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 Knowledge for the CommonWealth

Can Overfeeding Phosphorus Lead to Polluted Waters?

Dairy Pipeline: January 2000

Gerald M. (Jerry) Jones
Extension Dairy Scientist
Virginia Tech
(540) 231- 4764

Is there a chance that you are overfeeding phosphorus which could contribute to water pollution? Scientists compared data from the Cornell University dairy farm for 1994 and 1979 and found that increased concentrations of nitrogen (N) and phosphorus (P) in well water over this period was attributed to increased purchases of these nutrients in feed. When more phosphorus is imported in feed or fertilizer than is exported through milk or animals, the excess is lost from the farm as surface runoff or it accumulates in soils. Cornell milked between 360 to 400 cows over the 15 years. Of 5 wells located in corn fields, nitrate nitrogen concentration doubled in four of them and increased in the 5th by 40%. The N concentration in water from the well located near the center of the heavily farmed area exceeded EPA's accepted standard of 10 ppm. The soil phosphorus level increased in corn and alfalfa fields because of P in manure. Milk yield per cow increased from 14,964 lb. to 22,559 lb. and feed purchased to support the higher milk yield was the major source of increased N and P. Although N and P imports through fertilizer purchases were reduced, feed imports increased the balance of nutrients beyond those needed for milk production. Scientists at the University of Wisconsin estimate that US dairy producers are feeding about 25% more P than recommended by the National Research Council, who compiles nutrient requirements for dairy cattle and other animal species. The published requirements in the US are higher than other countries such as Germany, England, and Netherlands. Many herds feed rations averaging 0.48% P or higher; however, research shows that levels of 0.37-0.40% are adequate for high producing cows without negative effect on milk production, animal health, or reproduction. Lower ration P will reduce P output to the environment which will consequently reduce the amount of land required to spread manure. If you are overfeeding nutrients, there's a very high chance that the excess is working into the water table.

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