AGRI-BRIEFS   AGRONOMIC NEWS ITEMS

From Agronomists of the Potash & Phosphate Institute 655 Engineering Drive, Suite 110 Norcross, Georgia 30092-2837 Phone (770) 447-0335

Spring 2002, No. 6

The need for phosphorus, potassium, sulfur, and magnesium, and the appropriate rates should be determined from recent soil test results, based on site-specific soil samples. Most forage producers know to avoid soil sampling in livestock feeding areas, and areas near water and shade, where animals congregate and soil nutrient levels may be elevated because of animal wastes.

Experienced forage producers recognize that appropriate nutrient rates depend on grazing needs and hay yield goals. Perennial cool season forage grasses will yield between 3 and 6 tons per acre, with nitrogen rates from 90 to 150 pounds per acre and balanced inputs of phosphorus, potassium, and other nutrients. Producers sometimes rely on a “nitrogen-only” forage program and fail to consider the harvest removal of phosphorus and potassium. As a consequence, soil test levels may decline over time, leading to disappointing yields and inefficient nitrogen use.

Perennial warm season forage grasses like bermudagrass also respond to fertilization and often yield from 4 to 8 tons per acre per year. Bermudagrass, bahiagrass, and dallisgrass will take up about 46 pounds of nitrogen, 12 pounds of P₂O₅, and 35 to 50 pounds of K₂O per ton of forage. As with cool season forage grasses, warm season forage grasses should be grazed, or hayed about every four weeks, to capture both yield and good forage quality.

Required phosphorus and potassium may be applied with the first nitrogen application (if not applied last fall) as the grass breaks dormancy and begins to grow. Nitrogen, and any other needed nutrients, should be applied at or before green-up, before rapid growth begins. The initial nitrogen rate at green-up often ranges from 50 to 100 pounds per acre. Additional nitrogen should be applied at 50 to 100 pounds per acre about every four to six weeks afterward, depending on grazing needs and haying objectives. Forage producers should recognize that less nitrogen, phosphorus, and potassium may be needed in grazed forage systems, compared to hay, haylage, and silage systems, because a portion of the nutrients are returned to the soil by grazing animals as urine and manure.

If forage producers have not collected and analyzed soil samples in the last two to three years, they should do so now, then be ready to fertilize at, or just before, forage green-up. Good livestock performance and weight gains depend on good forage management, starting with optimum soil fertility and plant nutrition.

Contact a Certified Crop Adviser, fertilizer dealer, or a county Extension agent for more information on research-based fertilizer recommendations for forage grasses and optimum forage management.