Whole-farm approach handles phosphorus on dairy farms


MADISON, Wis. – What goes in must come out, and in the case of phosphorus, too much has been going into cows and coming out on farmlands.

Good and the bad. New federal rules aimed at curbing runoff pollution are going to affect many farms.

The bad news: surveys show phosphorus levels in most farm soils are too high, and if they conduct business as usual, many dairy farms will not be able to comply with the proposed phosphorus rules.

The good news: proper whole-farm phosphorus management will allow most of those farms to meet the regulations, according to Mark Powell, an agroecologist at the USDA-ARS Dairy Forage Research Center.

Continued study. Powell has studied how phosphorus management in one dairy system component (e.g. feed) affects other system components (soils and crops), and how whole-farm phosphorus management can help producers comply with forthcoming nutrient management regulations.

In 1999, 75 percent of the major soils in Wisconsin tested above high (24 ppm) and 50 percent tested greater than excessively high (38 ppm) in phosphorus levels, according to UW-Madison studies.

On dairy farms, these increases usually occurred because imports of phosphorus in feed and fertilizer exceeded exports in milk, cattle, and surplus grain or hay.

Explanation. Many of the environmental problems facing animal agriculture are due to the separation of livestock production from its feed supply, Powell said.

Swine and poultry operations usually import their feed, and new phosphorus regulations will pose major hurdles for those industries.

On the other hand, most Midwest dairy operations raise most of their own feed and recycle manure through cropland.

Powell’s research has shown that most state dairy farms have stocking rates of less than 0.44 cows per acre, the threshold value for self-sufficiency in forage and grain production.

Self-sufficiency means that a farm has adequate land to recycle its manure phosphorus through crops.

Self-sufficient stocking rates will vary from farm to farm. Farms feeding recommended levels of phosphorus and spreading manure on all their available cropland can maintain higher stocking rates without increasing phosphorus runoff than farms feeding phosphorus excessively and spreading manure on only parts of their cropland, Powell notes.

“On many dairy farms, the phosphorus problem originates not so much from excessive stocking rates, but rather from a combination of high dietary phosphorus levels and inadequate utilization of available cropland for manure spreading,” Powell said.

Finding a balance. Balancing phosphorus inputs and outputs through proper feed, fertilizer and manure management is the first step toward reducing soil phosphorus buildup and runoff phosphorus losses from dairy farms.

Farmers and their nutrient management consultants need to look at the whole-farm nutrient package, Powell said, and develop ways to manage nutrients more efficiently to increase profits and conform to nutrient management regulations.

Recommendations. The National Research Council recommends that diets for high-producing cows contain 0.38 percent phosphorus, with 0.48 percent recommended for the first three weeks of lactation.

However, when he surveyed Wisconsin farms, Powell found that the phosphorus content of dairy diets ranged from 0.23 percent to 0.85 percent phosphorus.

About 85 percent of the surveyed dairy farms fed phosphorus in excess of NRC requirements, and more than half of all cows were fed phosphorus in excess of 0.38 percent.

If new rules restrict manure application to cropland to prevent phosphorus accumulation, supplementing dairy diets with inorganic phosphorus will increase the cropland requirement for manure phosphorus recycling dramatically, Powell said.

“Balancing phosphorus inputs and outputs through integrated feed, fertilizer and manure management is quickly becoming the principal regulatory challenge facing the U.S. dairy industry,” Powell said.

Critical information. “Feed consultants and veterinarians need to know that their dietary phosphorus recommendations could very well be the most critical element of a farmer’s ability to comply with nutrient management regulations, especially for farmers having limited cropland area upon which they can spread manure.

“The link between dietary practices and water quality impairment needs to be incorporated into whole-farm nutrient management planning.”


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