ENID, Okla. —
As producers prepare for the next grazing season on summer-introduced perennial pastures, they need to understand the relationships with adding fertilizer into that forage production system.
There are two ways to approach adding fertilizer. If the amount of fertilizer to be applied (maybe dictated by a dollar amount) is determined, then you adjust stocking rate to meet that fertilizer application and forage produced. If the stocking rate is determined, then you adjust the fertilizer rate to meet the forage needs of the pasture. Below are a few calculations for determining your fertility needs.
Let us start with the determined stocking rate because that would be the standard practice most producers in northwest Oklahoma would use. A 1,200-pound cow will consume roughly 3 percent of her body weight in dry matter forage per day. Her daily consumption would be 36 pounds of dry matter. Over a 365-day grazing period, this cow would eat approximately 13,140 dry matter pounds. If our bermuda grass pasture is in good condition and on good soils, it will produce about 1,500 pounds of dry matter forage per acre, of which our cow will consume approximately 70 percent.
Without fertilizer, one cow is going to need 12.51 acres to meet her forage demand. If our planned stocking rate is five acres per cow for the 365-day grazing period, then the producer needs to add fertilizer to increase forage production to meet that need. Through research at Oklahoma State University, for every 50 pounds of nitrogen fertilizer added to bermuda grass, we should increase forage production by 2,000 pounds. Given these parameters, our producer’s needs to fertilize his bermuda grass pasture with 131 pounds of actual nitrogen. Phosphorus, potassium and other nutrients might also be needed if the soil test results required their addition.
At 60 cents per pound of actual nitrogen, the fertilizer requirement to reach the one-cow-per-five-acre stocking rate would be $78.78 plus application costs. If the plan was to only spend $50 per acre on fertility, then you would have to increase the number of acres to meet the forage demands.
More likely, a producer has in mind a fixed dollar amount he is willing to spend on soil fertility. As we look at the previous example, our fertilizer program is $50 per acre. We apply that amount to our 1,500 pound per acre bermuda grass. At 60 cents per pound of nitrogen, we would be applying 83.3 pounds of actual nitrogen. That 83 pounds of actual nitrogen fertilizer will produce 3,320 pounds of forage. At 70 percent utilization, the stocking rate would now be 5.56 acres per cow. This example also recognizes that phosphorus, potassium and all other nutrients are adequate to reach our bermuda grass yield goals.
Based on animal weight and land productivity, land required per animal can vary greatly. Your Oklahoma Cooperative Extension Service ag educator can walk you through forage production equations similar to this. You also can utilize OSU Extension Fact Sheet 2584, Forage Budgeting Guidelines, to work through similar forage budgeting formulas.
Gribble is Oklahoma Cooperative Extension Service northwest area agronomist.