Years ago, we used to hear that most cows were not fully ready to breed until around 40 days after calving. Some farmers would breed cows if they were just past 40 days in milk, but over time most seemed to wait until 50 days before breeding.
    For the last couple of decades, longer voluntary waiting periods have been advocated by many, and now VWPs between 60 and 80 days are much more common. A longer VWP is often recommended as a way to improve reproductive performance, since some studies have shown greater fertility as VWP increases. Longer VWPs have also been advocated to reduce the numbers of cows that have short lactations, thereby increasing profitability. The assumption is that cows are being dried up when income over feed cost is still high, and that total lifetime days dry increase as lactations get shorter.     
    Furthermore, reproductive performance has improved in many herds following the adoption of timed A.I. programs, resulting in a greater percentage of cows pregnant on first breeding, which has supported the argument that too many cows have short lactations in some herds.
    But, do we really know the economic value of longer VWPs? Only a few studies have looked at this question, and the results have been inconsistent. Two recent papers in the Journal of Dairy Science looked more closely at this question and found some interesting results. These papers were of several published from work done beginning in 2014 using three commercial farms in New York.
    Cows were inseminated following enrollment in a double-ovsynch protocol. Second and greater services were based on heat detection and ovsynch protocol or both. One group of animals was inseminated following a 60-day VWP and another was inseminated following an 88-day VWP. All cows remained in the experiment for up to 18 months unless they left the herd due to sale or death. rBST was used in all three herds. Returns were calculated as cash flow per cow including the lactation and dry periods. Cash flow was also calculated per slot for an 18-month period, which meant that every leaving cow was replaced by a heifer to accurately reflect what happens on commercial farms. Modeling was also completed to estimate returns for various input pricing scenarios. In a separate analysis, cash flow per cow was calculated for only the animals that became pregnant on the first breeding.
    First service conception rate was better for primiparous cows in the 88-day VWP group compared to the 60-day VWP group (55 percent vs 45 percent) but there was no differences between groups of multiparous cows. Lactation length was 22 days longer for primiparous cows in the 88-day group versus the 60-day group, but there was no difference in lactation length of multiparous cows in the 88-day group and 60-day group. There was a greater risk of culling for multiparous cows in the 88-day group versus the 60-day group (35 percent vs 29 percent), but no difference between groups of primiparous cows. However, primiparous cows in the 60-day group were slightly more likely to be culled in the subsequent lactation than primiparous cows in the 88-day group.
    Extending the VWP from 60 to 88 days resulted in a net gain of $89 per cow per lactation for the primiparous cows and no gain or loss per lactation for the multiparous cows. When evaluating returns per slot for an 18-month period, extending the VWP from 60 to 88 days in primiparous cows resulted in a net gain of $68 per animal slot over 18 months, and extending the VWP in multiparous cows resulted in a net loss of $85 per slot over 18 months.
    Prices for a variety of inputs such as milk price, cull price and replacement cost were changed and results were predicted per 18-month slot using a computer simulation. Changing input costs changed the returns for both primparous and multiparous cows, but in all simulations extending the VWP for first lactation cows was profitable while extending the VWP for older cows showed negative returns when calculations were based per slot. When cash flow per cow was calculated for the animals that became pregnant at the first breeding the relationship was the same; i.e. it was more profitable to extend the VWP to 88 days in primiparous cows and more profitable to breed older cows with a 60-day VWP.
    The main reason why extending the VWP in older cows was not profitable was because it resulted in greater culling. A secondary reason is that there was no improvement in reproductive performance at first breeding.
    In recent years, there have been articles advising dairy farmers to reduce culling hopefully to increase the age of their herds and reduce herd replacement costs. There have also been articles recommending increasing the VWP to increase profitability due to increased lactation length. However, this recent analysis suggests that increasing VWP in multiparous cows does not result in longer lactations because of the effect of increasing culling and that the net economic effect of the longer VWP in older cows is negative. Based on this analysis, increasing the VWP in multiparous cows will result in greater culling rates and net economic loss. Increasing the VWP in primiparous cows could increase profitability, but remember that this study was done with cows supplemented with BST, so the effects might be less in today’s herds. The bottom line is to be careful when considering an increase in VWP; it may not be as simple as it seems.
    Jim Bennett is a dairy veterinarian at Northern Valley Dairy Production Medicine Center in Plainview, Minn. He and his wife, Pam, have four children. Jim can be reached at bennettnvac@gmail.com with comments or questions.