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 has become a serious source of pollution.
It impacts the eutrophication of lakes and streams, whereby excess nutrients such as nitrogen and phosphorus cause excessive growth of aquatic plant life. This 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, with 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.
Almost every reaction about the transfer of energy in the body and the acid-base buffer systems in the blood involves the mineral. It is 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.
It 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 phosphorus increases the absorption of P in the intestine. High levels of calcium relative to phosphorus in the diet can reduce the efficiency of P absorption.
On the other hand, cows that suffer from hypocalcemia (low blood calcium) may also become deficient in phosphorus because, while parathyroid hormone (PTH) increases the mobilization of both calcium and phosphorus 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.
Toxicity is rare because cattle are quite efficient in excreting excess P in manure and saliva.
Excessive phosphorus intake can interfere with both calcium and magnesium absorption. If adequate calcium is present in the diet, the estimated maximum tolerable level of phosphorus for cattle diets is around 1% of the daily dry matter intake.
Phosphorus requirements are somewhat difficult to quantify since dietary factors such as Ca:P ratio; dietary concentrations of aluminum, calcium, iron, magnesium, manganese, potassium and fat all impact P absorption. The source of the P (grains, forages, or mineral supplements) also plays a role.
In general, a dietary concentration of 0.30 to 0.34% should be sufficient for normal growth and development of heifers.
A 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. You can determine the P requirement for a lactating cow as follows:
This 71-grams is the amount cows must absorb from the diet.
The Agri-King ration program has individual phosphorus absorption values for each feed ingredient. It balances for absorbable P, not total P.
Assuming absorption of approximately 67%, the daily intake would have to be at least 106 grams.
This does not account for the phosphorus (30 to 90 g) recycled through saliva. Assuming that 30 grams would be ingested from saliva daily, then only 76 grams would be required in the diet.
If the total feed intake was 55 pounds, then a concentration of 0.30% P in the diet would be required; if the intake was only 48 pounds, 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. Feeding higher levels did not improve reproductive performance.
Also, concentrations of between 0.32 and 0.42% phosphorus in the diet proved optimal for milk production.
Since it 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 added to all rations. AK
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