By Dr. Gene P. Gengelbach, Ph.D., P.A.S.
Phosphorus (P) is a macro-mineral that is necessary for proper growth and development. However, in many areas of this country and around the world excess phosphorus in the surface water is looked on as a serious source of pollution. Phosphorus is involved in the eutrophication of lakes and streams, whereby excess nutrients like nitrogen and phosphorus cause excessive growth of aquatic plant life which can reduce light penetration, and the decomposition of this excess organic matter can deplete oxygen in the water. Therefore, we must understand what an animal’s phosphorus requirement is and then balance the diet accordingly.
Phosphorus is required for a wide variety of functions within the body. Phosphorus has more known biologic functions than any other mineral element. Even though approximately 80% of the phosphorus in the body is located in bones and teeth, P is found in every cell in the body. It is involved in almost every reaction about the transfer of energy in the body and the acid-base buffer systems in the blood. It is found as a component of cell membranes as well as in nucleic acids in the cell nucleus.
Phosphorus concentration in the blood plasma is closely regulated, normally 6 to 8 mg/dl in growing animals and 4 to 6 mg/dl in adults. Phosphorus is absorbed into the blood from the small intestine; excess phosphorus is secreted into saliva so it can either be reabsorbed or else eliminated in the feces (see Figure 1). The secretion of 1,25-dihydroxyvitamin D due to low levels of dietary P increases the absorption of P in the intestine. High levels of calcium relative to P in the diet can reduce the efficiency of phosphorus absorption. On the other hand, cows that suffer from hypocalcemia (low blood Ca) may also become deficient in P because while parathyroid hormone (PTH) increases the mobilization of both Ca and P from the bone, it also increases the excretion of P by the kidney and in the saliva.
Phosphorus deficiency is most commonly found in animals consuming forage from soils deficient in P or consuming excessively mature forages or crop residues low in phosphorus. Signs of deficiency include poor appetite, reduced growth rate or milk production, and general unthriftiness. Severe phosphorus deficiency can cause bone malformation, specifical rickets in growing animals and osteomalacia in adults. Phosphorus toxicity is rare because cattle are quite efficient in excreting excess P in manure and saliva. Excessive P intake can interfere with both calcium and magnesium absorption. If adequate calcium is present in the diet, the maximum tolerable level of P for cattle diets is estimated to be around 1% of the daily dry matter intake.
Phosphorus requirements are somewhat difficult to quantify since P absorption can be affected by dietary factors such as Ca:P ratio, dietary concentrations of aluminum, calcium, iron, magnesium, manganese, potassium and fat and by the source of the phosphorus (grains, forages, or mineral supplements). In general, a dietary concentration of 0.30 to 0.34% phosphorus should be sufficient for normal growth and development of heifers. A recent study at the University of Wisconsin (Bjelland, et al., 2011. J. Dairy Sci., 94:6233-6242) evaluated diets containing either 0.30 or 0.40% phosphorus fed to Holstein and Holstein-Jersey cross heifers from 4 to 22 months of age. Both groups had similar rates of gain, hip height and width, body length, heart girth, cannon bone circumference, and pelvic area. Services per conception and age at pregnancy were not different between groups. During their first lactation, both heifer groups had similar production levels of milk, fat, and protein and similar days open and services per conception. The authors concluded that there was no advantage to feeding 0.40 vs. 0.30% phosphorus to growing and breeding heifers. Growing and finishing beef steers and heifers also have P requirements in the .24 to .30% range, while beef cows’ requirements are at the lower end of that range.
Lactating dairy cows require a maintenance level of phosphorus plus the amount of P secreted in milk, in addition to the extra P required for growth or pregnancy. The P requirement for a lactating cow can be determined as follows:
This 71-gram amount is the amount that needs to be absorbed from the diet.
The Agri-King ration program has individual P absorption values for each feed ingredient (it balances for absorbable P, not total P). Assuming that approximately 67% of the dietary P is absorbed, the daily intake would have to be at least 106 grams. This does not account for the phosphorus (30 to 90 g) that is recycled through saliva. Assuming that 30 grams would be ingested from saliva daily, then only 76 grams would be required to be in the diet. If the total feed intake was 55 lbs., then a concentration of 0.30% P in the diet would be required; if the intake was only 48 lbs. the required concentration would be approximately 0.35%. In a summary of all pertinent research trials, the 2001 Nutrient Requirements of Dairy Cattle (7th Revised Edition, National Research Council) reported that as long as P concentration in the ration of lactating cows was above 0.32%, reproductive performance was normal, and was not improved by feeding higher levels. Also, milk production was optimized with concentrations of between 0.32 and 0.42% phosphorus in the diet.
Since phosphorus is generally the most expensive macro mineral added to the ration and considering the environmental concerns about excessive P in manure, we have a responsibility to carefully examine the amount of phosphorus that is added to all rations. AK