Responsibly Meeting Phosphorus Requirements of Dairy Cattle

Dr. Ronald L. Boman
USU Extension Dairy Specialist


At the 2002 Utah Dairy Seminars we were privileged to have Dr. Larry Satter (world renowned dairy researcher from Wisconsin) teach us about the problems with over-feeding phosphorus in dairy rations. Dr. Satter visited Utah three years ago to speak about the same issue at the Intermountain Nutrition Conference, but this time he came with additional research information. The problem with over-feeding phosphorus is two-fold. First, it is an unnecessary expense which should be avoided, especially with the current low milk prices. Second, dairy producers are having to account for phosphorus in their Comprehensive Nutrient Management Plans. Applying manure to cropland based on phosphorus content instead of nitrogen will essentially double the amount of land required. Environmental concerns with phosphorus are associated with pollution of streams and lakes. Excess phosphorus in water causes algae and aquatic plants to grow very rapidly. Decomposition of the algae and aquatic plants consumes the oxygen in the water and adversely affects fish and other aquatic animal and plant life, as well as the use of this affected water for culinary purposes.

Dr. Satter dispelled the �myth� that phosphorus (P) above the current 2001 NRC recommendations (0.32 to 0.38%) for dairy cattle will enhance reproductive performance. Dr. Satter summarized 13 research trials with lactating dairy cows fed a low (0.32 to 0.39% of DM) or high phosphorus (0.40 to 0.61% of DM) ration. There were 390 cows at each level of phosphorus. There were no differences in reproductive parameters of these two groups. In another research trial a total of 267 cows were fitted with a radiotelemetric transmitter (Heatwatch) and blood samples were taken weekly for progesterone analysis. Diets assigned at calving contained either 0.37% P or 0.57% P. There were no differences in 16 measures of reproductive performance between the two levels of dietary P.

What about milk production response to dietary phosphorus level? Dr. Satter presented data from 9 research trials with 262 animals per group offered dietary P levels of either 0.34% or 0.47%. Average milk production (4 trials were complete lactations) averaged 66.9 and 66.7 lbs/day for the respective P levels.

The Rest of the Story:

Phosphorus fed in excess of the requirement is excreted, mainly in the feces, but also in the urine. Reducing P content of average U.S. dairy diets from 0.45% P to the average NRC recommendation of 0.36% P represents a 20% reduction in dietary P and a 25 to 30% reduction in manure P. Reducing dietary P concentration not only reduces P content of manure, but it reduces the vulnerability of P in manure of being solubilized in run off water following field application.

The 2001 NRC model used P absorption coefficients (availability) of 64 and 70% for forages and concentrates, respectively. Dr. Satter�s laboratory results suggest that the true digestibility or availability is in the range of 70 to 85%. So it makes sense for nutritionists to bring dietary phosphorus levels in line with NRC recommendations (which are adequate to excessive) and for dairy producers to insist that this happens.

The full text of Dr. Satter�s handout will be available on the USU Extension web site at

http://extension.usu.edu/coop/ag/livestock/dairy/index.htm
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If I can be of further assistance, please feel free to contact me at (435) 797-2163 or ronb@ext.usu.edu.

References:

(1) Knowlton, Katherine F. and Rick Kohn. 1999. �We�ve got to stop overfeeding phosphorus.� Hoard�s Dairyman. June, p. 437.

(2) Satter, L.D. and Z. Wu. 2000. �Phosphorus: Nutritional Management of Phosphorus in Dairy and Beef Rations.� Proceedings 2nd Annual Intermountain Nutrition Conference, Salt Lake City, Utah.

(3) Satter, L.D. 2002. �Meeting Phosphorus Requirements of Ruminants in an Environmentally Responsible Way.� Handout at Utah Dairy Seminars.

(4) Wu, Z., L.D. Satter, A.J. Blohowiak, R.H. Stauffacher, and J.H. Wilson. 2001. Milk production, estimated phosphorus excretion, and bone characteristics of dairy cows fed different amounts of phosphorus for two or three years.� J. Dairy Sci. 84:1738-1748. ©