Farm Horizons, June 2013

Early-season weed control recommendations

By Dave Schwartz
Certified crop advisor, Gold Country Seed

The growing season started out cool and wet this spring, delaying planting in much of Wright and Meeker counties.

Where I live, south of Litchfield, most of the corn was planted by May 17, but growers were waiting for dry weather to finish planting their soybeans.

In early June, corn and soybean growers are usually busy spraying for weeds, so I thought I would share some thoughts on weed control.

• When spraying post-emergence herbicides, in general, it is best to spray too early rather than too late. Weeds compete for light, moisture, and nutrients. A common recommendation for the timing of spraying post-emergence chemicals is to make the herbicide application in corn before weeds reach 4 inches in height, and spray soybeans before weeds reach 6 to 8 inches in height. The “critical period of competition” varies quite a bit, depending on weed density, soil moisture, and weed species present. So, the worst combination of these factors is high weed populations, combined with droughty weather and the crop competing with broadleaf weeds. If weed pressure is heavy, get herbicides on early to protect grain yield.

• Most growers have moved to a weed control program that includes two herbicide applications. The application of a preemergent or early post product reduces weed pressure early in the season. This gives growers a wider window to make the second application, which is often Glyphosate. University studies have shown growers can expect better weed control and more profit by making two herbicide applications. Developing a weed control plan that utilizes herbicides with two modes of action will extend the life of Glyphosate on a farm.

• Growers need to know growth stages for corn and soybeans when applying post-emergence herbicides other than Glyphosate, so the herbicide is applied at the correct time. Early in the season, it is fairly easy to identify stages of growth for corn and soybeans. Corn is at the V1 stage when the first leaf of a corn plant has completely unrolled. The first leaf has a rounded leaf tip. All leaves – after the first leaf – have a pointed leaf tip. V2 is the stage when the second leaf unrolls, and it will have a pointed leaf tip. In soybeans, cotyledons first emerge from the soil, followed by two true leaves, called unifoliolate. Then, the first trifoliolate leaves emerge and this is referred to as the V1 stage. When the second trifoliolate leaves emerge, the plant is at the V2 stage, etc.

• To reduce the potential for Glyphosate-resistant weeds, be sure to use recommended rates and apply Glyphosate when weeds are young and easier to control. Broadleaf weeds are becoming more tolerant to Glyphosate, so using full rates of Glyphosate will strengthen a grower’s weed control program. As I have shared with many growers, Glyphosate is a silver bullet for weed control, so it needs to be used wisely. Weed control costs will rise significantly when Glyphosate is no longer effective.

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