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Research delivers result around partitioning of dietary nitrogen

GENETIC DISCOVERY: CRV Ambreed’s R&D Scientist Phil Beatson says cows bred for lower levels of MUN are expected to excrete less nitrogen in their urine which will, in turn, reduce the amount of nitrogen leached from grazed pasture. Research also shows a proportion of the nitrogen diverted away from the cow’s urine goes into milk protein. Image credit: CRV Ambreed

CRV Ambreed’s genetic research into reducing nitrogen leaching on New Zealand dairy farms has identified that when cows are bred for low milk urea concentration, a proportion of the nitrogen diverted away from the cow’s urea goes into milk protein.

This finding gives CRV Ambreed further confidence that breeding cows for low milk urea concentration will not only reduce the amount of nitrogen excreted in their urine, but will also increase the efficiency with which dietary nitrogen is used for milk protein production.

The search to understand precisely how animals partition the nitrogen they consume has been the subject of decades of research into making cows more efficient in the way they use dietary protein.
The relationship between milk urea and the percentage of protein in milk identified by CRV Ambreed should give scientists new leads for that work.

The dairy herd improvement company has begun marketing semen from more than 20 existing, high-performing bulls under a LowN Sires™ brand.

The bulls are desirable for traditional traits as well as being genetically superior in their ability to reduce concentration of milk urea nitrogen (MUN) in their daughters.

MUN is a measure of the amount of nitrogen contained as milk urea and there’s very strong international evidence of a direct connection between MUN and the amount of nitrogen excreted in urine when cows are fed varying diets – cows with low MUN excrete less nitrogen as urine.

CRV Ambreed has shown it is possible to breed cows that genetically have lower MUN.

These cows are then expected to excrete less nitrogen as urine and consequently less nitrogen will be leached into groundwater.

Calculations by CRV Ambreed show a reduction of 20% in leaching within 20 years is possible by using genetics to breed cows with lower levels of MUN.

Now, further analysis indicates where at least some the nitrogen goes if it’s not going into urea.

This finding is thought to be a world-first in demonstrating variation between cows of differing genotypes for how they partition dietary nitrogen eaten in the form of protein.

CRV Ambreed R&D Scientist Phil Beatson says dairy cattle convert nitrogen consumed into five areas: milk (protein + urea); growth (muscle); dung; gases; and urine.

“From an environmental perspective, we saw urine as being the big issue because of its impact on water quality,” Beatson says.

“And we asked: can we breed cows that have reduced MUN, and if so do these cows excrete less nitrogen as urine?

“Now we know that we can reduce MUN through breeding, and that these low-MUN cows will partition more dietary nitrogen from milk urea towards milk protein. This strongly indicates that low-MUN cows will excrete less nitrogen as urine because they divert some nitrogen away from milk urea and into milk protein.”

This means not only can LowN Sires™ bulls and a low-MUN approach be used to breed for environmental gains, but also possibly for an increase in animal efficiency.

Beatson says modelling and genetic analysis of the LowN Sires™ bulls suggests about 25% of the nitrogen that will be diverted away from urine in their daughters will go into milk protein.

Higher percentages of milk protein is good news for milk processing companies desiring less water in the drying process.

He understands the low-MUN and high percentage protein genetic link is a world-first and could therefore be a clue to understanding how animals partition the nitrogen they are fed.

“Animal nutritionists will be extremely interested in our finding as there have been decades of research into nitrogen-use efficiency.”

Beatson says this discovery could be the tip of the research iceberg.

“A huge effort has been invested over the past 70 years trying to understand nitrogen partitioning and that’s produced some interesting trends but nothing conclusive. Now New Zealand scientists may target groups of animals that are known to be diverse for MUN to investigate differences in how they partition dietary nitrogen.”

He says while breeding versus feeding cows are different avenues in terms of reducing nitrogen excreted as urine, the two are expected to be additive.

“In other words, genetic gains will add to gains from better feeding.”

It may also have implications for more than dairy cows.

“This research gives us confidence that cows bred for reduced-MUN will partition nitrogen differently and this is likely to be true for all ruminants.”

Phase 2 of the research will study groups of animals genetically different for MUN to understand more precisely the relationship between reducing MUN and reducing nitrogen in urine.

How low MUN and LowN Sires™ works

The primary cause of nitrogen leached into the ground and waterways comes from the cow’s urine having very high concentration of nitrogen and being deposited in small patches.

Some of the nitrogen excreted is converted to gas, some is taken up by plants, and a substantial amount is leached, with soil-type affecting the proportion that is leached.

CRV Ambreed, with the input of other researchers, has spent five years investigating the genetics of Milk Urea Nitrogen (MUN).

The rationale has been that if it’s possible to reduce MUN through traditional genetic selection means, and providing the relationship between MUN and amount of nitrogen excreted in urine holds, then the genetically improved animals for MUN will excrete less urinary nitrogen and hence leaching per animal and per hectare can be reduced.

“Cows bred for lower levels of MUN are expected to excrete less nitrogen in their urine which will, in turn, reduce the amount of nitrogen leached when cows are grazed on pasture,” CRV Ambreed R&D Scientist Phil Beatson says.

“It could potentially save New Zealand 10 million kilograms in nitrogen leaching a year within 10 years, based on the national herd number of 6.5 million dairy cattle. Farmers who start a breeding programme for low-MUN now, add another tool to their farming systems to manage nitrate leaching and are looking at potential nitrogen leaching reductions of 10-12% by 2025.”

Beatson says that’s significant and it comes with minimal or no disruption to normal farm management.

Genetic studies have found that MUN is a heritable trait and have even stated that it is possible to reduce MUN through genetic selection.

But that avenue has not been pursued overseas where alternative farming systems mean cows are not on pasture as much as they are in New Zealand and nitrogen leaching from urine patches is possibly not as important an issue as it is here.

Beatson’s research was designed to identify superior bulls for MUN with the specific aim being to provide low-MUN bulls to breed cows that excrete less nitrogen in their urine: in line with overseas studies he has identified some bulls that will breed daughters that have desired low levels of MUN, while at the other end of the scale some bulls will breed daughters with high levels of MUN.

CRV Ambreed is the first organisation in New Zealand, and possibly the world, to market bulls with low-MUN genetics with the aim being to provide a long-term genetic solution to nitrogen leaching.

Since 2012, CRV Ambreed has measured MUN concentration in 650,000 milk samples and analysed them to understand how strongly the trait is inherited, and to create a MUN breeding value for all cows measured as well as sires of the cows.

MUN cow breeding values (BVs) can be made available to farmers who herd test and herd record with CRV Ambreed meaning they can effectively manage their breeding programmes around low-MUN.

Source: CRV Ambreed

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