Protein for dairy cows – how low can you go?

Protein nutrition in the dairy herd is receiving more attention due to the cost of protein supplementation and environmental effects.

Dairy cows are relatively inefficient at utilising dietary nitrogen, with approximately 28% being incorporated into milk protein and the rest being excreted through urine and faeces into the environment. The efficiency of incorporating dietary nitrogen into milk protein can be improved by feeding diets lower in protein than what typically tend to be found commercially in practice.

It is widely accepted that feeding higher levels of dietary protein, the higher the milk yield, partly due to a positive effect on dry matter intake. However, many trials have shown that reducing crude protein from typical levels of 17-18% to 16% in the dry matter does not affect milk yield or composition. Only when diets are below 15% crude protein, does milk yield start to be affected, partly through a reduction in dry matter intake, reduced fibre digestion and a deficiency in metabolisable protein supply.

Feeding lower protein diets to dairy cows will require the expertise of a good nutritionist to ensure that metabolisable protein supply is as close to requirements as possible and energy supply is not limiting to the rumen microbes. Energy is key to maximising protein efficiency with lower protein diets. Utilisation of protein is optimised with intestinal digestible energy sources compared to rumen fermentable energy sources i.e. a shift from fibre to non-fibre carbohydrates (e.g. maize grain). Ensure that any dietary change is carried out gradually to give the rumen microbes time to adapt, so that production is not compromised by sudden changes.

Milk composition, fertility and health do not appear to be affected by lower protein diets and it is far more likely that high protein diets will have a detrimental effect, with elevated levels of plasma urea being associated with a lower uterine pH, affecting embryo survival.

As well as improving the efficiency of protein utilisation, cost of production can also be reduced. By lowering the crude protein percentage in the ration from 17% to 16%, a saving of up to 10p/cow/day could be made (based on current protein prices), equating to £450/month for a 150 cow herd.


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