Managing nutrient losses in the dairy cattle diet

Nitrogen and phosphorus losses can be managed through improving the accuracy and digestibility of the diet and reducing feed waste.

Nutrient losses can be managed through improving the accuracy of the diet offered, improving digestibility of the diet consumed (thus reducing manure excretion) and reducing the amount of feed waste. The main nutrients of environmental concern are nitrogen and phosphorus.

Nitrogen is a component of dietary protein and is essential in cattle diets. Nitrogen can volatize into the air as ammonia and nitrates can pollute ground and surface water. Phosphorus is important to animals as a key component of bone and cell membranes. Phosphorus binds to soil and can be carried to surface water. Phosphorus can cause excessive growth of algae, which uses large amounts of oxygen resulting in fish kills.

Improving formulation accuracy

Accuracy of the diet offered can be improved by ensuring that you are meeting, not exceeding, the needs of the animal. Grouping animals by nutritional requirements (age, production level and body condition) will minimize over- or under-feeding valuable nutrients.

An essential part of improving accuracy is to work with a nutritionist. Nutritionists should have feeds analyzed for composition routinely (monthly or whenever there is a change), especially forages and by-product feeds in which phosphorus and nitrogen concentrations can vary greatly. Nutritionists should balance rations to NRC (2001) recommendations for protein and phosphorus since improvement in lactational performance and reproduction is not observed at higher feeding rates.

For mature, lactating Holstein dairy cows, approximately 1 gram of phosphorus for each pound of milk produced is sufficient. Based on typical amounts of feed intake, phosphorus concentrations in rations would range from 0.32 to 0.38% (dry matter basis). Balancing to the amino acid requirement can further improve the accuracy of the ration by reducing the crude protein concentration.

One way to evaluate status and adequacy of dietary protein nutrition is to monitor milk urea nitrogen (MUN) concentrations. MUN is a general indicator of amount of dietary protein fed in excess of requirement or dietary protein with poor amino acid profile or quality. Updated recommendations state that under typical production situations herd average for MUN should range between 10-12 mg/dL of milk for Holsteins.

Improving digestibility

Any undigested feed nutrients excreted in feces represent lost nutrients and lost profit. Using a processor on corn silage and grinding concentrate feeds can greatly increase the amount of feed digested and therefore reduce nutrient loss. Although 100% diet digestibility cannot be achieved, the digestibility of the selected feeds should be considered, especially for forages. Commercial feed testing laboratories can test the neutral detergent fiber (NDF) digestibility of forages which can be impacted by many things including: variety, growing season, maturity, fermentation, preservation and feed out. Measuring NDF digestibility not only gives a means to compare forages, but ration balancing can be more precise and result in more predictable animal performance.

Management considerations

As milk production increases, dietary nutrient requirements increase at a slower rate, thereby increasing nutrient utilization efficiency per unit of milk produced. Management factors that increase milk production and improve nitrogen efficiency include the manipulation of photoperiod by the addition of artificial lighting and milking three times per day versus two.

Feed closely to the amount the cows are consuming and if possible feed the refusals to other animals. Ensure that the equipment used to present feed to animals is not causing excess feed wastage. This is commonly seen with feed mixers that are overly full and feed troughs that are not large enough to accommodate the amount of feed in them.


Feeding nutrients in excess of what the animal requires will increase purchased feed costs and the land base needed for manure application. In addition, energy often is required by the animal to get rid of nutrients she does not need. Using a combination of strategies to improve formulation and feeding accuracy, feed digestibility and production will conserve nitrogen and phosphorus. These changes can have a big impact on the environment and on your bottom line.

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