Research Highlights

Managing Weeds Can Be Challenging in Early Planted Soybeans

By Sarah Hill

More and more soybean growers are trying to bump up their planting date, even if by only a couple of days, to capture the potential increased yield. However, an earlier planting date can lead to unintended consequences, such as needing different strategies to control weeds.

Early Emergence—for Soybeans & Weeds

“The traditional planting window is May or even June,” says Travis Legleiter, extension assistant professor and researcher at the University of Kentucky. “Most growers are putting down residuals right around the same time, so it works well. In our first year of research, we found that if soybeans are planted in March or early April and a residual is applied at the same time, that residual is gone by the time waterhemp starts emerging.”

This planting date and weed control research project, led by Legleiter, is funded by the Kentucky Soybean Promotion Board. 

When conditions are right for the soybean crop to emerge earlier, weeds will emerge earlier, too. Many soybean growers use a pre-emergence herbicide to control troublesome weeds like Palmer amaranth and waterhemp that emerge at the same time as soybeans. Soybean seedlings are competitive in a high-density population, Legleiter says, and can bounce back with timely weed control application. 

“If you can keep the field clean early on, it will help the soybeans canopy sooner,” he says. “With early planted soybeans, it could be multiple weeks before emergence, so it takes longer to get to canopy. Even if soybeans aren’t competing with weeds in March and/or April, they’re taking longer to put on leaves.”

The payoff for planting soybeans early should be evaluated on a field-by-field basis, according to Legleiter. Research has shown that early planted soybeans have higher yield potential but battling waterhemp all growing season may dampen that potential gain. 

“In fields with waterhemp pressure, if you have the right products and systems in place, you can make early planting of soybeans work,” he says. “If you have really tough waterhemp problems at a high density, there may be a benefit to waiting.”

Residual Herbicide Pros & Cons

Soybeans don’t grow much during those cooler late spring months, and the timing of applying residual herbicide is critical, according to Legleiter. Stretching out the timeframe that weeds need to be controlled requires additional input costs and creates ideal conditions for crop injury from herbicides.

“If growers are thinking about planting in April, they should consider using a true overlapping residual and putting on another residual later, before the first one breaks,” he explains. 

Soil type also plays a factor in weed control by influencing the type of herbicide a grower should apply and application rate. Legleiter’s team found that all herbicides effectively controlled the weeds, implying that farmers can choose whichever herbicide best suits their soil.

“Coarse soils with lower organic matter are more prone to crop injury on soybeans when residual herbicides are used, especially if they’ve received a lot of rain,” Legleiter says. “Fine soils with higher organic matter have a lower risk of crop injury, although the risk is still there.”

Fall vs. Spring Burndown

Another option for soybean growers to consider is prepping the field with a fall burndown, Legleiter says. However, this presents another challenge—getting winter annual weeds burned down before planting.

“For weeds like marestail that have a fall emergence pattern, there’s a benefit to having a clean field to plant into come early spring,” he says. “It really depends on the type of weeds you’re dealing with. With fields that have a lot of waterhemp, a fall burndown could be detrimental to weed control. Marestail can be controlled with a burndown in either fall or spring.”

Clean fields result in soil temperatures warming more quickly, which is good for soybeans, but again, also makes excellent conditions for weeds to pop up. 

“When we don’t do a fall burndown, winter annual cover keeps summer annual weeds like waterhemp from emerging,” Legleiter says. “For fields with waterhemp, we suggest doing a burndown right before planting.”

Weather conditions can have a major impact on the timing of a burndown. If conditions are warm and sunny, then a burndown will work well, Legleiter says, and growers can get into the fields to plant soybeans early. Cool, wet, and cloudy conditions will make a burndown much less successful, preventing the grower from planting early.

Additional Resources:

Dicamba Alternatives for Palmer Amaranth and Waterhemp Control In RR2X Soybean – SRIN article

Published: Jan 30, 2023