Supplemental P May Not Pay In Dairy Diets

In response to water quality laws passed in the 1970s, manufacturers reformulated laundry detergents to remove the phosphorus. In the near future, water quality concerns will prompt new regulations for dairy farms.

You can’t reformulate dairy cows. But you can reformulate their rations to contain less phosphorus … and you should, if you’re feeding phosphorus at more than .4 percent of ration dry matter, according to a University of Wisconsin-Madison researcher. Your cows will perform just as well, you’ll reduce the amount of phosphorus that runs off to our waterways, and you’ll save some money on phosphorus supplements, according to Larry Satter, a dairy nutritionist at the USDA Dairy Forage Research Center.

Most Wisconsin dairy farms can expect to deal with phosphorus nutrient management in the future, as government regulations restrict phosphorus runoff from agricultural lands. Current manure-application guidelines, based on crop nitrogen needs, allow phosphorus to build up and run off from farmlands. Future guidelines will probably be based on phosphorus, rather than nitrogen.

In the meantime, dairy farmers might want to look at the diets they”re feeding, Satter says.

Satter recently completed a nationwide survey that, along with several other surveys, indicates that most producers are still feeding diets containing .45 to .5 percent phosphorus. “This is too much,” Satter stresses. “It”s an absolute waste, and it”s contributing to deterioration of our surface waters.”

Research at the USDFRC and College of Agricultural and Life Sciences supports the move to lower dietary phosphorus levels. In a recent trial, Satter and his colleagues measured milk production and phosphorus levels in the manure of Holstein cows. The cows ate either a base diet containing .31 percent phosphorus (which is lower than a typical unsupplemented dairy diet), or diets supplemented with sodium monophosphate to contain .40 or .49 percent phosphorus (dry-matter basis).

Milk yields for the 308-day lactation were 23,738 pounds for the .31 group, and about 24,500 pounds for the supplemented groups. Feeding .31 percent phosphorus appeared to be borderline deficient; yields were lower in the .31 group due to decreased production in late lactation, the study showed. These results agree with several other studies showing that dietary phosphorus levels between .33 and .39 percent are adequate for moderate to high producing cows, Satter says.

Phosphorus content in manure increased as dietary phosphorus increased in Satter”s study. Cows fed .31 percent phosphorus conserved the nutrient by minimizing phosphorus excretion in manure and urine; cows receiving phosphorus supplements excreted more phosphorus through these routes.

High-phosphorus dairy diets seem to result in trouble-prone manure. CALS soil scientists Mark Powell and Larry Bundy found a four to five-fold increase in phosphorus runoff when the manure came from cows fed diets with .49 percent phosphorus compared with .31 percent phosphorus. This happened even though the total amount of phosphorus in manure applied to plots was the same.

More than a dozen modern studies have shown that supplementing phosphorus levels in typical dairy diets does not improve reproductive performance. The phosphorus/reproduction link is based on early 20th-century studies of grazing cattle in South Africa, and studies from the early 1950s in Scotland. These animals ate extremely poor diets with little or no supplemental grain. When fed diets with supplemental phosphorus, their reproductive performance improved. Dairy cows in Wisconsin never see these skimpy diets, however.

Satter is currently monitoring the breeding records of five cooperating herds (averaging 23,000 to 28,000 pounds of milk) that feed phosphorus at about .4 percent, the level recommended by the National Research Council. In a future study, Satter and reproductive physiologist Milo Wiltbank will monitor expression of estrus in lactating cows fed either .37 or .48 percent dietary phosphorus.

Most Wisconsin dairy diets using homegrown feeds contain .35. to .39 percent phosphorus, according to Satter. For example, a diet of alfalfa, corn silage, corn and soybean meal containing 16 to 18 percent protein will typically contain .35 to .38 percent phosphorus. Many byproducts are rich phosphorus sources. Some producers who rely heavily on byproduct feeds may be at .45 to .48 percent phosphorus without supplementation, Satter says, while those feeding byproducts at more typical levels would be at about .4 percent phosphorus. The University of Wisconsin dairy herds today get very little supplemental phosphorus today, he notes.

Most dairy farmers now feed about .5 percent phosphorus; cutting back to .38 to .4 percent reduces the phosphorus in manure by about 30 percent. Diets containing .35 percent phosphorus still offer a generous safety margin, since most studies show that problems in dairy cows don”t begin to show up until dietary phosphorus levels fall below .3 percent, Satter notes.

Below .25 percent dietary phosphorus, rumen bugs run out of phosphorus and milk production drops. At very low phosphorus levels, .15 percent to .2 percent, reproduction can suffer. Beef cows might encounter these levels grazing corn stalks or winter pasture, for example. “But dairy cows aren”t ever going to hit these low levels on any dairy diet we could put together,” Satter says.

“We still have ample land area in Wisconsin to utilize dairy manure, unlike some states. North Carolina and Maryland, for example, have saturated their soils and will probably have to depopulate some of their livestock and poultry populations,” he says. “We have time to act, but if we fail to act by reducing excessive phosphorus in our livestock rations, we may find ourselves in the same fix as North Carolina and Maryland.”