Every day most New Zealand dairy farmers’ bulk milk test report informs them of the Milk Urea concentration (MU) in their bulk milk sample.
But are those farmers, and indeed other industry groups, making the most of this potentially vital piece of information? This article addresses how MU may be used more effectively by both farmers and farm monitoring services.
MU values are provided to farmers supplying Fonterra, Open Country Dairy, Synlait and Westland Milk Products. The values are measured in the daily bulk sample by the milk testing laboratory along with % fat, % protein and somatic cell count.
The average MU value in New Zealand is around 30 units.
MU is an indicator of how much dietary nitrogen (consumed by the cow as plant protein) is not used for production and therefore wasted: the higher the MU value, the higher the proportion that is surplus. Internationally, farmers and advisors on cow nutrition have used MU values to monitor whether dietary protein levels in cow diets are deficient, adequate, or excessive. Subsequently, protein content of diets may be modified so that protein is fed at optimum levels.
Such fine-tuning of diet is more difficult for New Zealand systems where most of the cow’s intake is pasture whose protein content varies markedly due to factors like plant species, stage of plant growth, fertiliser inputs and season. As a result, MU values for New Zealand pasture-fed herds vary from farm to farm, paddock to paddock, and day to day – values reportedly can range from 14 to 44 units.
However, for New Zealand farming, the daily bulk milk MU value may have a further and just as important use – to predict very precisely the amount of nitrogen being excreted as urine by the herd each day.
Nitrogen eaten as protein is used to synthesize milk and muscle protein while excess nitrogen is excreted in urine, faeces and possibly gases. In fact, in the milking herd only 15-20% of dietary nitrogen is used for milk protein synthesis and the remainder excreted. About 45% of dietary nitrogen is excreted as urine. And it is the association between MU and the amount of nitrogen excreted as urine each day that becomes central to this article.
Many studies have reported a direct link between MU and the amount of nitrogen excreted as Urinary Nitrogen (UN) by cows in their urine each day. This is illustrated by the following graph (adapted from Spek JW, 2013. Variation of milk urea in dairy cattle, p 37. Ph D thesis – Wageningen University):
The graph shows that for each 1-unit increase in MU there is an increase of around 7 grams of UN excreted in urine. For the average New Zealand herd with a MU of 30, on average each cow will be urinating:
30 units of MU x 7 grams UN per unit of MU = 210 grams nitrogen as UN per cow per day.
On the other hand, a herd at MU 14 will be urinating 98 grams nitrogen per cow per day, and a herd at MU 44 will be urinating 308 grams nitrogen as UN per cow per day.
Note: while there is significant between-cow variability in the MU-UN relationship, on a herd basis the bulk milk MU represents the average MU of all the herd and that day’s bulk milk MU–UN relationship is expected to be very robust.
The amount of nitrogen excreted by the herd each day as UN can be calculated as:
UN = MU x 7 x number cows (grams per day).
If a herd of 500 cows has MU of 30 then the amount of nitrogen hitting the ground in urine is:
30 x 7 x 500 = 105,000 grams N per day (105 kg N per day).
The key point is that this relationship between MU and UN is extremely useful to predict the amount of nitrogen hitting the ground, and could form the basis of mitigation strategies to reduce leaching.
It’s widely accepted that the primary source of nitrogen that is leached into groundwater is from urine patches. If we reduce the amount of nitrogen hitting the ground as urine, then the amount of nitrogen leached will decline. Importantly, reducing urinary nitrogen will also reduce the amount of the greenhouse gases ammonia and nitrous oxide which “evaporate” from the urine patch. Reducing urinary nitrogen is critical to reducing both leaching and greenhouse gas emissions.
The first step to reducing urinary nitrogen is to know how much urinary nitrogen is being excreted. It is extraordinarily expensive to directly measure urinary nitrogen but it’s almost certain that bulk milk MU very precisely predicts urinary nitrogen. It’s likely that the MU-urinary nitrogen relationship found in international studies will hold in New Zealand – or at least there will be a direct relationship with perhaps a slightly different slope (in other words instead of 7 grams of urinary nitrogen per unit of MU the best value for New Zealand may be 6 or 8 grams). There is opportunity for the New Zealand dairy industry along with researchers and farm monitoring bodies to explore the MU-UN relationship under a range of feeding systems with the aim of using MU to predict urinary nitrogen. Once the relationships for New Zealand conditions are confirmed, all farmers will be able to use MU to accurately estimate the amount of nitrogen hitting the ground as urine on their farms every day.
In turn the farm monitoring programmes will have an accurate predictor of urinary nitrogen load per farm which would be a direct and transparent starting point to mitigate leaching.
The value of information that can be gleaned from MU should not be overlooked. The relatively small investment in research to verify bulk milk MU-UN relationships in New Zealand farming systems may return huge environmental dividends