Source: Plant Health Initiative

Weeds can be one of the biggest challenges a soybean grower will face. Weeds compete with soybeans for light, moisture, and nutrients, with the early-season competition being most critical. Most of the yield reduction occurs during the first six weeks after planting.

Know your weeds
Annual weeds are those weeds that live for a single growing season and die at the end of the year after producing seed. Examples of annual weeds are crabgrass, lambsquarters, and pigweed. Some biennial species produce seeds in their second year and then die. Perennial weeds live for several years and regenerate shoots each year from underground roots and rhizomes. Quackgrass, horsetail, plantain, thistle, and ragweed are examples of perennial weeds. Weeds are also classified as either broadleaf weeds or grasses.

Choose control methods that are most effective for your specific weed problem. Common weeds that occur in the North Central states include common and giant ragweed, Eastern black nightshade, waterhemp, woolly cupgrass, yellow foxtail, large crabgrass, and others. Get to know the weed species that occur in your fields and keep track of them with field maps. There are many excellent resources to help you identify weed seedlings and older weeds your fields.

The introduction of Roundup Ready soybean in 1996 dramatically changed weed management in soybeans. Roundup Ready varieties are genetically altered to tolerate applications of Roudup and other herbicides containing the active ingredient glyphosate. Many growers came to rely almost exclusively on this method of weed control. Glyphosate provides broad-spectrum weed control, and it is better than many other herbicides at controlling larger weeds. It is also considered to have low environmental and human health risks.

Unfortunately, glyphosate-resistant weeds are a result of systems that rely heavily on glyphosate. To date, nine species of weeds are known to be resistant to glyphosate in the United States. The need to diversify weed management programs and protect the value of glyphosate, and other herbicide active ingredients, is now an important aspect of weed management.

Weeds with resistance to multiple active ingredients is a growing problem. Lambsquarterswith resistance to triazines, ALS inhibitors, and insensitivity to glyphosate makes this weed a serious threat to soybean production. Waterhemp is another common weed in the Midwest which has known biotypes resistant to ALS-inhibitors, PPO, triazine and glyphosate.