Extension by Aaron Berger, UNLWritten by Aaron Berger
Mature cows should excrete 100 percent of the nutrients they consume in terms of nitrogen, phosphorus and potassium.
For example, take 100 cows fed 30 pounds per head per day of 17 percent protein alfalfa hay that is .03 percent phosphorus and 2.4 percent potassium on an as-fed basis. What is the value of the nutrients in the manure and urine that are available to the meadow or field where the manure is deposited?
Two thousand pounds of alfalfa hay, multiplied by .17 crude protein, equals 340 pounds of protein. Nitrogen multiplied by 6.25 equals crude protein, and by taking 340 pounds of crude protein and dividing by 6.25 gives 54.4 pounds of nitrogen in the ton of hay. Only about 35 percent of the nitrogen in manure and urine is available to be used, and the balance is lost to volatilization as ammonia. Using 54.4 pounds of nitrogen in the feed and multiplying it by 35 percent equals 19 pounds of nitrogen available for use by growing plants out of that ton of alfalfa hay.
The availability for phosphorus and potassium in manure and urine from feed consumed is 100 percent. To find the value of phosphorus and potassium, take 2,000 multiplied by .0003 equals six of phosphorus, and 2,000 multiplied by .024 equals 48 pounds of potassium.
In one ton of alfalfa hay, there are approximately 19 pounds of nitrogen, six pounds of phosphorus and 52 pounds of potassium that are applied to the ground in the manure and urine where the hay is fed. The fertilizer nutrient value of these minerals in a ton of hay at 0.70 per pound of N ($13.30), .65 per pound of P ($4.20) and .50 per pound of K ($26) would, in total, equal $43.50 per ton.
It is common to see weed problems develop on rangeland where cattle are fed during the winter months, as the nutrients from the hay are often concentrated in feed areas and the availability of nitrogen, especially in rangeland situations, encourages weed growth, so look for an opportunity to feed cattle on ground where the nutrients can be utilized on meadows or for growing annual forages.
Fertilizer prices appear to be likely to continue to go up in price. Finding ways to effectively utilize and recycle nutrients will continue to be increasingly important. Strategically thinking about how to capture the value of nutrients in harvested forage is one way to reduce the need for purchased fertilizer.
For more information on calculating the nutrient value of harvested feeds, visit extension.missouri.edu/p/G2083.