– Hank Bartholomew, Southern Ohio Grazing Coordinator
The use of nitrogen fertilizers (N) on pastures can generate a great emotional debate among some groups of graziers. My philosophy is that N fertilizer is a tool that we can use to assist in managing forage supply. I am reluctant to discard any tools although some graziers may choose to do so. N fertilizer is also an input that requires some of our cold hard cash to purchase. As with all inputs, we must be frugal and manage purchased inputs closely.
Nitrogen can be a source of pollution in both ground water and runoff and may come from legumes, animal manure/urine and N fertilizers. Research at the USDA-ARS hydrological station near Cochocton indicates that N rates per year would need to be in the 200-400 lb./ac./yr. range before we would find high nitrate rates in runoff or groundwater from pastures.
Legumes should be our primary source of N in our cool season pastures with small amounts of N applied at strategic times to boost forage growth. These comments apply to cool season grasses and legumes only (ie: bluegrass, orchardgrass, tall fescue, clovers, birdsfoot trefoil, etc.). N from legumes does feed the grasses in pasture by one of three methods. First, small amounts of N may leak from the legumes that grasses can capture. This is most likely a minor source. Second, nitrogen is released from legume roots as old roots die and are replaced by new ones. Soil organisms then break down the plant material releasing N for plant use. A third source is excess nitrogen from legumes in pasture that is consumed by animals and then redistributed by manure and urine. This is a very important source of N and improved grazing management results in more uniform distribution of animal waste.
Some graziers are reluctant to apply N fertilizer because they believe it will reduce the clover stands in their pastures. This is true when large amounts of N are applied in the spring which results in tall, over grown pastures that shade out legumes. It is not true at other times when grazing height is managed in the 4 to 8 inch range. It is light not the N which reduces clovers in a sward, although with high continued rates of N grasses have a competitive advantage.
Some graziers will apply small amounts of N (20-30 lbs.) to a portion of their pastures in March to jump start spring growth and allow earlier grazing. All N rates are given in lbs. of actual N per acre. 100 lb. of urea (46-0-0) per acre equals 46 lbs. of actual N per acre. Urea or 28% N solutions are inexpensive sources of N that can be used during the cool times of the year. It is important to vary the paddocks that receive early spring N treatment from year to year. Be cautious with N applications in the spring as they can make your spring flush problems worse.
The next time you might consider N application is during late June or early July when spring flush and the reproductive stage of cool season grasses is over. Moderate amounts of N (30-50 lb.) can be applied to help in overcoming the summer slump of cool season pastures. If you are consistently short on pasture during the summer months, nitrogen in early summer can boost production. Including warm season legumes or grasses such as alfalfa, marion lespedeza, or switchgrass are other alternatives to get through the summer slump.
The time of year when almost everyone can benefit from N fertilization is during late summer. The more forage we can grow during the latter part of the growing season and carry forward into fall, the less stored feed we will need for winter supplementation. We recommend 30-50 lb. of N for most grasses and 50-70 lb. for tall fescue, the premier grass for winter grazing. Recent research has shown that Orchardgrass stockpiles very well but may not hold it’s quality as well as fescue.
Nitrogen sources are very important for summer applications. Urea or 28% N solutions are easily lost if 0.5 inch of rain isn’t received in 3 or 4 days. We lost 40% of the urea applied when rain was 5 days coming after application in a N trial in Perry county last summer. Ammonium nitrate (34-0-0), Ammonium Sulphate (21-0-0-21) and Diammonium Phosphate (DAP-18-46-0) are non-volatile sources that can be applied anytime without significant losses during hot, dry weather. DAP should only be used on pastures where the phosphorus is needed., but the N in DAP is very inexpensive if you were going to buy P fertilizers anyway. If you do plan to apply P use DAP when your pastures are going to need a boost. Although cost per lb. of N is higher for ammonium nitrate the reduced chance of loss make it my preferred source for summer applications. Ammonium nitrate can be hard to find during the summer in some locations. Locate your source well before you need it.
There is no point in applying N and growing more forage than your animals can utilize. Vary your rates and timing depending on your previous experiences and stocking rates.