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Weed Impact on Crop Stand

As you drive down the interstate and back country roads, you probably notice fields of purple or green scattered throughout the countryside. They are the first signs of vegetative growth in the spring, when temperatures begin to warm. These fields may look like beautiful flower gardens to some, but it can be a weedy nightmare for a farmer. These winter annual weeds are usually more defined in no-till or minimum tillage fields. A winter annual seed germinates in the fall, and flowers in early spring. The young weed seedlings are very cold hardy and able to survive most winters in the Midwest. The past couple of winters have been fairly mild and have allowed ample growth of these winter annual weed species.

Henbit and purple deadnettle are the purple flowering weeds that are found in many fields. Purple deadnettle is more commonly found in the southern counties of Illinois and henbit to the north. Common chickweed is another common winter annual that flourishes in many fields. Controlling these weed species either in the fall or spring can go a long way to establishing a good seedbed in the spring. The following illustrations show the identity of these weeds.

Henbit and Purple Deadnettle

Photo courtesy of NC State University

Common Chickweed

Photo courtesy of University of Illinois

The approach you take to control these winter annuals can be the difference in a timely planting. Using a residual herbicide in post spray applications can help reduce weed growth late in the growing season. Fall herbicide applications after harvest or a “burndown”, will also aid in keeping winter annuals in check. Looking at the present time, there is still an option for controlling these weeds. A spring burndown application can still be effective, as long as weather allows for an application in the next week or two. Spring tillage can be an effective tool for weed removal, but can also be a challenge. Starting with a clean field is the best practice to establish a good crop stand and eliminate future problems that may persist.

Winter annual weeds present many management challenges. Dense weed populations slow the warming and drying of the soil in the spring. As a result, fields often have lower soil temperatures than fields without weeds. Heavy vegetation will also reduce the seed to soil contact. These factors can greatly diminish plant stands. There is also an increased risk of pest damage in fields infested with winter annuals. Six winter annual weeds serve as alternative hosts to Soybean Cyst Nematode (SCN). These weeds include purple deadnettle, henbit, field pennycress, Shepard’s purse, small-flowered bittercress, and common chickweed. Winter annual weed control is a good practice for the management of SCN. Black cutworms, wireworms, seed corn maggots, and various species of white grubs can also be seen in high populations in these fields. Black cutworms overwinter in the southern states and as the moths move north in the spring, they lay eggs in fields with dense vegetation. Severe stand reductions can result when larger larvae cut plant seedlings below the growing point. White grub larvae overwinter deeper in the soil and surface as temperatures warm. The grubs survive by feeding on the roots of the weed species and eventually the crop roots. An abundant amount of pests were able to survive due to the mild winter conditions. A soil insecticide should be considered when the populations of these pests are at high levels.

Cutworm Damage to Corn

Photo courtesy of University of Minnesota Extension

White Grub Feeding

Photo courtesy of University of Nebraska

A clean field at planting can go a long way towards maximizing plant stand and crop yield. Managing winter annual weeds will allow for timely planting, help achieve good plant stands, and aid in the suppression of pest populations. A weed-free field from the start will have a lasting impact through the remainder of the crops lifecycle.

Chase Heap

Harvest-Max Associates

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