Bulk tank urea might prove useful feed tool
Are bulk milk urea results
worth noting – and do they
indicate poor fertility? That
was the subject of a recently
study. Emma Penny reports
HIGH urea levels many only start to affect fertility when all other aspects of herd management are as good as possible, a study funded by the Milk Development Council and undertaken by ADAS nutritionist Bruce Cottrill has concluded.
The possible link between high urea levels and fertility in dairy cows is a topic of intense debate. Since milk market deregulation, availability of bulk milk urea levels has increased producer interest in the topic.
Dr Cottrill says a number of studies have suggested a link between high urea levels and infertility. Two Scandinavian investigations have shown that bulk tank results showing high urea levels are linked with low conception rates. Conversely, a Norwegian study showed that there was a link between urea levels and fertility in one region of the country, but no link in three other regions studied. "We wanted to see whether there was a link between bulk tank urea levels and fertility; could bulk tank results be used as a management tool for UK producers?" says Dr Cottrill. "Questions have been raised about the validity of bulk milk urea, so in addition to the bulk tank study we also decided to look at urea levels in individual cows at service.
"There are several theories besides high urea affecting fertility. One is that ammonia is the problem, not urea, while some also believe that a change in pH at service has a negative effect on sperm activity. Whichever of those options, it seems that conditions at service has a critical effect on fertility."
The bulk tank study looked at milk urea results from 475 herds, coupled with their NMR fertility records to ensure reliable data was available for each herd. Milk urea and fertility results were compared for four month periods in winter and summer to provide as wide a spread of data as possible.
On average, results showed that bulk milk urea levels were consistently highest in August, and lowest in February.
"We looked at conception rates and bulk milk ureas on each farm. We then took out all herds serving fewer than 15 cows in any one month to avoid distorting any trends," he says. "When we calculated results we found there was no relationship between bulk tank urea levels and fertility at all."
Although slightly disappointed, Dr Cottrill was not surprised. "Bulk tank results do account for all cows in the herd, which is a wide spread. Perhaps we would have obtained slightly more meaningful results with very tight calving periods."
Next, bulk tank milk urea levels were examined on a monthly basis to see whether big changes in urea levels between months affected fertility. "Again, there was no correlation. Even where levels rose by 200mg/litre or fell by 150mg/litre, there was no effect on fertility."
With bulk tank urea levels bearing no relation to fertility, Dr Cottrill began the second part of the study, looking at individual milk urea levels in 437 cows on 11 farms.
Urea levels were measured at only first service to avoid complicating results with other factors. All cows were subsequently PDd to see whether they were in calf.
"Even the results of this survey appeared inconclusive. Milk urea levels for successful and unsuccessful services were exactly the same. This was not what we expected at all, but with hindsight the results may be clearer," he says.
Only in one herd was there a significant difference between milk urea levels and fertility. Despite the highest milk urea levels in the study, this herd had the highest conception rate. On this farm, cows successfully conceiving had a significantly lower milk urea level than those requiring a second service.
"Pregnancy rate in all other herds in the study was about 50% to first service; in this herd it was 66%. While all farms in the study – one of which was ADAS Bridgets – were well managed, it may be that this herds breeding management was better, allowing the effects of the high urea levels to be seen.
"A host of factors affect fertility, but the high degree of management in this individual herd means fertility is as good as possible. Where this is the case, it may be that high milk urea levels do start to have an noticeable effect on fertility."
In most herds, other factors such as poor heat detection will affect fertility performance. But Dr Cottrill suggests that where fertility management is as good as it can be, then the effect of high urea levels starts to become apparent.
"Care is needed in interpreting these results as it is only one herd. But it appears that milk urea levels can have an effect on fertility, but only where all other factors are correct; it is not an over-riding fertility factor on most farms," he says.
But he suggests that despite the lack of a link between bulk milk urea levels and fertility that bulk tank results should still be studied carefully. "They can give a good indication as to whether rations are balanced."
Work in the US on milk urea levels has focused almost exclusively on its link with feed management and rationing, says Dr Cottrill.
"High milk urea levels suggest that rations are too high in protein. Excess protein is excreted in urine and milk, and the energy cost associated with excreting it further upsets the ration balance."
As a guide, American researchers have calculated that where bulk tank milk urea levels rise from 300mg/litre to 450mg/litre, the energy cost of excreting excess urea is 2.5 MJ ME – equivalent to 0.5 litres of milk a day. *
Bulk tank milk urea levels can play a useful role in ration management.
Only when fertility management is as good as possible do effects of high milk urea levels start to become apparent in dairy cows.
• Noticeably affects fertility only when everything else correct.
• Bulk tank results used as ration balance guide.