Why does my soybean variety have “STS” or “SR” listed with it and what does it mean for my planting season? In 1994 sulfonylurea-tolerant soybeans (STS), then later sulfonylurea ready (SR), were introduced to help growers control troublesome broadleaf weeds. These STS or SR traits are native, non-GM traits, that make the soybean more tolerant to sulfonylurea herbicides. Sulfonylurea herbicides work by inhibiting the ALS (acetolactate synthase) enzyme, which plays an important role in forming necessary proteins for the plant’s growth and development. This causes the weed—or non-STS soybean plant—to starve because it cannot get the necessary proteins for growth.
Why should I care?
Considering an STS or SR soybean will help combat carry-over that some growers may experience from the prior year’s herbicide program or in a double-crop situation following a wheat crop that used a sulfonylurea product for weed control. High soil pH, soil type, lack of rainfall and time of application are some of the main reasons a sulfonylurea can carry over to the next crop.
How does carry-over occur?
Sulfonylurea mainly degrades by a chemical process called hydrolysis, which is the chemical breakdown of a product due to reaction with water. Thus, the later into the season—and subsequently drier weather—this class of herbicides is applied, the higher the potential carry-over to the following crop. Cooler weather and higher pH soils also slow down degradation, which could increase the potential residual herbicide in the soil. A wide range of persistence in the soil happens within this same chemical family. Some is short-lived, and some carry-over may last longer than ideal.
Here in Illinois, many are blessed with high organic matter (OM) soils. OM soils have a net negative charge which can bind these herbicides to the soil and take longer to degrade. Clay particles also have a net negative charge and will bind sulfonylurea. Additionally, these herbicides bind more tightly to the soil colloids in dry weather, increasing the opportunity for carry-over.
What can I use?
Even though sulfonylurea chemistries can be applied to corn and soybeans, different products are labeled for each. There are multiple products that can be used in an STS or SR trait platform. Tank mixing your existing herbicide program with products like Classic® or Pursuit® can help with dicot weed control and nutsedge. However, the development of ALS-resistant weeds like kochia, waterhemp, shattercane and cocklebur can limit the efficacy of these products. Scouting your fields and knowing your weed populations are key, and as always, be sure to read and follow pesticide label directions.