Research Note
Nitrogen: N-Efficiency Makes a Difference
Protein sources today remain (relatively) inexpensive. Amino acid supplementation is increasingly flexible and cost-effective. Why care about nitrogen efficiency in the dairy cow?
First, the cow excretes nitrogen that is not used for productive functions, such as milk or tissue. Outside the cow, this “fugitive N” becomes fertilizer and a source of pollution in the form of ammonia and nitrous oxide. Besides downstream environmental impacts, fugitive N from cows contributes to non-farm neighbor disputes and anti-dairy NIMBYism (Not-In-My-Back-Yard-ism).
However, improving dietary N-efficiency may also offer production benefits, including balancing rumen function at a lower level of blood urea nitrogen (BUN). There may be potential to safely lower milk urea N (MUN) to 7-8 mg/dL while maintaining or increasing milk protein.
“The potential today to improve N-efficiency is significant,” says Cornell’s Dr. Mike Van Amburgh, “given more precise estimates of ruminal N requirements and post-ruminal amino acid requirements.”
Van Amburgh notes that the concept of N-efficiency in lactating dairy cattle dates back nearly 40 years, with a primary focus on the amount of “intake N” transferred into milk N, also known as “productive N” (comprising maintenance, tissue, fetus, and milk). Early work modified the form and amount of intake N and reduced the amount of excreted N, which lessens environmental impact but does not significantly change N-efficiency.
Optimum N-efficiency requires accounting for rumen N requirements and all supplies of amino acids (AA). Work conducted a decade ago on the Cornell Net Carbohydrate and Protein System (CNCPS v7) estimated the overall efficiency of use of the essential AA (EAA) on a metabolizable protein (MP) basis at 73%.
“However,” Van Amburgh suggests, “further increasing EAA efficiency in high-producing lactating dairy cattle is not likely because we need to operate at an optimum efficiency, not a maximum.
“Moreover, we need non-essential AA (NEAA) for optimum energy metabolism and many functions not related to N output, including energy substrates not captured in net protein yield.
“Ultimately, improving overall N-efficiency comes down to reducing the amount of N excreted in the urine, which shows overfeeding of nutrients that cannot be used for productive functions.”
Van Amburgh acknowledges that improving N-efficiency may have greater appeal in more densely populated dairy regions, for example, parts of the Tri-States as well as New York, Pennsylvania, and New England states. “Meanwhile, out in the field we’re still debunking the myth that more protein in the diet results in more protein in the milk.”
Researchers are currently organizing field trials to test higher N-efficiency diets. Learn more at the upcoming Cornell Nutrition Conference.
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