Earlier weed control in corn — preferably pre-emergence — may help maximize costly nitrogen (N) use this season. When weed control is delayed in season, the amount of N needed to reach the best yields increases, Extension specialists say.

“My recommendation is to control weeds early to use N most efficiently,” says Carrie Laboski, University of Wisconsin soil fertility specialist. “Applying more N to compensate for poor weed control is expensive and has potential risks. The extra N, while taken up by weeds, will be released back into the soil and may or may not be captured by another crop. The N might move into surface or groundwater.”

In addition, the majority of growers relying on a postemergence weed-control program in corn are not using the program as intended. Delaying weed control beyond a 4-6-in. weed height can result in corn grain yield losses of 7-20%.

About 75% of surveyed Wisconsin cornfields treated with postemergence herbicides were treated after the recommended time, says Chris Boerboom, Wisconsin Extension weed specialist. Such late treatment created a 6.5% yield loss, based on average weed densities and weed heights. This translates to an average $39/acre loss, (based on a 150-bu. yield potential and $4/bu. price), Boerboom says. He adds that economic losses would be even greater with higher-yielding corn.

TO DETERMINE THE best weed-control timing recommendations and N rates to optimize economic corn yields, Laboski and Boerboom compared the amount of N needed to produce an optimum crop with weeds controlled by a pre-emergence herbicide, weeds controlled at the 4-in. height and again at 12 in. The study ran two seasons.

The amount of extra N required for the crop varied by weed height at treatment. In 2006, weeds controlled early in the season required about 100 lbs. N for an economic optimum N rate (EONR) to produce a 220-bu. corn crop. Waiting until the weeds were 4 in. tall required 20 lbs. more N, while waiting to control weeds until they were 12 in. meant 164 lbs. N/acre. (The EONR represents a 0.1 N-to-corn price ratio with 40¢/lb. N and $4 corn.)

“Corn yields with the pre-emergence weed-control program and when weeds were controlled at the 4-in. stage were similar in both years,” Laboski adds. “However, the additional eight days of weed competition until the 12-in. height was reached for weed control reduced corn yields at N rates that maximized yield with earlier weed control. At the 200-lb./acre rate, corn yields were similar for all weed-control timings.”

To calculate economics during the study, the pair used a $4/bu. corn price, a 90¢/lb. N price, a pre-emergence herbicide price and application cost of $40/acre and a postemergence herbicide price and application cost of $22/acre.

“Partial returns in 2006 were greatest with the pre-emergence herbicide at moderate N rates. They were very similar to when weeds were controlled at 4 in.,” Laboski says. “If the pre-emergence weed-control program was more expensive, the 4-in. weed-control timing with a glyphosate application would have provided the highest return at the optimal N rate. The economic return for controlling weeds at 12 in. was greatly reduced, although it increased with increasing N rates.”

The same tests were run in 2007, and again Laboski says partial returns were greatest with pre-emergence herbicides at low N rates. Returns when the weeds were controlled at 4 in. were less than the pre-emergence treatment over the range of low N rates. Returns were similar after the economic optimum N rate was reached, and returns declined with the cost of additional N.

Partial returns were also dramatically lower with the 12-in. weed-control timing, even when the high N rates compensated for the extended weed competition. Had the partial budgets been based on an N price of 60¢/lb., the results would be similar, although partial returns would be proportionately greater at the higher N rates.

Farmers weighing whether they should consider pre-emerge vs. 4-in. weed-control timing will find either is more profitable than waiting until 12 in. to control weeds.

“Overall, this data supports that an investment in pre-emergence weed control may be more profitable than applying N at rates above economic optimum rates,” says Boerboom. “Weed management is not the most expensive input for corn. Nitrogen may be most costly, but weed management and N are linked because of weed competition. The N removed by weeds can be replaced to restore corn yield, but the cost is expensive. While cost to control weeds with a pre-emergence herbicide to prevent weed competition for N is greater than the cost of spraying glyphosate postemergence, it can be worth it.”

While the study's results are convincing, applying the findings to individual farms requires additional grower management. Laboski points out that university N-rate-response databases use data from both small-plot and replicated field strip sites with good N management where there is very little N loss and good weed control. As such, the N-rate guidelines — in particular the maximum return to N (MRTN) approach used in the Midwest — assumes good N management. When an individual grower reduces N rates to match current economics using MRTN, good N management must be present.

“If a grower usually applies 160 lbs. N/acre and has delayed weed control, that grower may have never noticed a yield loss because the total amount of N applied was really greater than what the crop needed,” Laboski explains. “But now, if the grower reduces the rate to 130 lbs. N/acre and delays weed control, the grower may see lighter green corn.

“In that situation, the N rate applied was not enough for both the corn and the weeds,” she says. “The problem wasn't reducing the N rate, it was delayed weed control.”