Fertilization is just as important for forages as it is for corn and soybeans to maximize productivity. Soil sampling and testing is also just as important to guide fertilizer decisions, such as applying the right rate per acre for a pasture or hayfield.
“You should take soil samples once a year from hayfields and every three years from pastures,” advises Brian Lang, an Iowa State University Extension field crop specialist at Decorah in northeast Iowa. Hayfields need to be checked more frequently than pasture since large amounts of nutrients are removed in the harvested hay.
Grass and legume growth in pasture and hayfields is stimulated by fertilizer. While nitrogen is the primary nutrient most commonly needed for grasses, phosphorus and potassium also may be deficient in the soil and needed for increased growth. Lang, along with ISU Extension colleagues Rebecca Vittetoe and Denise Schwab, offer the following information and recommendations.
Nitrogen applications can either be a one-time, annual application or can be split-applied. Suggested N application rates for single application are:
- Kentucky bluegrass in April, 60 to 100 pounds N per acre
- Tall cool-season grasses in April, 80 to 120 pounds N per acre
- Warm-season grasses in late April to early May, 80 to 150 pounds N per acre
Suggested N application rates for split applications:
- Kentucky bluegrass in early spring (March to April), 60 to 80 pounds N per acre; late spring (May to early June), additional 30 to 40 pounds N per acre (optional); and late summer (August to September), additional 30 to 40 pounds N per acre
- Tall cool-season grasses in early spring (March to April), 80 to 120 pounds N per acre; late spring (May to early June), additional 40 to 60 pounds N per acre (optional); and late summer (August to September), additional 40 to 60 pounds N per acre
The tall cool-season grasses are orchardgrass, smooth bromegrass, reed canarygrass and tall fescue. For pastures or hayfields with tall fescue, high N application rates increase the risk of fescue toxicosis, which adversely affects cattle, sheep and other grazing animals.
For legume-grass mixed pastures or hayfields, if the stand is less than a third legume, treat it as a grass pasture or hayfield. If the stand is more than a third legume, no nitrogen is recommended. Also, for legume-grass mixed pastures or hayfields, high or frequent applications of fertilizer N (particularly spring N applications) will make the grass component more competitive and limit the amount of legumes in the mixture. The grass will grow more aggressively and crowd the legume. To encourage a greater legume presence, use modest N application rates and limit application to summer or fall.
Phosphorus, potassium and lime
Forage plants also respond to added phosphorus and potassium when soils have low or very low P and K test levels. Sampling soil and having it tested is the only way to know what those levels are and to determine how much P and K fertilizer needs to be applied.
While pastures typically have little nutrient removal, that doesn’t mean that the redistribution of those nutrients in the cattle manure is equal. Nutrient levels tend to be higher near shade and watering sites where cattle gather. Also, how grazing is managed (continuously, rotational, strip, etc.) can impact nutrient redistribution. Taking these factors into account is important when sampling soil in pastures.
For hayfields or pastures that have hay baled or otherwise removed, it’s important to apply additional P and K fertilizer based on crop nutrient removal. For example, each ton of smooth bromegrass hay harvested at 15% moisture removes 7.9 pounds of phosphate (P2O5) and 41 pounds of potash (K2O) per acre. More information on crop nutrient removal for other forage crops can be found in Table 3 of A General Guide for Crop Nutrient and Limestone Recommendations in Iowa, PM1688.
Timing of P and K applications can be flexible; however, it may be more convenient to apply P and K fertilizer in the fall or spring along with the N fertilizer application.
Soil pH can also impact forage productivity. Apply lime where needed, according to soil sampling and soil test results. It is recommended for grass-based hayfields and pastures a soil pH of around 6 should be maintained. To encourage and maintain legumes, try to maintain a pH of 6.5 for clovers and bird’s-foot trefoil, and a pH of 6.9 for alfalfa.