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United States Department of Agriculture

Agricultural Research Service

Title: Dairy Diet Effects on Phosphorus Cycles of Cropland

Authors
item Powell, J - UNIV OF WISCONSIN MADISON
item Wu, Z - UNIV OF WISCONSIN MADISON
item Satter, Larry

Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: November 11, 2000
Publication Date: N/A

Interpretive Summary: Many dairy producers feed excessive amounts of phosphorus (P) to their dairy cows. This results in higher than necessary levels of P in the manure and increases the risk of environmental pollution. The purpose of this study was to examine how excessive P supplementation of dairy cows affects the land required for recycling manure P back through crops, and the ability of dairy farms to recycle manure P in view of new regulations that limit the land application of manure based on crop P requirements. Using average crop yields, it requires about 1.5 acres of land to utilize the P excreted in manure by a dairy cow producing 20,000 lbs of milk per year if the cow is fed only the amount of P required. If P is fed 25% in excess of the requirement, a very typical situation in the United States, then the cropland required to safely utilize the manure P is increased by 38%, or to a little more than 2 acres per cow. This study illustrates how important an nimpact excessive P feeding can have on the land required to utilize the manure P in an environmentally safe way. Dairy farmers can use this information as they develop nutrient management plans to comply with oncoming state and federal regulations regarding nutrient management.

Technical Abstract: The repeated importation of feed and fertilizer has resulted in soil nutrient accumulation on dairy farms. Many dairy producers feed phosphorus (P) 25 to 30% in excess of what is required for maximum milk production. There is a need for integrated feed, manure and land management strategies that decrease environmental risks while maintaining farm profits. This paper examines how the excessive supplementation of a dairy cow diet with affects the land required for recycling manure P through crops, and the ability of dairy farms to recycle manure P in view of new regulations that limit the land application of manure based on crop P requirements. Approximately 0.60 ha of cropland is required to recycle manure P excreted by a lactating cow fed a P adequate diet. The addition of supplemental P to raise dietary P from 3.8 g kg**-1 of diet dry matter, a level adequate for lactating cows, to 4.8 g kg**-1, typical of diets currently fed in North America, increases the cropland needed for recycling manure by 38%. The decision on how much dietary P to feed profoundly affects how many cows and for how long a farm can support milk production before all cropland attains excessive levels of P. For example, the annual addition of 6.8 kg P per cow to increase dietary P from 3.8 to 4.8 g kg**-1 for a 90 cow herd on a 80 ha farm would create excessive soil P levels in all fields within 40 years. A strategy of reducing dietary P would be a win-win situation for dairy producers, since both feed costs and the amount of manure P that has to be recycled are reduced. Strong partnerships among researchers, extension agents, feed, and fertilizer consultants are needed to provide integrated approaches to nutrient management on dairy farms and reduce negative environmental impacts.

Last Modified: 4/18/2014
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