COVID-19 Response

For information about fall semester instruction and campus operations, please visit

During this time, please contact us at

Whole-Farm Approach Handles Phosphorus On Dairy Farms

What goes in must come out, and in the case of phosphorus, too much has been going into cows and coming out on farmlands. New state and federal rules aimed at curbing runoff pollution are going to affect many Wisconsin farms.

The bad news: surveys show that phosphorus levels in most Wisconsin 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.

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. He discussed his findings Jan. 15 at the Wisconsin Fertilizer, Aglime & Pest Management Conference.

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.

Many of the environmental problems facing animal agriculture are due to the separation of livestock production from its feed supply, Powell says. Swine and poultry operations usually import their feed, and new phosphorus regulations will pose major hurdles for those industries.

On the other hand, most Wisconsin 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 says.

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 says, and develop ways to manage nutrients more efficiently to increase profits and conform to nutrient management regulations.

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.

More dietary phosphorus produces more manure phosphorus. As far as crops are concerned, manure has too much phosphorus and not enough nitrogen. Repeatedly applying manure to meet the nitrogen needs of crops will cause phosphorus to accumulate in the soil, increasing the risk of runoff. 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 says.

Farms that produce manure phosphorus in excess of crop phosphorus requirements need to amend feed and/or fertilizer practices, seek additional land for manure application, export manure, and/or reduce animal numbers on their farms if they are to achieve phosphorus balance, Powell says. Amending feed and fertilizer practices is the simplest, quickest and cheapest solution for most Wisconsin dairy farmers, he points out.

On Wisconsin farms where manure phosphorus exceeded crop phosphorus requirements, lactating cows were fed, on average, 30 percent more phosphorus than NRC recommends for their level of milk production. Adopting NRC”s dietary phosphorus recommendations would reduce the number of farms and amount of land in positive phosphorus balance by two-thirds, he says.

Some dairy farmers could eliminate phosphorus supplements but still have problems, because common protein supplements contain a wide range of phosphorus concentrations. For example, meat and bone meal has a protein:phosphorus ratio of 11 to 1; corn gluten meal”s ratio is 108 to 1. On dairy farms with high soil phosphorus levels, low-phosphorus protein supplements could reduce manure phosphorus and the land required for manure application.

“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 says. “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.”