The critical period for weed control typically occurs within the first four to six weeks after planting. This implies that crop yields will not be affected by weeds that emerge with or soon after the crop if these weeds are controlled within four to six weeks of planting. Also, weeds that emerge after this critical period normally will not affect yields due to the crop's head start.

Waterhemp in flower.

Current management programs favor weed species with a late or prolonged emergence period. Waterhemp is an example of a weed that escapes control by emerging after control tactics have been implemented. Many growers have reported new flushes of waterhemp or other weeds in both corn and soybean in recent weeks, and are considering the need to control these late-season weeds. Decisions whether to treat these fields need to be based on economic returns.

Weeds that emerge after the crop is established are much less competitive than weeds that emerge with the crop. Corn yield loss attributable to redroot pigweed competition decreased from 12 percent with plants that emerged at the same time as corn, to 3 percent with plants that emerged at the 4- to 7-leaf stage. Seed production by the late-emerging pigweed was reduced 90 percent compared to pigweed that emerged at the same time as corn.

In most situations, weeds that emerge during late June or July will have little impact on crop production, although many of these weeds will become visible later in the season, especially in soybean.

Growers also need to consider whether the weeds can be effectively controlled and the impact of the herbicide treatment on crop yields. At this time of year, weeds usually don't get noticed until they become fairly large. The combination of large weeds and high temperatures will reduce the effectiveness of most herbicide treatments. In addition, postemergence herbicides are more likely to cause significant yield losses when applied to crops in the reproductive stage. Many herbicides prohibit application to crops in reproductive stages of growth.

In summary, late-emerging weeds may be appearing in fields across the state. Although the initial reaction often is to implement some control strategy for these weeds, careful evaluation usually will show that these treatments are not warranted due to the reduced competitiveness of late-emerging weeds, ineffectiveness of late-season treatments, and potential negative impacts of herbicides on flowering crops.