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Managing Nurtrients in Dairy Cattle Diets

By Faith Cullens, MSU Extension dairy educator  (.(JavaScript must be enabled to view this email address))

Livestock producers can manage nutrient losses by improving the accuracy of the diet offered, improving the 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 amino acids that are vital to the maintenance, growth, reproduction and lactation of dairy cattle. Nitrogen is a nutrient of concern because it can volatize into the air as ammonia, and nitrates can pollute groundwater and surface water.

Phosphorus is also important in animals’ diets because it is a key component of bone and cell membranes. When excreted, phosphorus binds to soil and can be carried to surface water. When not managed properly, phosphorus can cause excessive growth of algae and other aquatic plants, which uses large amount of oxygen. Low oxygen levels can result in fish kills.

Improving formulation accuracy

A first step in ensuring that you are meeting, not exceeding, the needs of the animals is grouping animals by nutritional requirement (age, production level and body condition) so that you are not over- or underfeeding valuable nutrients.
You can improve the accuracy of your feed formulations by working with a nutritionist. The nutritionist should have feed composition analyzed routinely (monthly or whenever there is a change), especially forages and byproduct feeds in which phosphorus and nitrogen concentrations can vary greatly. The nutritionist should balance rations to National Research Council (NRC) recommendations for protein and phosphorus. Higher feeding rates do not improve lactational performance and reproduction.

For mature, lactating Holstein dairy cows, approximately 1 gram of phosphorus for each pound of milk produced is sufficient. This level is provided with typical feed intakes and 0.32 to 0.38 percent ration phosphorus (dry matter basis). Balancing to the amino acid requirement of the target animal can further improve the accuracy of the ration by reducing the crude protein concentration. This area of nitrogen management is developing, and nutritionists may choose to utilize protected amino acids to target specific amino acids.

One way to keep an eye on the adequacy of dietary protein nutrition is to monitor milk urea nitrogen (MUN) concentrations. This inexpensive analysis is routinely done along with the determination of milk components. MUN is a general indicator of the 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, the herd average should range between 10 and 12 mg/dL of milk for Holsteins (Kohn, 2007).

Improving digestibility

Any undigested feed nutrients that are excreted in feces as part of manure 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 you can never achieve 100 percent diet digestibility, you can maximize digestibility by paying close attention to the feeds you select—especially forages. Commercial feed testing laboratories can test the neutral detergent fiber (NDF) digestibility of your forages. This can be affected by many things, including variety, growing season, maturity, fermentation, preservation and feed handling. Measuring NDF digestibility not only gives a means to compare forages but makes ration balancing more precise and animal performance more predictable.

Management considerations

As milk production increases, dietary nutrient requirements increase at a slower rate as nutrient utilization efficiency per unit of milk produced increases. Management factors that increase milk production and improve nitrogen efficiency include the manipulation of photoperiod by the addition of artificial lighting, milking three times per day versus two and using bovine growth hormone (Jonker et al., 2002).
To reduce waste, feed close to the amount the cows are consuming, and if possible, feed the refusals to other animals. Make sure that the equipment that you are using to present feed to animals is not causing excess feed waste. This is commonly seen when feed mixers and feed troughs are overfilled.


Feeding nutrients in excess of what the animal requires increases costs as well as the land base needed for manure application. In addition, energy is often 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 milk production will conserve nitrogen and phosphorus. These changes can have a positive impact on the environment and your bottom line.

This article was published in the 2009 Manure $ense guide. To download the entire guide, click here.