Low-N cows give higher protein concentration

Sheryl Brown

Milk from cows with lower milk urea nitrogen concentration (MUN) contains a higher proportion of milk protein.

It appears up to 25% of the nitrogen being diverted away from milk urea is going into milk protein, CRV Ambreed research and development manager Phil Beatson says.

Each litre of milk from low-MUN cows contains less nitrogen in milk urea and more nitrogen as protein.

These results reinforce breeding nitrogen-efficient cows is a win-win for the dairy industry, Beatson says.

“It’s a double-edged win. We believe that decreasing MUN will not only decrease dairying’s environmental footprint through reducing the amount of nitrogen excreted in urine and thereafter leached into waterways, but also increase nitrogen-use efficiency for protein production.”

Earlier this year CRV Ambreed published breeding values for MUN, which is known to have a relationship with urine nitrogen outputs.

Further analyses of the MUN data has confirmed a favourable relationship between MUN and milk protein percentage, he says.

The genetic correlation is -.22. This negative correlation shows as MUN gets lower, the percentage of milk protein percentage gets higher.

‘We’ve found a moderate relationship and it’s in a favourable direction. It gives us high confidence that these cows are using nitrogen better for both the environment and protein production.’

“It’s a big finding. This is possibly the first time that this favourable relationship has been reported.

“Right from the first analyses of the MUN data we were confident that MUN could be reduced by breeding. But then that raised the question: what do the low-MUN animals do with the nitrogen not lost as milk urea? The fact that a proportion is being used for milk protein production is strongly indicating that there is variation between animals in the way they partition dietary nitrogen eaten as protein.”

It’s a strong indicator that lower-MUN cows use nitrogen differently than other cows, Beatson says.

“We’ve found a moderate relationship and it’s in a favourable direction. It gives us high confidence that these cows are using nitrogen better for both the environment and protein production.”

CRV Ambreed and AgResearch scientists have analysed herd test data on 650,000 milk samples collected from 130,000 cows. The genetic analysis has involved MUN, percentage protein, percentage fat, milk yield, percentage lactose and somatic cell score.

Dairy cattle convert dietary nitrogen into milk (protein and urea), growth (muscle), faeces, gases and urine.

“In our case we wanted to develop genetics to reduce dairying’s impact on water quality through reducing the amount of nitrogen excreted as urine. But if the nitrogen is not going into urine it has to be going somewhere else.”

While the CRV Ambreed work indicates some of the nitrogen is diverted from MUN to protein, the MUN – %Protein relationship is of moderate strength and not all of the diverted nitrogen is likely to end up in milk protein. However, it’s possible that nitrogen is also being diverted from MUN into faeces, Beatson says.

“If a proportion is going to faeces it’s also a great result because that nitrogen has a better chance to be used by plants rather than being leached.”

CRV Ambreed is liaising with New Zealand research organisations to put together a partnership research programme to further investigate the MUN-urine nitrogen relationship.

The Phase 2 research will likely include a group of cows that is genetically high for MUN and a group that is genetically low for MUN. These cows will be fed the same diet and monitored for nitrogen outputs, including urine, protein, milk urea, faeces and green house gases.

“We’ll be looking at everything that goes in and everything that goes out.”

*CRV Ambreed launched its LowN Sires team earlier this year. Visit www.crv4all.co.nz/lownsires/

To read more about the CRV Ambreed research see Dairy Exporter, April 2017, Page 12.


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