Friday, March 29, 2024

Milk urea reflects protein in grass

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The link between dietary protein intake and milk urea levels is not clear-cut, consultant Dr Terry Hughes says.
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“Milk urea is like Mt Cook, up and down,” he told a DairyNZ FeedRight workshop near Hawera in April.

“A cow can be mobilising protein, it can be re-circulating protein from yesterday and back into the rumen today, plus more coming in – if you can predict that you are absolutely brilliant.”

DairyNZ scientist Dr Jane Kay said when a cow eats pasture, the protein is converted to ammonia in the rumen. The microbes or bugs in the rumen use that ammonia to make the microbial protein that fuels milk production and other animal functions. Excess ammonia is absorbed across the rumen wall into the bloodstream, converting to urea in the liver. That urea ends up in either urine or in the milk.

In a grass-fed system, high milk urea readings do not automatically translate into bad news.

“In our pasture-based systems we have quite high levels of crude protein,” she said.

“It’s an association with milk urea levels and the amount of protein in the pasture – but it is not detrimental to cow health, and it is not detrimental to reproduction or fertility.”

While older research undertaken in total mixed ration systems found a link between high milk urea and poor reproduction, other studies conducted on grass-based systems in New Zealand and Scandinavia found that higher bulk milk urea levels were associated with improved reproductive performance.

“What we were actually seeing was that cows that were better fed, having higher milk urea levels, actually had better reproduction – there was definitely no advantage in trying to lower it. What we’re seeing is that in spring we can have milk urea levels of 40, 50, 60 milligrams a decilitre – that’s just a reflection of the amount of protein in the grass.”

Kay said there was no advantage in attempting to lower the milk urea level by introducing supplements high in carbohydrates or low in protein to “catch” more nitrogen in the rumen.

“In NZ our cows are adapted to those high protein levels. It’s an incredibly efficient process in the animal converting that ammonia to urea.”

On Fonterra tanker dockets the milk urea value used is a three-day rolling average. Kay said that was to try and introduce a level of consistency into the results given the tendency towards significant variation on a day-to-day basis.

The FeedRight presenters told farmers that milk urea results should not be used in isolation to make decisions about diet, with Lincoln University lecturer Dr Jim Gibbs saying it should be used as a broad brush indicator.

Kay said there was just a 70% association between milk urea levels and crude protein content in the diet. This made developing rules-of-thumb difficult. She suggested that in high-input systems, if a protein supplement was being fed a milk urea reading regularly in excess of 30mg a decilitre could suggest a need to review the diet. At the lower end of the spectrum, a declining trend where levels reach less than 15mg a decilitre could trigger a review of the crude protein level in the diet.

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