Pigweed control in soybeans used to be easy before the development and increase in glyphosate resistance, but those days are now in the past. Many fields have been overgrown with pigweed in recent years, however the pigweeds weren’t quite as bad in 2017 as in previous years. Grower adoption of new management strategies and technologies (with maybe a little luck from weather) has probably helped with pigweed management, but control continues to be a challenge.
Figure 1. Glyphosate-resistant Palmer amaranth in soybeans. Photo by Dallas Peterson, K-State Research and Extension.
Part of the difficulty in achieving season-long pigweed control is their biological characteristics. Waterhemp and Palmer amaranth are both prolific seed producers, so many fields have extremely high seedbanks due to past control failures. The pigweeds also have a fair amount of seed dormancy and will start to germinate in April, but continue to germinate through the entire growing season. Once emerged, plants can grow rapidly, so the window of opportunity for effective postemergence control can be very short.
The most successful and consistent pigweed management programs in soybeans will be an integrated approach that utilizes a diversity of herbicides and timely applications, along with good cultural practices. One of the keys to managing pigweeds is to stay ahead of them. There are a number of good residual soybean herbicides for preplant and preemergence pigweed control. However, they require good rainfall for activation and rarely provide complete season-long control. In a no-till situation, you may want to consider a split application of residual herbicides, with a preplant application in mid- to late-April followed by another application at planting time. This approach provides a better chance for activation and control of early emerging pigweed, along with extended residual control later into the season.
Postemergence treatments need to be applied before pigweeds exceed 3 to 4 inches tall. Whereas, glyphosate used to control large pigweeds, no other postemergence herbicide is consistently effective once pigweeds exceed 3 inches. An overlapping residual also can be included in the postemergence treatment, especially if a split application wasn’t used before and at planting time. Sequential postemergence treatments also may be an option depending on product guidelines.
There are many different herbicide options that can be fit into the program. In addition, several different herbicide resistant traits are currently available, or likely will be available in soybeans in the future. Each trait has its unique strengths and weaknesses, but a multiple pronged approach utilizing a diversity of herbicides and timely applications will be critical for success regardless of trait technology. A program approach utilizing different herbicide modes of action will also help sustain the effectiveness of the new technologies further into the future.
Dallas Peterson, Weed Management Specialist
Doug Shoup, Southeast Area Crops and Soils Specialist