Small is Big: Early Weed Control to Preserve Yield

I will start by saying, if you have fields with weed problems, the sooner you adopt an early residual program in addition to your glyphosate weed control system in your corn and soybean, the more dollars per acre you will have at the end of the year.

Over the years, the lack of early weed control in a glyphosate-only control program has created hidden yield losses in glyphosate resistant corn and soybeans. In addition, it promotes glyphosate resistance in our weed profile (but that is a subject all its own.) The temptation to wait until there is significant weed growth to spray, with the hopes of having a one-pass system, is outdated and must be reconsidered.

Weeds competing for moisture, nutrients and light in the early stages of crop growth are more detrimental than many growers realize. Small corn is more sensitive than small soybean plants to early weed competition, but both are hurt significantly to reach their maximum potential. The first 2-4 weeks are critical and sometimes our first application of glyphosate can be delayed due to work load or weather conditions. The use of residual pre-emerge chemicals should be considered to ensure early control in both crops.

In corn, the best practice is to control weeds before they get 2-3” tall. This critical period is when many growers are planting soybeans. The only effective way to do this is an early residual application. Many studies have shown a yield loss of .5-2.5 bu/day after the 3rd leaf tip emerges on corn with high weed densities present. If weeds emerge at the same time as the crop, corn is vulnerable to yield losses anywhere from emergence (VE) to 12” tall. Soybeans are vulnerable to yield loss also. Studies have shown 5% yield losses as early as the V1 stage of soybean growth and 20% loss at the V5 stage of growth with significant weed density. One study in Ontario indicated delaying weed control past ¾ bu/A/day reduction of yield for each day after 4 weeks.

Row width, moisture available for growth, weed density, and nitrogen levels all contribute to the timing of the critical period for early weed control in both crops. In corn, row width does not play as significant role in the establishment of early weed growth since corn does not canopy as aggressively as soybean. Nitrogen fertilizer not only helps the corn but it also helps the weeds grow more rapidly. There is not a significant difference in early weed suppression in corn row widths of 20”, 22” or 30”. Narrow row soybeans (7.5” and 15”) can significantly suppress weed growth due to early crop canopy, verses 30” rows.

“Small is Big”. Uncontrolled weeds in small corn and soybeans can mean big losses.