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Dairy cows must follow the law of conservation of mass. What this means is that everything that goes into a cow must eventually come out. Nutrients consumed by a cow are secreted in milk, excreted in manure, or retained in the body (either in the cow or in her developing fetus). The nutrients secreted in milk are shipped off the farm several times each week, and most of the nutrients in the cow or her calf eventually leave the farm as a bull calf or a culled dairy cow. The excreted nutrients, although not as valuable as the nutrients in milk have substantial value as fertilizer. The most important nutrients when we consider both the cow and the crops are nitrogen (in nutrition, we call this crude protein), potassium, and phosphorus.
Based on the average milk yield in the U.S., an average lactating Holstein cow eats about 48 lbs./day of dry matter. Survey data on the nutrient composition of an average diet are not available (surveys generally provide data from high-producing herds), but assuming an average cow is fed to her approximate nutrient requirements (plus a little extra), a typical diet probably has about 16% crude protein (2.6% N), 0.38% P, and about 1.2% K. These concentrations will vary greatly from farm to farm, and actual farm data should be used when determining manure application rates.
About 25% to 30% of the N consumed by a cow is secreted in milk. Using the above assumptions, an average cow will excrete about 0.9 lbs./day of N in manure. Excretion of manure N increases linearly as N (i.e., protein) intake increases; the amount of N excreted per day could easily vary by +/- 25% (0.67 to 1.3 lbs./day) across herds because of diet composition, feed intake, and milk production and composition. Form of dietary protein affects the route of excretion (feces or urine) much more than it affects the amount of N excreted. Substantial losses of N from manure can occur during its handling, storage, and field application, which means that the amount of N in manure that is applied to crops is usually much less than what is excreted by the cow.
If P is fed close to requirements, about 70% of the P consumed is excreted in manure, which means that the average cow will excrete about 0.13 lbs./day of P. As P intake increases, the amount of P excreted in manure increases linearly.
Depending on the concentration of K in the diet, 70% to 80% of the K consumed by a cow is excreted in manure. Based on the assumptions above, the average cow would excrete about 0.4 lbs./day of K. As with all other nutrients, as intake of K increases, manure excretion increases.
Based on these assumptions, the average cow excretes manure with a P:K:N ratio of about 1 unit of P to 3.1 units of K to 6.9 units of N. Because of substantial losses of N, the concentration of N relative to P is much lower in manure when applied to crops (discussed below).
The fertilizer needs of crops depend on the crop, yield potential, soil characteristics, and the crop grown on the soil the previous year. Fertilizer recommendations are given as lbs./acre, but that value can vary greatly depending on yield potential, soil, crop, etc. However, the ratio of P:K:N (as actual P, K, and N, not P2O5 or potash) is more similar across yield potential and crops, and ratios can be used to determine proper application rates for manure (see example below). For corn grain, an average recommendation for P:K:N fertilization is 1:5:6 (for every 1 unit of P, you need to apply 5 units of K and 6 units of N). If the corn followed alfalfa, then only 3 or 4 units of N are needed per unit of P. For corn silage, the ratio is 1:6:5, and for wheat, it is about 1:7:4. If the crop is a legume such as soybeans or alfalfa, then very little, if any, N fertilizer is recommended. Nutrients applied in different ratios than those above will cause buildup of soil nutrients. In some cases, this is a good thing (for example, soil with low P will benefit from some P accumulation), but if soil is already adequate in the nutrient, additional buildup can cause environmental problems and eventually crop issues. Assuming no loss of N during manure storage (which will NOT happen), manure from a typical lactating cow has a P:N ratio that is reasonably close to the needs for non-legume plants. Manure is low in K relative to P and N (fresh manure), which means supplemental K fertilizer will often be needed.
In this example, the farm is growing corn silage with an expected yield of 20 tons/acre (7 tons of dry matter), the crop grown the previous year was wheat, and the farm has average soil characteristics (soil P, K, cation exchange capacity, etc.). The standard N, P, and K fertilizer recommendations would be about 28 lbs. P (65 lbs. of P2O5), 170 lbs. of K (200 lbs. of K2O), and 140 lbs. of N/acre. If manure had a P:K:N ratio of 1 to 3.1 to 3.5 (assuming half the N in manure is lost) and you applied manure to meet the P requirement of the crop (28 lbs.), you would apply only 87 lbs. of K (28 x 3.1) and 98 lbs. of N, both of which are well below the requirements of the crop. This means additional K and N fertilizer would be needed. If you applied that same manure but at a rate to meet the K needs of the crop (i.e., 170 lbs.), then 54 lbs. of P (170/3.1) and about 190 lbs. of N (170 x (3.5/3.1) would be applied per acre. Over time, this would result in substantial soil buildup of P.
Feeding adequate but not excessive amounts of N (protein) and P to dairy cows will result in manure that is closest to what most crops require. Preventing loss of manure N by proper manure handling, storage, and soil application and by feeding the correct forms and amounts of protein will improve the fertilizer value of the manure. Maintaining MUN in the acceptable range not only can reduce feed costs, it can also improve the fertilizer value of the manure.
Department of Animal Sciences
Ohio Agricultural Research and Development Center
The Ohio State University