Research Shows Small Timeframe to Spray Effectively

Workload management prevents yield loss due to weed competition. Compiled by staff

Published on: Apr 5, 2005

Like comedians, total post-emergence weed control relies on a well-timed delivery. To prevent yield loss, weeds must be controlled before they are 2 to 4 inches tall, depending on weed species, density, soil fertility and moisture. However, post-emergence spray timing is as unpredictable as the weather, and the impact of a late application is not evident until the crop is harvested.

Research compiled by Ohio State University from across the Corn Belt showed that 4-inch weeds caused an average 6% yield loss. As weeds grew, so did the yield loss. Michigan State University research showed that waiting 23 to 25 days after planting to control weeds caused up to an 8 to 10% yield loss.

Based on this research, weed control programs that rely on total-post, non-residual herbicides must have the first application made roughly three weeks after corn planting to prevent yield loss. Corn planted at the end of April should be sprayed by mid-May. Corn planted in early May needs to be sprayed before Memorial Day.

But to achieve timely spraying, weather must cooperate. "Spring rains are vital to crop development," says Tom Christensen, business development manager for Syngenta Crop Protection. "However, they also keep growers and custom applicators out of the field - sometimes when weed control is critical."

For example, National Agricultural Statistics Service data shows that on average, roughly half the corn acres in Illinois and Iowa are planted by May 5, as well as 63% of Missouri corn acres.

"According to university research, that corn should be sprayed between May 19 and May 26 to prevent yield loss from weed competition in a post-emergence program," Christensen explains. "But the big question is, will fields be too wet to enter during that time period?"

Using the 10-year average rainfall data, Christensen estimated how long fields would be accessible on a weekly basis through the spring.

"Our estimates are based on the calculations done by state agricultural statistics services," he explains. "They are reasonable averages, but actual days available for fieldwork depend on soil type and other factors, as well."

The formula he used figured that a certain amount of rain would prevent fieldwork for a given number of days.


Number of days a field is inaccessible

Less than .25 inches

.5 days

.25 to 1 inch

1 day

1 to 3 inches

2 days

More than 3 inches

3 days

The week of May 20 is typically wet in Illinois, eastern Iowa and Missouri, limiting days available to get in the field and making timely post-emergence applications difficult for those corn acres planted at the beginning of the month, based on calculations with this formula.

"Many areas of the Corn Belt have less than half the week available to be in the field, based on these calculations," Christensen notes. "Not only does corn need to be sprayed at that time to prevent yield loss, but many growers in these areas will be planting soybeans, as well. Our maps show that these areas are drier the following week, but cornfields without residual herbicide protection already have weeds competing for resources and stealing yield."

Christensen says that growers need to be realistic about the amount of work they can do and when. "Workload management is one of the greatest challenges growers face, and the data we’ve collected shows that there simply isn’t time to spray all your crops post-emergence, manage soybean crops and avoid yield losses," he says. "And custom applicators face the same challenges with the number of acres they serve."

Pre-emergence residual herbicides can ease the workload pressure, he continues. These weed control programs prevent weeds from competing with the young crop for nutrients, light and moisture from the beginning. Plus, these herbicides can be applied earlier in the season, during times that are traditionally drier.

"We can’t control the weather, but we can manage crops and spread workloads to minimize risk of yield loss," he adds. "Pre-emergence herbicide programs give more flexibility than total post-emergence programs and don’t rely on exact timing of delivery for success."