Cows do not have a Crude Protein (CP) requirement, Varga et al (2007). Yet, for many years CP has been used when formulating diets, and this often continues to be the case. Cow’s have a requirement for Amino Acids (AA), not protein, and the supply of the AA is predominantly produced by the cow herself in Microbial Protein (MicP) in the rumen. Together with Rumen Undegradable Protein (RUP) and Endogenous Protein (EP), this makes up the supply of Metabolisable Protein (MP) to the small intestine. Both MP and the component AAs are then absorbed by the small intestine and used for protein synthesis and meeting the needs of the cow for: maintenance, growth, reproduction and lactation.
In total, cows require twenty AAs, of which ten are Essential Amino Acids (EAA) that she cannot produce herself in the quantities needed. The other ten AAs are Non-Essential Amino Acids (NEAA) that the cow also requires, but can be produced in the sufficient quantities to meet requirements. An AA in the shortest supply becomes the first-limiting EAA, and within dairy cow diets it is usually two: Lysine and Methionine. Histidine has also been identified as one of the most significant EAAs especially on grass silage based diets.
As mentioned, the total AA supply digested in the small intestine comes from a combination of MicP (55/60%), RUP (approx.40%) and EP (approx.5%). The sum of the three constitutes the MP supply. Nutritional models are able to predict the cow’s MP requirement which is dependent on the following factors: body weight, breed, yield and milk quality. This means that it’s not possible to formulate rations to supply MP with the correct AA profile to match the cow’s EAA needs, and in doing so reduce the amount of CP being fed. Crude protein is often over fed in order to meet the non-specific EAA requirements.
Microbial Protein (MicP) has an AA profile similar to milk, and is thus considered a true protein of exceptional quality. It is therefore essential to maximise rumen MicP production through the synchronisation of RDP and rumen carbohydrate supply to increase the overall nitrogen efficiency.
Table 1: Lysine, methionine and histidine content in animal products, bacteria and typical dairy feedstuffs expressed as a % of crude protein
(Schwab et al., 2001)
As discussed, Lysine and Methionine are two of the first-limiting EAAs required by the milking cow. It therefore makes sense to maximise MicP, increasing the supply of both AAs to the small intestine in the form of MP. This, in turn, improves the animal’s performance through increasing the utilisation of all other AA’s. A study by Varga (2007) at Penn State University found this to be correct. They formulated two diets: one at 18% CP, and the other at 16% CP (with no additional protected Lysine or Methionine apart from that supplied by the raw materials). The study found Nitrogen utilisation efficiency (NUE) improved using the lower CP diet by 4.6%, with an additional 2.72kgs of milk produced per cow, along with an improvement in milk components. A more recent study published in the Journal Animal by Edouard et al., (2016) demonstrated that dietary CP could be reduced from 17% to 12% in late lactation cows without any effect on milk quality or milk yield during the 2 week trial period. In this case NUE went from 22% to 31%.
So, by considering increasing Lysine and Methionine levels in MP during diet formulation, rations can be formulated at 16% Crude Protein whilst maintaining and even increasing milk yield and component levels (Schwab et al. 2007).
It is as suggested by Prof.Chuck Schwab:
“Wouldn’t you rather take something out of the diet that is an excess and put something in that will give you more milk?”
Now there is a question!
Dairy Technical Specialist
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