Diet Affects Manure Nutrient Content of Dairy Waste

The nitrogen, phosphorus and potassium (N, P, K) content of dairy manure will vary depending on the animal's growth stage, production performance and ration formulation.

Dave Fischer, University of Illinois Extension animal systems educator, says the dietary effects of dairy cattle rations as it relates to N and P excretion needs to be better understood by producers and industry personnel.

Researchers from the University of Florida have observed that the predicted excretion of nitrogen, phosphorus and potassium from dairy waste decreases on a per unit basis as milk production increases. Thus, it is known that high-producing dairy cows are more efficient in converting feed nutrients into milk nutrients. High-producing dairy herds approach a 30 percent conversion of feed nitrogen, and exceed a 30 percent conversion of feed phosphorus into milk. The end result is fewer nutrients are excreted per unit of milk produced in herds that follow top dietary nutrition management.

A 17 to 19 percent crude protein diet is recommended to meet the requirements in early lactation rations while 14 to 16 percent crude protein is needed in mid- to late lactation. A low rumen degradable protein source (high by-pass) can be beneficial in early lactation rations for high-producing cows. In addition, to avoid excess nitrogen excretion, limit urea to 0.2 pound per cow per day in the first 200 days in lactation.

Phosphorus is an important mineral element which is key to energy metabolism and essential for other body functions and body fluids in the dairy animal. Phosphorus requirements of dairy cows vary according to level of production, body size and stage of gestation. However, excess amounts of phosphorus in the dairy cow ration will also have a direct increase in phosphorus content of the feces. The level of phosphorus in milk appears to remain constant with different levels of phosphorus in the ration. Fischer says once cows attain a balance of phosphorus for body function and needs, the amount of phosphorus excreted in the feces will increase.
Research indicates that 0.41 to 0.38 percent of ration dry matter phosphorus for lactating dairy cows is enough for optimum performance. Restricting dietary intake of phosphorus to the National Research Council allowances will help reduce the amount of phosphorus excreted in the dairy manure. In addition, properly balanced diets will provide for optimum production without overfeeding or underfeeding.

Overfeeding dairy cows can be costly to the producer as well as the environment. Feed nutrients should not be fed unless they provide an increase in milk production or an improvement in the health of the animal. Excess feed nutrients not being used for body function or transformed into milk nutrients will be excreted in the manure. Balancing the feeding ration to meet the needs of the dairy animal requires feed testing, feeding based on production, and monitoring dry matter feed intake.

Fischer says a sound manure nutrient management plan for allocating manure nutrients to crop production is as important as balancing the diet of the animal. Making the best use of available crop nutrients from dairy waste will provide for a cleaner environment while improving the profitability of the farming operation.