If you”re supplementing phosphorus in a typical Wisconsin dairy diet, you”re probably feeding too much of the mineral. Cutting back on supplementation won”t hurt production or herd health, and you”ll be doing a favor for water quality, according to researchers at the University of Wisconsin-Madison”s College of Agricultural and Life Sciences and the USDA Dairy Forage Research Center.

A recent field trial showed that reducing dietary phosphorus will reduce the total amount of phosphorus in dairy manure, and greatly reduce the amount of phosphorus that”s vulnerable to runoff, according to soil scientist Larry Bundy, soils graduate student Angela Ebeling and USDA soil scientist Mark Powell.

In this trial, Holstein cows and first-calf heifers were fed unsupplemented diets containing 0.32 percent phosphorus or diets supplemented with monosodium phosphate to contain 0.48 percent phosphorus.

Reducing dietary phosphorus by one-third reduced the amount of phosphorus in manure by one-half, according to Ebeling. It also reduced by a factor of 4 or 5 the amount of phosphorus that ran off from fields where the manure was applied. At 0.32 percent dietary phosphorus, manure contained 0.49 percent phosphorus. At 0.48 percent dietary phosphorus, manure contained 0.89 percent phosphorus, so feeding 50 percent more phosphorus produced manure containing 100 percent more phosphorus. Things got worse after the manure hit the ground, Ebeling reports.

In June 1999, the manures were surface-broadcast on silt-loam soils at the Arlington Agricultural Research Station, at either 25 tons or 9.4 tons per acre. The experimental plots weren”t tilled in 1999, and had corn residue from the previous year. The researchers collected runoff right after a simulated heavy rainfall, then planted corn in the plots.

Runoff from plots with low-phosphorus manure contained 0.27 parts per million of dissolved reactive phosphorus. Runoff from plots that received the same weight of high-phosphorus manure contained 2.87 parts per million — more than 10 times as much. High-phosphorus manure applied at the same phosphorus rate as low-phosphorus manure produced runoff containing 1.2 parts per million phosphorus — almost five times as much.

Results from an October rainfall simulation showed that dietary phosphorus concentrations influenced phosphorus concentrations and loads in runoff more than four months after the manure was applied. The high-phosphorus manure still produced runoff containing four times as much phosphorus as the low-phosphorus manure at equal manure weights. The researchers also monitored natural runoff from November 1999 to May 2000. The natural runoff showed the same trends as the simulated rainfalls.

High phosphorus concentrations in the runoff are probably due to the monosodium phosphate supplemented in the high-phosphorus diets, according to Ebeling.

“The marked differences in runoff phosphorus concentrations and loads between the low and high phosphorus manure treatments even when the manures were applied at equal phosphorus rates is likely due to differences in the phosphorus composition of the manures,” she says. Cows that eat more phosphorus than they need will excrete the unneeded phosphorus in their manure. The high-phosphorus manure contained and released more inorganic phosphorus than the low-phosphorus manure.

The National Research Council currently recommends 0.38 percent phosphorus for dairy diets. The CALS researchers used 0.32 percent. Ebeling says that a 0.38 percent phosphorus diet would still produce much less phosphorus runoff than a 0.48 percent diet.

A recent national survey by Larry Satter, a CALS dairy scientist at the U.S. Dairy Forage Research Center, showed that many farmers are still adding too much phosphorus to dairy diets.

Most dairy farmers now feed about 0.5 percent phosphorus; cutting back to 0.38 percent would reduce phosphorus in manure by about 30 percent. Diets containing as little as 0.35 percent phosphorus still offer a generous safety margin, Satter says, since most studies show that problems in dairy cows don”t begin to show up until dietary phosphorus levels fall below 0.3 percent.

Typical Wisconsin dairy diets using homegrown feeds contain 0.35 to 0.39 percent phosphorus without supplementation, according to Satter. For example, a diet of alfalfa, corn silage, corn and soybean meal containing 16 to 18 percent protein will typically contain 0.35 to 0.38 percent phosphorus.

Phosphorus that runs off from land-applied manure encourages weed and algae growth in surface waters. When the plants die, they rot and stink, and can deplete oxygen to fish-killing levels. State and federal regulators have targeted non-point phosphorus pollution, and farmers are going to see new regulations aimed at reducing phosphorus runoff from farm fields.

“I”m excited about the results of this study because they are directly applicable on the farm,” Ebeling says. “Producers can start immediately by controlling the nutrient flow on their individual farms. If producers nationwide could work on this seemingly small scale by reducing unneeded supplemental phosphorus in dairy diets, then the environment will be positively affected on the global scale.”

This research was supported by the USDA”s Agricultural Systems Research Program, the Wisconsin Fertilizer Research Fund, the UW Nonpoint Pollution and Demonstration Project, and CALS.