By Luis Moraes | Ohio State University, Department of Animal Sciences
Lactating cows require substantial amounts of dietary protein to maintain physiological functions associated with maintenance and to yield protein in milk. In fact, amino acids (AA) that form proteins are the required nutrients and not proteins per se. But we will use the notation of protein requirements as it is often used for diet formulation.
Animals also have a protein requirement for any body weight gain and for the protein needs associated with pregnancy. Lactation protein requirements are often the largest component, especially for high producing cows.
For example, the cow Ever-Green-View My Gold-ET recently set the single-lactation national milk production record with 77,480 pounds of milk (365-day record) and an impressive 2,055 pounds of protein. Of course, this amazing cow is not a true representation of protein yields across U.S. herds.
On average, Holstein cows enrolled in the 2015 herd production-testing programs yielded 710 pounds of protein in milk (source: http://www.farmanddairy.com).
The lactation requirement
Feeding enough (but not in excess) protein to meet the lactation requirement relies on two important steps. First, we need to be able to estimate how much protein the cow (or group of cows) we are formulating the diet for will output in milk.
In other words, we need to know a target protein yield so we can determine the protein requirement.
Second, we need to be able to determine the availability of protein in feeds for different levels of feeding or production. A final component that combines both steps is the need for an efficiency with which the supplied protein is utilized to synthesize milk protein.
Back in the days, both the protein content of feeds and the dietary percentage guidelines were set on a crude protein (CP) basis, that is 6.25 times the nitrogen content. Ruminant nutrition research over the past decades has greatly increased our ability to understand the protein needs of different physiological functions.
Our understanding of the availability of different protein fractions in feeds has also increased dramatically. For instance, feeding systems worldwide and, consequently, many diet formulation software now define protein requirements on a metabolizable protein (MP) basis.
Metabolizable protein
The MP is defined in the Northern American feeding system (NRC, 2001) as “the true protein that is digested postruminally and the component AA absorbed by the intestine.” Therefore, the MP supply is basically the sum of three components: ruminally synthesized microbial protein, ruminally undegraded feed protein and endogenous protein.
The two main contributors to the MP supply are microbial protein and ruminally undegraded feed protein. However, not all microbial CP is true protein and digestible.
Likewise, not all rumen undegraded protein is digestible at the small intestine. Thus, not all microbial and rumen undegraded CP contribute to the true protein digested postruminally.
Further, part of the MP is going to be used for maintenance, pregnancy and growth. The rest, named the MP supply available for lactation, is available for yielding protein in milk, although this process is not 100 percent efficient.
The efficiency of utilization of MP for lactation is defined by the ratio: protein yield / MP supply available for milk production. The efficiency tells us how much protein the cow will actually output in milk for each unit of MP. The NRC (2001) assumes a constant 0.67 (or 67 percent) efficiency of utilizing MP for lactation.
However, recent studies have shown that the efficiency actually decreases with the MP supply. In particular, researchers have suggested that the efficiency may decrease in a nonlinearly with MP supply.
Biological reasons
Many biological reasons have been attributed to a decrease in efficiency with increased supply, for example, changes in mammary blood flow and increased oxidation and AA metabolism in non-mammary tissues.
The key point is that the efficiency of utilizing MP changes with the feeding level. It is higher at lower MP supplies and it decreases as MP supply increases.
In the next week’s issue, we will discuss how this change in efficiency affects the requirement of MP for lactation. In short, a decrease in efficiency increases the amount of MP needed for one pound of protein outputted in milk.
We will also discuss how researchers at Ohio State are tackling this issue using nutritional modeling.
