Are We Feeding Too Much Phosphorus?

The practice of feeding elevated phosphorus (P) levels has come under scrutiny recently as concern grows over the impact that livestock feeding practices have on manure nutrient concentrations and the environment. Concern over the land base necessary to utilize the phosphorus in manure as a crop nutrient has resulted in various livestock species reevaluating the amount of phosphorus required in feeds. Swine research has shown that phosphorus levels fed to pigs commonly exceed the requirement. The development of the commercial phytase enzyme has improved the availability of P found in feedstuffs thus allowing the amount of supplemental phosphorus in pig diets to be decreased.

A survey of university extension specialists, nutrition consultants, and feed industry nutritionists by Dr. Larry Satter of the United States Dairy Forage Research Center in Wisconsin showed that dairy cows are fed phosphorus well in excess of NRC requirements. The average P level in the diet dry matter was 0.48% with some cows receiving 0.6%. The NRC recommends 0.34 to 0.41% (dry matter basis). Higher levels of 0.48% are recommended only during the first 3 weeks of lactation to compensate for the reduced feed intake which usually occurs at this time. Dr. Satter asks the question "Is the NRC recommendation out of line, or are producers simply feeding more than they need?"

To determine a requirement one must look at P availability in feedstuffs and the normal losses of P (ie. the maintenance requirement). The majority of P in grains is found as phytate phosphorus. This is a problem for monogastrics because they lack the phytate enzyme which would allow them to make better use of naturally occurring P. Ruminants, however, utilize phytate phosphorus due to the phytate enzyme produced by rumen microbes. The estimates of phosphorus availability used in the current NRC is 50%. In 1993, the working group for developing dairy feeding standards in Germany adopted a value of 70% for phosphorus availability and the USDA Dairy Forage Research Center feels this is a fair value to use. If we accept a higher availability of naturally occurring P, it is not necessary to supplement as much P to the diet. There are also significant differences in the maintenance and production requirements as calculated by NRC and the German group.

It is ironic to note that, in spite of differences in P availability, maintenance and production requirements, both NRC and the recently revised German recommendations arrive at the same conclusions. Cows producing 110 lbs. milk/day require 0.41% P(NRC) or 0.4%P (German). Cows producing 66 lbs. milk/day require 0.37% or 0.35% P (NRC or German, respectively). Why then are P levels being fed that are, on average, 20 to 25% higher than recommended? Dr. Satter estimates this overfeeding is costing the US dairy industry $100 million annually.

A lack of faith in feeding standards because of the uncertainties discussed above has likely led to overformulation as a safety margin. However, the single largest factor resulting in higher P levels is likely the belief that feeding P in excess of the recommendation will improve reproduction. While there is an occasional observation that reproduction improves with elevated P levels, the overwhelming evidence is that P has no effect on reproductive performance as long as P levels are adequate to sustain rumen microbial growth. If microbial growth is reduced due to very low P (<0.25%), digestibility and energy supply may be negatively impacted causing an indirect effect on reproduction. A research project published in the Journal of Dairy Science (October, 1999) showed that feeding 0.28% P was sufficient for cows producing 20,000 lbs. per lactation. No effect on reproductive traits was noted when compared to higher feeding levels. The 0.24% level resulted in reduced dry matter intake, milk production, and body weight and was clearly inadequate to meet the P requirement of high yielding dairy cows. These results concur with the results of 13 trials summarized by Dr. Satter which looked at reproductive traits such as days to first estrus, days open, services for conception, days to first AI and pregnancy rate.

What impact will reducing dietary P levels have on the environment? Reducing dietary P from 0.48% to 0.4% will reduce phosphorus in the manure by 25 to 30%. This reduction will result in 25 to 30 % less land required for manure disposal if P content becomes the determining factor for the application rate of manure. Both the NRC feeding standards committee and those in Europe are currently reevaluating their P recommendations in response to concern over concentrations in the manure. The day may soon come when overfeeding of critical nutrients, simply as a "safety margin", is not acceptable.