Milk urea (MU) is a by-product of the breakdown of dietary protein. Nitrogen and protein digestion in the rumen releases ammonia. If a diet has excess rumen degradable protein, excess ammonia is produced. This excess ammonia is absorbed into the bloodstream and converted in the liver to urea.
High milk urea levels indicate that there is an inefficient use of protein in the diet. This can lead to reduced fertility, impacting the uterine environment. High levels can also cause an increase in ammonia emissions as most of the urea is excreted in the cow’s urine.
There are many ways which MU’s can be tested such as Infrared Technology and using a Spectrophotometer. MU figures often vary between dairies, depending on their lab.
Housed cattle: MU levels can range between 220-280mg/l, with the optimum at 250mgl/l.
Cattle at grass: According to AHDB’s Dairy Scientist, when cows go out to grass, milk urea levels are likely to increase, sometime by more than 350mg/litre, staying high until grass quality starts to change, where protein fall to levels around 20%.
Health and fertility impacts
There is contradicting and limited research on the relationship high milk ureas have on fertility and the health of the cow, however:
- Are cows cycling every 21 days? High urea levels can reduce cows’ likelihood of coming a bulling.
- Are cows cycling every 30 days? High ureas can make a harsh environment for oocyst implantation, so if she is cycling every 30 days, she may be having failed implantation.
- If everything is all good don’t worry about slightly high milk urea’s; they are only an indication.
Whilst the trial work is inconclusive, it should be noted that excess urea in the system requires energy for excretion which is wasteful and costly.
What affects milk ureas?
The main influence on MUs is diet and protein within the diet. Other facts include water intake, cow condition, stage of lactation, season, rumen health and liver function. In terms of the diet:
- Low milk urea is linked to a positive NFEPB (carbohydrate-rich or lacking in fermentable protein).
- High milk urea linked to a negative NFEPB (protein-rich or lacking in carbohydrates).
If NFEPB is 0, milk urea should be around 18. For every +100 NFEPB, we add 2 to the milk urea.
So, at the target of 200 NFEPB, you are looking for 22 mg/dl Milk urea.
What can we do to affect it?
Even if cows are milking well, it is worth checking if cows need more readily available energy to balance the high degradable protein levels, especially when at grass.
NWF Opti Rumen and NWF Ultra Buf can be used to help mitigate high urea levels. We can assume that the inclusion of NWF Opti Rumen, will reduce the diet’s NFEPB by 35%.
Milk urea levels can be useful for helping to diagnose herd problems and to identify opportunities to improve protein supply. Although the milk urea levels may be in the acceptable range, suggesting an adequate supply of protein the balance of that protein may not be correct. Milk urea can provide a useful tool but needs to be incorporated into the overall feeding management system rather than be relied on as a sole guide to protein requirement.
References: AHDB| DairyNZ| Gustafsson & Carlsson, 1993| Hutjens & Barmone, 1995| Butler et al; 1996 Cottrill., et al, 2002