Is it time to review VWP?
AT a recent Farm Advisory Service (FAS) meeting on dairy herd fertility, kindly hosted by Margo and Willie Webster of Mitchell (Drumdreel) Partners at Drumdreel in Fife, one of the topics of conversation centred around what the voluntary waiting period (VWP) should be.
Within the farmer group, this ranged from 42 to 60 days, with a number of higher yielding herds having a VWP of 42 days. Stevie Rolfe of SR Cattle Services, who attended the meeting and works closely with the Websters carrying out pregnancy scanning, stressed the importance of knowing the conception rate to first service and using that as a useful indicator to set the VWP.
VWP is defined as the period after calving during which cows are not bred, even if they have already been seen in oestrus.
It is recommended that the VWP should not be less than 40 days to allow the uterus to recover from calving, eliminate any infection, return to its normal size (involute) and resume ovarian activity. After this stage, the cow should become more fertile as she moves through the period of negative energy balance and passes peak milk yield. Typically, fertility will continue to increase until about 100 days in milk.
Deciding on the VWP is a fine balance between serving cows as soon as possible after calving but allowing sufficient time to ensure good conception rates. Conceive too early and you run the risk of having to dry off cows still giving plenty of milk. On the other hand, getting the cow back in calf too late means that she will spend a greater proportion of time in late lactation where milk production is less profitable in relation to her keeping cost and there is a greater period of time between lactation peaks, lowering lifetime production.
If a herd has an excellent submission rate (heat detection rate), the VWP can be extended. On the other hand, a herd with a poor submission rate would be advised to breed earlier if possible. Submission rates should ideally be more than 60%, any less and there will be more missed opportunities to breed cows.
Conception rates to first service should also be factored into the length of the VWP. Excellent conception rates to first service may allow a delay in breeding after calving, however, conception rates would need to increase by 6 to 8% points if the time to first service is delayed by 10 days.
Look at conception rates in cows bred less than 50 days compared to those bred between 50-90 days. If rates are significantly poorer in the early bred cows, consider increasing the VWP. A good target to aim for is 40% conception rate to first service.
For herds with automated heat detection systems, the decision of when to re-breed can be made on an individual basis. For example, high yielding cows slow to start cycling post-calving can be easily identified and given longer before starting rebreeding.
Factors can be taken into consideration such as the number of oestrus cycles since calving, the intensity of oestrus and any health issues, as well as having a milk yield benchmark and cows being in acceptable body condition. A longer VWP has been shown to be beneficial for conception rates in first lactation animals and their more persistent lactation curve means they are more efficient milk producers in later lactation than cows.
Although shortening the VWP may help improve the calving interval and improve 100 day in-calf rates by getting some cows to conceive earlier, factors affecting fertility should be reviewed as a whole, as it is likely that poor performance will be improved by a combination of factors, not just changing the VWP.
Factors that can lead to sub-optimal fertility include inadequate feed space, excessive body condition loss post-calving (from over conditioned cows during the dry period), poor heat detection and high lameness prevalence in the herd. UK average 100 day in-calf rates are 50% but 65% can be achieved in well-managed herds.
Stevie said: “For higher yielding herds (typically more than 8500litres) it may be worth increasing the VWP from 42 days, especially if conception to first service is below the 40% target.”
On the back of the meeting, the Websters, along with a few other farmers, said they would consider extending the VWP with a view to improving their herd’s fertility.