Breeding to reduce the amount of Nitrogen hitting paddocks


New Zealand’s 4.8 million milking cows excrete 1000 tonnes of nitrogen a day in their urine and 200 tonnes of that ultimately ends up leaching into groundwater.

CRV Ambreed has determined the daily numbers using existing information related to milk urea concentration in daily bulk milk reports. The nitrogen numbers are large and the herd improvement company says it’s important to see them reduced.

CRV Ambreed’s head of R&D Phil Beatson says CRV Ambreed is encouraging conversations about how New Zealand dairy farmers could make use of the Milk Urea concentration (MU) value on their daily bulk milk reports to calculate the amount of nitrogen their herd is excreting in urine and take steps to address that.

Beatson says 1000 tonnes a day is a huge amount of nitrogen, but what’s more important is that about 20% of that nitrogen is leached through our soils and ultimately ends up in ground water – that is 200 tonnes a day.

“This is an issue we must address now to safeguard future generations of New Zealanders,” Beatson says. “It is important to realise that not all farms are the same in terms of milk urea, nitrogen excretion and leaching from their cows. Reducing milk urea, nitrogen excretion and leaching is achievable – and quickly.”

CRV Ambreed believes there is powerful information in the daily bulk milk reports received by most farmers to calculate the amount of nitrogen a herd is excreting in urine: the milk urea concentration (MU) reported each day to farmers in their bulk milk test report is a very accurate indicator of the amount of nitrogen excreted each day per cow in urine. Read the technical article with calculations here.

MU is an indicator of how much dietary nitrogen (consumed by the cow as plant protein) is not being used for production and is excreted. Nitrogen excreted in the urine is particularly important because the high concentration of nitrogen in urine coupled with the small area of the urine patch means the plants and soil cannot cope with the nitrogen, and across all New Zealand about 20% is leached into groundwater.

Research from overseas and in New Zealand shows a direct relationship between MU and grams urinary nitrogen excreted per day per cow, Beatson says. On average the relationship between MU and urinary nitrogen is about 7 grams urinary nitrogen per 1 unit of MU. The average MU in New Zealand is 30 units, so the average cow is excreting 7 x 30 which is 210 grams of urinary nitrogen a day. With 4.8 million cows in milk, nationally there is 1000 tonnes of nitrogen hitting the ground in urine every day, he says.

Beatson uses an example of a herd of 500 cows whose bulk milk MU on a day is 30 units, combined with the best estimate of the MU-UN relationship of 7 grams urinary nitrogen per unit of MU. On average, each cow is peeing out 7 x 30 = 210 grams urinary nitrogen and the herd is excreting 500 x 210 grams = 105,000 grams nitrogen in urine per day, a total of 105kg a day on that farm.

He says the effective way to deal with the nitrogen leaching issues is to reduce the amount of nitrogen hitting the ground, and that means changes in the way we breed and feed cows.

Beatson says the MU value in bulk milk reports holds the key to this. Although the MU and urinary nitrogen relationship differs slightly from cow to cow when individual cows are considered, when a group of cows (eg a herd) is considered, the average MU of the group relates very tightly to the MU-urinary nitrogen relationship of 7 grams urinary nitrogen per unit of MU. This means that the bulk milk MU is a good predictor of urinary nitrogen on a per cow basis, and, when we multiply by number of cows, on a per herd basis, he says.

While the average New Zealand herd sits around the MU 30-unit mark, some herds have a much lower average MU and some have much higher average MU.

Beatson uses examples of adjacent farms with similar stocking rates: one herd averages MU 22 and another MU 38. “Levels around MU 22 are low and would tell us that this herd is fed a well-balanced diet and that the cows might also have desirable genetic make-up for low MU,” he says. “But those at the higher level of MU 38 will likely be receiving a diet with excess nitrogen. The herd with bulk milk MU 22 is excreting 154 grams urinary nitrogen per cow per day, while the herd with bulk milk MU 38 is excreting 266 grams urinary nitrogen per cow per day.”

He says reducing the amount of urinary nitrogen is the only impactful way to safeguard groundwater quality for future generations. “It’s the duty of all of us to adopt farming practices that reduce the amount of nitrogen going into groundwater and waterways. Plantings, carbon banks etc are helpful to reduce phosphorus and bacterial contamination of waterways but have minimal effect in reducing nitrogen leaching into groundwater.”

He’s encouraging farmers who get bulk milk reports from Fonterra, Open Country Dairy, Synlait and Westland Milk Products to use the MU information to start tracking their MU on a daily basis to determine what practices influence the MU value from day to day and with an overall aim of reducing MU and therefore urinary nitrogen.

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 is the primary source of nitrogen leaching. The relationship between bulk milk MU and urinary nitrogen is a very accurate measure of how much nitrogen is hitting the ground as urine on a herd basis and therefore it follows that MU is very likely to be a good predictor for how much leaching is actually taking place on farms, he says. Use of their bulk milk MU reading should therefore help farmers form the basis of strategies to reduce leaching.

CRV Ambreed Managing Director Angus Haslett says the firm is committed to helping New Zealand cement its position as a sustainable dairying country. “We believe it’s important to bring these conversations to the fore to help the country move towards an even greener industry.”

He says predictive modelling tools are currently the only way New Zealand can estimate nitrogen output on farms. “If we are really serious about our environment, we know we need to be more accurate in what’s being put onto the land, and we believe these daily bulk milk reports go a long way to helping us do that.”


CRV Ambreed and low-MUN cows

CRV Ambreed has shown it is possible to breed cows that genetically have lower Milk Urea Nitrogen concentration (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.

“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.

He says farmers who breed lower-MUN cows and understand a herd’s MU levels (and therefore its nitrogen output) could make enormous gains in helping the environment.

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 input from 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.

“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.

Beatson 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.”

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


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