When Nick Harre began his undergraduate education in plant and soil science at Southern Illinois University Carbondale (SIUC), he didn’t anticipate that weeds would become a research passion. He was one of the first students to receive the Illinois Soybean Association’s Crop Sciences Scholarship in 2010.

The Nashville, Ill., native began working with then-SIUC weed scientist and professor Bryan Young. Now, as a Ph.D. student at Purdue University, Harre is still working with Young, researching herbicide-resistant weeds.

Weeds impact nutrient uptake

Harre’s research confirms that early-season weed control is one of the most effective ways to help maximize yield potential.

He evaluated both crop nutrient loss from living weeds and nutrient release from controlled weeds. Competition from weeds up to 4 inches only minimally affected nutrient acquisition by soybeans, while competition from weeds 8 inches or taller negatively affected acquisition. Soybean yields and grain oil content were reduced.

The research also revealed that decomposition and release of nutrients from larger weeds is a slower process than from smaller weeds. Delayed weed control can affect the mineral nutrition of crops even after dead weeds are lying on the soil surface, Harre says.

Longer control needed for waterhemp

Harre and Young summarized the results of a three-year research project funded by the Illinois Soybean Association, with research conducted at SIUC, Western Illinois University and the University of Illinois. The project focused on the effectiveness of soil residual herbicides in controlling waterhemp, as well as the influence on soybean yield.

“Our data clearly show something is better than nothing with regard to residual herbicide use for waterhemp control up to three weeks after planting,” Harre says. “As problematic weeds such as waterhemp emerge throughout the growing season, lengthier residual control is often desirable, especially for fields infested with waterhemp containing resistance to multiple post herbicides.”

Harre says a single group 14 herbicide (PPO-inhibitors such as Valor SX and Authority-based products) at the full-use rate will only provide sufficient control for a short period, while group 15 herbicides (long-chain fatty acid synthesis inhibitors such as Dual II Magnum and Warrant) are regularly effective on small-seeded broadleaves. Tank mixtures of both groups provide the most consistent, prolonged control of waterhemp.

Researchers also evaluated how soil residual combinations affected soybeans grown in a weed-free environment. Soybean injury, delays in vegetative and reproductive development and grain yield components were measured.

Growers are concerned about the yield impact of early-season herbicide injury, Harre says. He notes that no soil residual herbicide combination reduced yield when applied according to labeled rates.

Soybean stunting was observed early with group 14 herbicides but did not delay trifoliate development, and injuries at three weeks after planting did not affect yield either. Injury still present at six to nine weeks, however, was more likely to have an impact.

“Farmers should be concerned if injury lasts until that point and make wise residual herbicide selections,” he says. “It is better to have injured soybeans early with no yield loss implications than no soybean injury with a field full of weeds.”

Watch for weed resistance

In 2014, Harre took a statewide collection of giant ragweed samples to screen for resistance and found two distinct types.

“One response is similar to that seen in other glyphosate-resistant species such as waterhemp and Palmer amaranth,” he says. “The other is unusual. Plants sprayed with glyphosate turn necrotic within one day, which is much faster than usual. Plants may be sacrificing leaves so growing points can survive the application.”

Harre is currently evaluating how application variables and environmental factors influence glyphosate-resistant giant ragweed and how unique management strategies may be beneficial in controlling the strange form of resistance.

Article used with permission. Written by Barb Baylor Anderson Corn and Soybean Digest.