Rotationally grazed pastures in Wisconsin respond best to nitrogen (N) applications made in early May or early August, when temperatures are in the mid- to high 70s.

Relatively cool temps and adequate soil moisture offer the greatest response when applying N, according to an N rate and timing study by Dennis Cosgrove, University of Wisconsin-River Falls extension agronomist.

"Split nitrogen applications have long been recommended on rotationally grazed pastures," says Cosgrove. "However, until recently, the most efficient timing for these nitrogen applications had not been established." He conducted an N rate and timing study in 2004 and 2005 to test application strategies.

"The optimum time to apply nitrogen is early May and early August. Mid-June applications are not productive because grass growth is slowed by heat and drought," he says.

"It's important to realize that, in early May, pasture growth may already be greater than the animals' ability to utilize it. Nitrogen applications may exacerbate this problem. In order to capture this increased growth, pastures will likely need to be mechanically harvested or stocked with very high stocking rates," he adds.

The research was done at a single location in west-central Wisconsin. Optimum application times will need to be adjusted in other locations to correspond to times of greatest pasture growth rates.

For more information, including a spreadsheet based on the study data that can be used to estimate the potential return on nitrogen fertilization at various urea costs, go to www.uwrf.edu/grazing. Under “Fertilization,” view “Nitrogen Management in Rotationally Grazed Pasture.” For the spreadsheet, click on the software link.