The environmental protection agency (EPA) in the U.S. is obligated to monitor phosphorus concentration of the soil where dairy and swine manure is applied. Surveys reveal that U.S. dairymen have been overfeeding phosphorus to their cows for years. This means that surplus phosphorus is excreted into the manure and applied to the soil.

The phosphorus-to-nitrogen ratio of manure is about twice the ratio that crops normally require. If we apply manure to meet the nitrogen requirement of the crop, then the soil will accumulate the extra unused phosphorus. Excess phosphorus in the soil eventually runs off into streams and lakes. It causes excess growth of algae in lakes. The EPA is now establishing programs to closely monitor phosphorus in soil. A higher concentration of phosphorus in manure means that more phosphorus will accumulate in soil. The easiest way to reduce phosphorus content of manure is to reduce phosphorus intake for cows.

The new 2001 National Research Council (NRC) Nutrient Requirements for Dairy Cattle has been published. NRC did an in-depth review of the research on phosphorus requirements for dairy cows. Using the new NRC model, a 1400 pound cow that is 100 days in milk and milking 100 pounds per day requires 69 grams of absorbable phosphorus. The model also predicts dry matter intake (DMI) for this cow at 58.8 pounds. Using an average absorption coefficient of 65%, the dietary phosphorus should be 106 grams or 0.4% of the ration dry matter. The 0.4% ration dry matter is important and it is lower than previous NRC recommendations. If we lower phosphorus levels in the ration, we can save significant money for dairymen. Reducing phosphorus from 0.5% to 0.4% of ration dry matter lowers the amounts of phosphorus fed by 0.05 pounds per cow per day. This is equal to 1/4 pound of dical per day. This adds up to approximately $1500 per year for each 100 cows.

Several milk marketing co-ops in the U.S. have adopted a policy that states that their members will feed the lowest level of phosphorus possible that is consistent with animal health and production. The members goal is to feed no supplement inorganic phosphorus in the ration. Members are encouraged to use recycled byproduct feeds to meet phosphorus requirements. Examples of excellent byproduct phosphorus sources are brewer's grains, distillers grains, and wheat midds. The ultimate goal is to lower phosphorus in the manure and to lower phosphorus in the soil.

The take home lesson from this paper should be to save dairymen money by lowering phosphorus to 0.4% of ration dry matter. Lower intakes means lower excretion and lower phosphorus build up in the soil.

Dr. Whitmore's comments: The state of South Dakota is developing a plan to monitor phosphorus concentration in the soil. We must lower phosphorus in our feed and manure. We may need 2 or 3 times more land to spread manure on if phosphorus is not reduced. We would have to pump the manure several miles to obtain this much land. This would be very expensive. My personal opinion is we can safely restrict phosphorus so long as DMI remains satisfactory. My guess is that soil phosphorus is an important issue in Japan also.

Parts of this article were published in Dairy Today, June/July, 2001, page 9.