Protein nutrition of the dairy cow



Protein nutrition of the dairy cow is complicated. When balancing rations for milk production, nutritionists previously only considered the crude protein requirement. More recently, factors such as undegradable intake protein (UIP), degradable intake protein (DIP), and soluble intake protein (SIP) have been considered when balancing rations. Balancing rations to meet the amino acid (AA) requirements of the lactating cow has recently gained interest.

Most of the cow's amino acid requirements are met in two ways:

  • Rumen fermentation.

  • Supplemental UIP.

In general, DIP is utilized by rumen microorganisms for growth. Rumen microbes have a limited life span before passing into the small intestine where they provide a very good protein source for the cow. Once the microbes reach the small intestine they are digested and absorbed just as any other protein. Microbial protein, which has an amino acid profile optimum for milk protein synthesis, can supply up to 50% of the cow's protein requirement. Consequently, maximizing rumen fermentation is very important.

Amino Acids
Amino acids, the building blocks of protein, are essentially chains of amino acids. Several different amino acids make up a protein. In the past, the cow's amino acid requirements weren't recognized because most of the needs were supplied through normal rumen fermentation. However, the amino acid needs of today's higher producing cows cannot be met through rumen fermentation alone and it is now accepted that the cow has amino acid requirements. There are ten essential amino acids (EAA), i.e. amino acids that must be supplied through dietary means. The requirements for most EAA are met through typical rations. The EAA most deficient in a ration in proportion to its requirement is termed "limiting amino acid." It is generally accepted that in most dairy rations, the most limiting amino acids are methionine (MET) and lysine (LYS). However, when the needs of MET and LYS are met, another amino acid can then potentially become limiting.

Determining the amino acid requirements of the cow can be very complicated. First, how are the amino acid requirements of the cow expressed? Several models have been proposed and express amino acid requirements as grams per day, percent of EAA, percent of metabolizable protein, etc. Also, just quantifying the AA presented to the small intestine is very difficult to determine. The contribution of AA from microbial protein is also very difficult to estimate and can be influenced by many factors, such as dietary protein level, SIP, non-structural carbohydrates, etc. Increasing dry matter intake (DMI) increases the flow of AA to the small intestine. Physical processing of a UIP source, such as heating soybeans, can influence the amount of protein escaping rumen degradation. The contribution of AA from UIP sources can also be complicated because of the variability of UIP within a feedstuff and the variability of the same feedstuff's UIP value among cows.

In spite of the challenges presented, progress has been made in determining the AA requirements of lactating cows. Simply increasing UIP in the ration will not work; therefore, the amino acid profile reaching the small intestine is very important. Santo et al. (1998) summarized 128 studies in which UIP was increased in rations. Milk production increased in only 17% of those studies. It is thought that for maximum milk production, the proper ratio of AA needed should be similar to that found in milk. It has been reported the AA content of the UIP fraction of a feedstuff will be similar to the AA content of the intact feed protein. Therefore, UIP sources should be chosen to supply a ratio of amino acids similar to milk protein.

Methionine and Lysine
Because MET and LYS are thought to be the most limiting AA in most dairy rations, considerable research has focused on these two AA. Different models proposed for LYS and MET seem to agree the ratio of LYS to MET should be 3:1. In a summary of six studies in which the LYS to MET ratio was less than 3:1, average milk production was reduced by 1.8 lb compared to controls. In a summary of studies in which the LYS:MET of 3:1 was met, milk production averaged 3.7 lb higher than controls. Protein yield and fat yield were also increased when the 3:1 ratio was met. Also, several studies indicate digestible LYS should be supplemented at 6.5% of the total protein to realize a response to supplemental MET.

Balancing Rations for AA
Even though AA requirements present a complicated issue, several common sense feeding management practices can be used to balance for AA.

FIRST.....  Maximize rumen fermentation. The LYS and MET (as a percent of EAA) content of milk is 16.0 and 5.5, respectively. Rumen microbes have a LYS and MET content of approximately 16.6 and 5.0, respectively. No feedstuff has a closer match with milk in LYS and MET content. Therefore, if LYS and MET are the two most limiting amino acids, then rumen microbial production must be maximized for top milk production.


SECOND....  Maximize dry matter intake. Increased DMI increases the flow of AA to the small intestine because rate of passage is increased. Many programs can estimate the MET and LYS requirement of the cow which estimate microbial protein flow to the small intestine. The ration can then be balanced for AA using UIP sources.

UIP sources should be considered that most closely match the ratio of AA found in milk. Good UIP sources to consider include heated soybeans, Menhaden fish meal, and flash dried blood meal. In addition, rumen protected amino acids, such as MET, are available for ration supplementation; however, return on investment should be monitored to determine their value in the ration.




ADM Alliance Nutrition, Inc. , a wholly owned subsidiary of the Archer Daniels Midland Company