How much protein do cows need?

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By Luis Moraes | Ohio State University, Department of Animal Sciences

Last week we talked about the efficiency with which lactating cows utilize metabolizable protein (MP) for lactation. We pointed out that the efficiency of utilizing MP for lactation changes with the MP supply. It is higher at lower MP supplies and it decreases as MP supply increases.

These changes in efficiency have a direct implication on how much MP a cow needs to produce milk.

For instance, the requirement of MP for lactation depends on the efficiency: We need to know how efficiently the cow will use the MP supplied to determine how much protein we need to feed.

In most feeding systems, including the Northern American (NRC, 2001), the MP requirement for lactation is computed by the protein yield divided by the efficiency. If the efficiency is assumed constant, the requirement of MP for one pound of milk is also constant.

Constant efficiency

As a matter of fact, the practical implication of assuming a constant 0.67 efficiency is that 1.5 pounds of MP is required for each pound of protein outputted in milk, regardless of the feeding level.

If a constant efficiency of MP utilization for lactation is unrealistic, a constant MP requirement (per unit of protein yield) also likely to be problematic.

Feeding systems from other countries have already shifted to the use of a variable efficiency, for example, in the Netherlands. However, the current Northern American feeding system for dairy cattle (NRC, 2001) still computes the MP requirement for lactation by dividing the milk protein yield by a constant 0.67.

Researchers at the Ohio State University Department of Animal Sciences, in collaboration with researchers from the University of California-Davis, Virginia Tech and the Agriculture and Agri-Food Canada, are working on a new model to tackle this issue.

The model predicts milk protein yield and calculates the MP requirement while accounting for a variable efficiency of MP utilization.

More efficient

A key feature of this model is that it accounts for the fact that cows more efficiently utilize MP at lower supplies and the more MP we supply, the amount of protein outputted in milk, per unit of MP, decreases.

The practical use of this model is to directly compute how much MP a cow needs for a given milk protein yield. The model user will plug in a milk protein yield and get a MP requirement adjusted for an efficiency that decreases with the MP supply.

As a general rule, the computed MP requirement for one pound of milk protein will increase with increasing levels of MP supply.

We are further refining the model to adjust the efficiency of MP utilization (and the MP requirement) by the AA composition of proteins and the dietary composition of other nutrients as well.

For example, one pound of MP that has an AA profile that is in better alignment with the AA required for lactation will be used more efficiently than one pound of MP that has a “worse” AA profile.

Not final

Results from this research are still not final. These models will only be available in a couple months and diet formulation software interested in using these models to move away from the constant efficiency will most likely have to wait.

The good news is that in the very near future we will be formulating diets that match more precisely the animal protein requirements with dietary supply.

These “more precise” diets have numerous benefits, for example, a potential increase in income over feed costs and a reduction of under- or over-feeding of nitrogen, reducing the amount of nitrogen excreted to the environment.

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