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
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
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
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
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,
"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 NRCís 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 farmerís 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."