Understanding information from bulk milk sample results


Every day farmers supplying Open Country Dairies, Fonterra, Synlait, Westland Milk Products and possibly other processors receive the Milk Urea concentration (MU) of their bulk milk sample which is also analysed for protein, fat and somatic cell content as the basis of their milk cheque. Across NZ the average MU value is 30 units.

MU gives us a good idea of how much Nitrogen is excreted in urine – this is called urinary nitrogen. Overseas plus limited NZ research shows a straight-line relationship between MU and grams of urinary nitrogen excreted per day per cow. On average the relationship between MU and urinary nitrogen is around 7 grams urinary nitrogen per 1 unit of MU. So, the average cow (with MU 30 units) is excreting 7 x 30 =210 grams of urinary nitrogen per day.

While there is variation from cow to cow in the relationship between MU and urinary nitrogen (in other words some cows differ from the best estimate of 7 grams urinary nitrogen per unit of MU), the average MU of a group of cows has a robust relationship to urinary nitrogen. This is what makes the bulk milk MU such a powerful tool: in effect the bulk milk MU represents the average MU for the herd. Now let’s consider a herd of 500 cows whose bulk milk MU on a day is 30 units and use the MU – urinary nitrogen relationship of 7 grams urinary nitrogen per unit of MU. Then on average each cow is urinating 7 x 30 = 210 grams nitrogen and the herd is excreting 500 x 210 grams = 105,000 grams nitrogen per day: that is 105 kg nitrogen per day.

And when that is extrapolated out to the 4.8 million cows in milk in NZ we get some significant numbers: the average cow in NZ excretes at least 210 g nitrogen per day; and with 4.8M cows in milk that means that every day 1M kilograms or 1000 tons of nitrogen is urinated onto NZ pastures from the milking herd alone. Over three years or 1000 days 1M tonnes of nitrogen is urinated on to NZ soils by dairy cows.

The issue with this urinary nitrogen is that nitrogen in the urine patch is at such high concentrations that it can’t all be used by plants and about 20% of urinary nitrogen leaches through the soil profile and into our waterways and groundwater. And that creates a massive challenge for NZInc: every day 200 tonnes of nitrogen from cow urine is leaching to end up in our groundwater.

The dairy industry is responding to this issue but primarily through methods which reduce its impact after the nitrogen has hit the ground in urine. By then the horse has bolted. I would argue that as an industry we should be addressing how farmers may reduce urinary nitrogen before it hits the ground: it can be done.

The amount of urinary nitrogen hitting the ground per ha depends on the amount of urinary nitrogen per cow and the stocking rate. The urinary nitrogen per cow per day = MU x 7; and urinary nitrogen per ha per day = MU x 7 x cows/ha. Over a year it’s: av MU x 7 x no. cows/farm area

So, the amount of urinary nitrogen hitting the ground may be reduced either: by reducing stocking rate (although this strategy may result in fewer cows being better fed and excreting more UN per cow and may compromise productivity); and/or by reducing MU and hence urinary nitrogen per cow. Reducing MU is the focus of this article.

For, although bulk MU averages 30 and the average herd excretes 210g N/cow in urine, some herds are managed so that bulk MU is much lower and urinary nitrogen excreted per cow per day is correspondingly lower. On the other hand, some herds have bulk MU values that are consistently higher than 30 and therefore excrete more urinary nitrogen per cow per day.


I am aware of adjoining farms run at similar stocking rates. Through a combination of skilful management practises, one has a season-average bulk MU of 20 units while the neighbouring farm averages around 40. While several important messages may be gleaned from systems with such diverse levels of MU, environmental impact is of most relevance to this article.

MU is an indication of how much dietary nitrogen is excess to requirement with this excess nitrogen being excreted in the cow’s urine. Levels of MU around 20 are low and indicate that the herd is fed a well-balanced diet and may be of desirable genetic makeup for low MU. On the other hand, the herd at MU of 40 is being fed diets with excess nitrogen, probably in the form of plant protein.

But let’s get back to the environmental consequences of the two systems. At a MU of 20, average excretion of urinary nitrogen is only 140g/cow/day – 66% of the national average. The impact of this to reduce leaching is potentially even greater – much greater!

Modelling the impact of reducing MU from 30 to 27 (a 10% reduction in MU and a corresponding 10% reduction in urinary nitrogen from 210 to 190 g/day) resulted in a reduction in leaching of 20%. The relationship between urinary nitrogen and nitrogen leached is a curve – as more urinary nitrogen is excreted then a higher and higher proportion is leached due to plants reaching a maximum nitrogen uptake. So, the reduction in nitrogen leached was 20% although the reduction in urinary nitrogen was 10%.

Now the farm at MU of 20 has reduced MU and urinary nitrogen not by 10% but by over 30%. The amount this farm is leaching may well be reduced by around 50% compared to an average farm with MU 30.

The farm at MU 40 is a totally different story. On average each cow is urinating 280 grams nitrogen per cow per day and leaching is likely to be perhaps 60% greater when compared to the average (MU 30) farm and will be leaching 3 to 4 times as much compared to the MU 20 farm.

Farming practises that reduce the amount of urinary nitrogen hitting the ground are essential in today’s world. Riparian plantings, carbon banks and the like may reduce phosphorus and microbial seepage into our waterways but has little impact on nitrogen leached into the groundwater. Reducing the amount of urinary nitrogen per area is the only way to safeguard groundwater quality for our future generations.

Farmers need to revise their feeding and breeding strategies to reduce MU as measured in their daily bulk milk sample, thereby reducing urinary nitrogen excreted and nitrogen leached.

If the dairy farming industry set and achieved a target MU average value of 22-24 as opposed to the current 30, while maintaining current cow numbers, the nitrogen leaching issue may be solved.

To read more about LowN Sires click here 

Phil Beatson

R&D Manager

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