Milk urea values provide us with a useful insight into the protein nutrition of the herd, but results should be interpreted carefully.
Much of the protein entering the rumen is degraded to provide protein for the rumen microbes. This rumen degradable protein comes in many forms, but a large amount will be present as ammonia. If the rumen bugs are supplied with sufficient fermentable energy and rumen conditions are favourable, they will be able to capture this ammonia and form it into highly valuable microbial protein. If the microbes are not able to capture the ammonia it will build up in the rumen and eventually pass through into the blood stream.
Ammonia is a toxic element within the cow, so is converted to a less harmful form of nitrogen compound, UREA, in the liver. It is essential that the liver is fully functioning otherwise the conversion of ammonia to urea is significantly reduced and the more toxic ammonia builds up leading to fertility, lameness and other issues.
A major risk to liver function is the build up of fat in the liver following periods of excessive weight loss. This tends to be mainly after calving, but can be anytime depending on energy supply.
High protein diets are most likely to lead to high milk urea’s and these diets need careful balancing. However, if milk urea levels are low on high protein diets, it is always worth considering how well the liver is processing the ammonia and whether there is a build up in the system.
One of the big issues which can arise from cows at grass is that fermentable energy supply to 
the rumen can be limited as sugar levels in the grass can halve during periods of duller weather.
This difference can be equivalent to a kilo of sugar (a bag) which reduces the supply of fermentable energy to the microbes, reducing their ability to capture the ammonia.
To further compound the issue at grass can be a lack of structural fibre in the diet. This reduces the cow’s need to chew the cud. Cudding is an important part of the saliva system which not only supplies acid neutralising sodium bicarbonate to the rumen, but is also recirculates the urea to the rumen for a further opportunity for conversion into microbial protein.
Grass crude protein levels are typically range between 22 and 26% in the dry matter, but the tips of the grass can be over 30% CP, so selective grazing can result in a very large over supply of crude protein in the diet. Levels of urea in the milk reflect the urea in the blood, so can therefore signal this excess in protein. However, this herd average masks what is happening to individual cows. A visual observation of dung pats across the field post grazing can highlight large variations in digestion, with some pats being firm whilst others are very runny, which can indicate selective grazing.
What should levels be?
Urea levels within the 0.03 (300 mg/l) range indicates a reasonable balance. Levels above and below this should be given consideration as to the variance.
Actions
- Ensure diets are well balanced for fermentable energy and rumen degradable protein.
- Watch for cudding activity in the cows. A good indicator is that two to three hours after cows go back out to grass after morning milking a large percentage of the herd will be laying down and a good 80% of those should be actively cudding. Spend 5-10 minutes watching. If cudding is poor, consider providing a long fibre source to the herd and/or strip grazing paddocks tighter.
- Avoid excessive weight loss within the herd by maintaining energy levels and dry matter intakes.
Our team are available for a review of milk urea
Call 01691 830741