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:
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, 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
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
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.
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
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