Reduce Ration Phosphorus / Save $$$ / Reduce Land Area for Applying Manure and Reduce Environmental Contamination

Dr. Ronald L. Boman
USU Extension Dairy Specialist

Phosphorus (P) is required by lactating cows for bone formation and maintenance, milk secretion, building muscle tissue, energy metabolism, and a host of other physiological and metabolic processes. In fact, phosphorus has more known biologic functions than any other mineral element. In the past, phosphorus was often fed to dairy cattle in excess of published requirements because high phosphorus diets were commonly believed to improve reproductive performance. This false perception likely originated from the observation that severe phosphorus deficiency impairs reproductive performance in range cattle that had dietary dry matter phosphorus levels of 0.25% or lower and at the same time were energy deficient. This dietary concentration is far below the concentration found in most feeds in modern dairy rations even without any P supplementation. Most by-product and high protein feeds contain from 0.60 to 1.2% P on a dry matter basis. However, beet pulp has very low levels of P (0.09%). Although severe P deficiency may impair reproductive performance, there is no research data to suggest a benefit from feeding P to dairy cows in excess of the 2001 NRC requirements (0.32 to 0.38% of dry matter intake). Wisconsin researchers summarized 13 trials where reproductive performance of dairy cows fed different levels of P was measured and found no relationship between reproductive performance and dietary P content. Another summary, by Dr. Larry Satter, of milk production response to dietary P level by seven groups of researchers compared average dietary P level of 0.34 and 0.57%. There were a total of 262 cows at each level of P and four of the trials were for complete lactations. There was no difference in milk production between the two levels of dietary P. On a practical basis, reducing the dietary P level from 0.45 to 0.35% of dry matter would save $1,800 to $2,000/year for a 100-cow herd (based on the cost of DiCal of $360 to $400/Ton). While most nutritional consultants are bringing their P formulations into line with current research and NRC recommendations, there are still a lot mineral packages with high levels of P. It is important for dairy producers to insist on having their nutritional consultants reduce the dietary P down to acceptable levels. Depending on the feeds being fed it may not be necessary to add any inorganic P to ensure dietary P levels of 0.32 to 0.38% of DM.

The cost savings of reducing the dietary P levels is only the tip of the iceberg. Essentially all of the dietary P above the animal�s requirement is excreted in the manure. Reducing dietary P can have a very significant effect on the amount of land required to effectively utilize manure P. It has been estimated that a cow consuming a ration of 0.35% P and producing 20,000 lbs of milk/year would excrete enough P in her manure that 1.3 acres of land would be needed to effectively utilize the P according to crop uptake. For the same cow receiving a dietary P of 0.45% it would require 1.9 acres of land for manure utilization, and a dietary P level of 0.55% would require 2.4 acres of land.

Public scrutiny of the impact of agricultural practices on the environment is growing. Animal agriculture, rightfully of wrongfully, is being blamed for much of the phosphorous contamination of our surface waters. Excess P in water causes algae populations to grow rapidly, or to �bloom�. The subsequent decomposition of the algae consumes dissolved oxygen in the water, which is a major factor affecting the growth and reproduction of fish and other aquatic animal life. These algae blooms can be caused by P in the manure getting into the surface water either as runoff or by leaching.

Reducing P to 0.32 to 0.38% (according to production level) of the ration dry matter is a win-win-win situation: First off, you save money because P is the most expensive of the macro minerals; second, you reduce the number of acres of land required for applying manure for efficient utilization of P; and finally you demonstrate to the public and to EPA that you are trying to be a good steward of the environment. These three benefits make reducing dietary P to recommended levels a �No Brainer�. ©