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CLIENT NEWS: Milk Urea readings could be the answer for determining nitrogen output – CRV Ambreed

CRV Ambreed says Milk Urea readings, supplied daily to most New Zealand dairy farmers, could provide a clear picture of exactly how much nitrogen herds are excreting each day in urine.

The herd improvement company was in the news in 2017 with the announcement about its team of LowN Sires bulls that are genetically superior in their ability to reduce concentration of Milk Urea Nitrogen (MUN) in their daughters.

MUN is the amount of nitrogen contained in Milk Urea and there’s a direct connection between MUN and the amount of nitrogen excreted in urine - cows with low MUN excrete less nitrogen as urine. 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.

CRV Ambreed’s head of R & D Phil Beatson says farmers are more used to talking about Milk Urea (MU) than MUN because MU values are presented to them every day on the bulk milk test report. Milk Urea concentration (MU) is an indicator of how much dietary nitrogen (consumed by the cow as plant protein) is not being used for production and is therefore wasted and excreted. Nitrogen excreted in the urine is particularly important because much of that nitrogen is leached into groundwater.

Beatson says about 45% of dietary nitrogen is excreted in urine. This amounts to each cow excreting about 75kg of nitrogen a year in urine and this urinary nitrogen (UN) is the primary source of nitrogen leaching. The relationship between bulk milk MU and UN is a very accurate measure of how much nitrogen is hitting the ground as urine on a herd basis. It follows that MU is very likely to be a good predictor for how much leaching is actually taking place on farms. Use of their bulk milk MU reading should therefore help farmers form the basis of strategies to reduce leaching.

He says MU values for pasture-fed cows in New Zealand vary but an overall drop of 2-3 points in MU values across the year would provide a 20% reduction in leaching. The average MU value in New Zealand is about 30 units, with daily herd test readings spread anywhere from 20-40 units, he says.

“The MU readings allow farmers to know, reliably, how much nitrogen their cows are peeing out, providing an accurate picture about the nitrogen load on their land. On the face of international and limited New Zealand evidence, these bulk MU readings appear to give farmers a very good estimate of the nitrogen load excreted as urinary nitrogen on a daily basis.”

Fonterra, Open Country Dairy, Synlait and Westland Milk Products provide daily MU values to their suppliers in bulk milk test reports which include percentage of fat and protein in the milk, plus somatic cell counts.

CRV Ambreed Managing Director Angus Haslett says it’s difficult to measure nitrogen output on New Zealand dairy farms and therefore regional councils rely on predictive modelling tools to provide farmers with leaching levels. These tools estimate nutrient movements on farms, based on modelling information related to feed and fertiliser, animal numbers, rainfall etc.

Haslett says for New Zealand to move to a greener dairy industry, it must move towards accurately predicting nitrogen loads, and utilise management, nutrition and genetics to reduce those loads.

“We think the MU data on the bulk milk test report provides an accurate indicator of the urinary nitrogen load. We are working with our partners to confirm the milk urea and urinary nitrogen relationship is robust to enable these important conversations to take place in New Zealand.”

CRV Ambreed estimates that a relatively small investment to confirm the MU and UN relationship could see results before the second half of 2018.

Links to protein research
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.

Further research has shown that when cows are bred for low-MUN through CRV Ambreed’s team of LowN Sires bulls, 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.

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.

“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.

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.” 

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.

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.

The genetic breakthrough is now the subject of a $21 million research project funded by the New Zealand government and led by DairyNZ.