James Locke, a soils and crops consultant with the Samuel Roberts Noble Foundation in Ardmore, Okla., notes that nitrogen rates for cereal-based winter pastures are determined by yield goal. He says to calculate a realistic yield goal based on potential seasonal productivity, stocking rate and corresponding forage demand. Using a few simple formulas, Locke demonstrates in a recent edition of Noble Foundation Ag News & Views how to determine total forage demand, forage demand per acre, and an appropriate nitrogen recommendation.
Cattle number x average cattle weight in pounds x 0.03 (percent intake) x total grazing days = total forage demand in pounds dry matter (total DM)
(Total DM ÷ 0.85 grazing efficiency) ÷ acres = forage demand in pounds DM per acre
(DM per acre - 1,000 pounds) ÷ 20 = recommended pounds actual nitrogen per acre
Locke notes that winter pasture will generally produce between 1,000 and 2,000 pounds DM per acre without added nitrogen, which is why 1,000 pounds is subtracted from the DM per acre. If the pasture is known to be a highly productive site without added nitrogen, adjust the formula deduction up to 2,000 pounds. The result is divided by 20 because the nitrogen response is about 20 pounds additional DM produced per pound of actual nitrogen applied.
Locke suggests that if fall and winter forage production is the priority, apply most or all of the nitrogen fertilizer at or near planting. If spring production is most important, apply about 30 percent of the nitrogen at or near planting and the remainder before the rapid growth phase in the spring.
Using example calculations for an operation planning to turn out 160 500-pound steers on 160 acres of good wheat pasture, Locke calculates the nitrogen rate needed to provide forage for 120 days. The steers are anticipated to gain approximately 2 pounds per head per day and come off pasture weighing 740 pounds. Their average weight is 620 pounds, which is the midpoint between the starting weight (500 pounds) and ending weight (740 pounds).
160 head x 620 pounds x 0.03 intake x 120 days = 357,120 pounds total DM demand
(357,120 total DM ÷ 0.85 grazing efficiency) ÷ 160 acres = 2,625 pounds DM per acre demand
(2,625 pounds DM per acre - 1,000 pounds) ÷ 20 = 81 pounds of actual nitrogen per acre