Grass is plentiful and for those that graze their milking cows there is a saving from less protein supplementation required. According to AHDB Dairy’s Forage for Knowledge, current grass quality data means that cows are likely getting a surplus of rumen degradable protein (RDP) from grass. Recent data shows that the protein in grass is around 21% in the dry matter, with the target for milking rations to be in the region of 16 – 17%.
While cows do not have a requirement for protein percentage, we have to meet their metabolisable protein requirements, which is made up of rumen degradable and rumen undegradable (bypass) protein. Much of the protein in grass is RDP and a surplus of this can lead to high levels of urea in the milk, due to excess RDP being broken down into ammonia in the rumen and then converted to urea in the liver. This is especially true where the rumen is lacking in fermentable carbohydrates, which help utilise RDP and convert it into microbial protein.
While various recommended targets of milk urea have been suggested from different sources, it is widely accepted that levels greater than 300mg/L or 0.03% are high, and that protein nutrition should be reviewed. Milk urea levels can be lowered either by reducing the supply of RDP, increasing the amount of rumen fermentable energy in the diet, or a combination of the two.
If milk urea is low (<150mg/L or < 0.015%) it is possible that there is a shortage of RDP in the diet and this could be restricting milk output, but this is more likely to be the case when cows are housed.
High clover grass leys will tend to have a higher protein content compared to pure perennial ryegrass mixtures and so the risk of high milk urea levels are even greater when cattle are grazed on high clover swards. Excess protein is inefficient as there is an energy cost to get rid of it and there may also be a detrimental effect on fertility. While the research in this area is conflicting, it has been shown that the pH of the uterus is affected by feeding an excess of RDP, which can lead to an unfavourable environment for embryo survival and reduce conception rates.
If milk urea levels are higher than desired, look at cycling behaviour. Firstly, high urea can reduce the likelihood of cows coming bulling. Secondly, if high milk ureas are impacting conception rates, this could be evident by cows cycling every 30 days if embryo implantation failed. Milk urea over 350mg/L have been associated with lower conception rates by 5-40% compared to milk urea levels in the normal range.
Milk urea is likely to be higher in pasture-based herds as opposed to housed herds or composite herds which are buffer fed with a TMR while at grass. If the high protein in grass is not correctly balanced with lower protein forages and supplements, the excess RDP can further exacerbate weight loss, also contributing to lower fertility.
Other influencing factors can be down to grass being grazed too soon after nitrogen fertiliser is spread. This may be the case when fertiliser is blanket spread or when grass availability is limited, and the rotation length is reduced. The risk is highest around 14 – 17 days after spreading but is also dependent on rainfall, N uptake and grass growth.
Although milk urea levels are only an approximate guide to protein nutrition, given the cost of feed, it is worth ensuring rations are correctly balanced for protein supply to optimise milk output and that there is sufficient readily available energy to balance high RDP supply when cows are at grass.
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