The Corn Feed

Dr. Larry E. Chase, Department of Animal Science, Cornell University

Protein is usually the largest component of the purchased feed bill on most dairy farms, and nitrogen (N) is the building block of proteins and amino acids in dairy rations. While dairy cattle require absorbable amino acids at the tissue level to support maintenance, milk production, growth and pregnancy, they do not really have a “protein” requirement. The amino acids required are derived from two sources. One is the microbial protein (MCP) synthesized in the rumen while the other is the amino acids in the rumen undegradable protein (RUP) fraction of feeds. The sum of the MCP and RUP is termed metabolizable protein (MP).

The trend in recent years has been to feed lower crude protein rations to dairy cattle, and there are  several reasons prompting it. First, feeding lower crude protein rations tends to lower feed costs, maintain milk production and improve profitability. A second reason is to improve the efficiency of N use in the cow and decrease N excretion to the environment, producing a win-win situation for both the dairy producer and society.

How can we monitor the efficiency of N use on dairy farms? To answer that, it is important to understand how a dairy cow uses feed N. Once consumed, there are basically two fates for  feed N.  Some N is contained and excreted in the milk, while the remaining N is excreted in  the manure. Dairy  cattle capture 20-40 percent of the feed N in the milk.

Determining the Efficiency of N Use

How can we determine the efficiency of N use on a dairy farm? One method is to calculate milk  nitrogen efficacy (MNE). The MNE value is the percent of the feed N consumed that is captured in the  milk. A value less than 30 percent indicates an opportunity for improvement by altering the  carbohydrate and protein components in the ration. A range of 30-35 percent is ideal, though a few herds may be greater than 35 percent. The highest efficiency values in research literature are 40-45 percent. Table 1 contains the calculations needed to determine MNE.

A second option is to monitor milk urea nitrogen (MUN). The major route of excretion of excess N is in the urine and milk. Since many dairy herds get daily milk component values from the milk plant, MUN is a readily available value that can be used to track N excretion. In the New York Precision Feed Management program, we are using a target range of 8-12 mg/dl for MUN. Values greater than 12 indicate an opportunity to fine-tune the ration in order to improve the efficiency of N use. If a herd is in the 8-12 range, there still may be some smaller opportunities to adjust the ration. There are a number of high-producing herds with routine MUN values of 7-9 that also have very good reproductive performance.

MNE and MUN are tools we can routinely use on dairy farms to evaluate the efficiency of N use. If we can improve the efficiency of N use, there is less N excreted back into the environment. More importantly, in most cases, this also improves profitability.

Table 1. Milk Nitrogen Efficiency Worksheet

Larry E. Chase is a professor and dairy nutrition extension specialist with Cornell University in Ithaca, N.Y. You can contact him at lec7@cornell.edu.

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