbicides are designed to save growers time by letting them make their herbicide application during a less hectic time of year. In addition, a fall application removes weeds while they are actively growing in the fall instead of towards the end of their life cycle in the spring. The removal of weeds before planting allows soil to warm and dry faster because sunlight is not being blocked by weed coverage.
Residual Control, Application Flexibility Increased from Spring-Applied Herbicides
Although fall-applied herbicides are good in theory, in practice this strategy can have some problems. For starters, a fall-applied soybean herbicide does not provide the residual control necessary to keep early-season weed pressure down. In some cases, fall applications are subject to decomposition by soil microorganisms over the winter, resulting in reduced protection in the spring during the critical soybean development stages. According to research published by Dr. Reid Smeda of the University of Missouri in his 2000 Fall Applied Herbicide Programs for Soybeans, residual control of fall-applied herbicides was not as good as spring-applied herbicides.
“Residual activity of fall-applied herbicides was short, and giant foxtail and common waterhemp control was approximately 30 to 60% lower compared to pre-plant herbicide applications," Smeda stated in a 2000 paper. “As a result, postemergence herbicide applications were necessary sooner for fall applied versus pre-plant treatments."
Don Porter, technical brand manager for Syngenta Crop Protection, agrees. “Even if a grower has spent money on a residual herbicide in the fall, in most cases he still has to come back and burn that field down again in the spring because residual products available for use with soybeans simply don’t last long enough to control every weed in that field from October/early November through planting. Even though you may see greater weed control out of a residual product over the winter, you’ve spent money that contributes nothing to weed control in the soybean crop when you need it the most."
This wider application window for a late-season glyphosate application can really pay off. According to Dr. Bill Johnson, associate professor of weed science at Purdue University, “a pre-emergence herbicide buys time earlier in the season, allowing growers to apply in a more convenient timeframe. It also helps by giving growers a wider application window for their late-season glyphosate pass, allowing them a later application in many cases. Finally, by applying a pre-emergence herbicide, growers minimize the likelihood of glyphosate resistance becoming a problem because of the different modes of action in pres."
In addition, pres help start the season off with a clean field, which means better yields at harvest. “In moderate- to heavy-density weed coverage, weeds can start reducing soybean yields as early as the V3 growth stage," says Johnson. “Weather delays and increased application time due to larger fields can delay application, which can be costly to an operation. By the time weeds are 12 to 16 inches tall, crops have already suffered a 10-20 percent yield loss."
Focus on Resistance Management
Most fall herbicides available for use with soybeans are ALS-inhibitors. However, ALS-resistant weeds, such as pigweed and waterhemp, are located in areas across the Midwest, leading growers and researchers to consider alternative modes of action. Many of the same weeds that are ALS-resistant are also glyphosate resistant, making effective weed control virtually impossible if using an ALS-inhibitor herbicide followed by glyphosate.
“For growers looking to control winter annuals throughout the off-season, a low-cost, non-residual product might be the best solution," says Porter. “Glyphosate or 2,4-D are all cost-effective weed control options to burndown winter weeds, but growers should consider saving their glyphosate