Effect of dry period length on reproduction during the subsequent lactation

Does the length of the dry period have an effect on reproduction? This question was addressed by R.D. Watters and co-workers in their study as reported in a paper with the title: Effect of dry period length on reproduction during the subsequent lactation, published in the Journal of Dairy Science, Volume 92 of 2009, page 3081 to 3090.

It is well-recognized that optimum reproductive efficiency results in increased profit per cow and improved overall efficiency in dairy operations. It is therefore of concern that it is observed that reproductive efficiency in high producing dairy herds is decreasing. One of the reasons for this observation is too low energy reserves because of high milk yields during the time when the cow must become pregnant for the next cycle. A possible way of addressing this problem is to shorten the dry period which would result in higher levels of feeding to the cow, since she would sooner receive the higher concentrate diets immediately before calving and at the onset of lactation. In previous studies it has been shown that days dry affected reproductive measures such as days to first after-calving ovulation, days open and success of artificial insemination (AI) as measured by pregnancy per AI, thereby supporting the feeding level argument. However, the results have not been convincing because of inadequate numbers of cows in the studies, which limit effective statistical comparisons.

In this study Watters and co-workers used a very large sample of a total of 780 cows. The cows were divided into two groups, the one group assigned to a targeted dry period length of 55 days (standard group) and the other group to a targeted dry period length of 34 days (shortened group). The feeding programs were according to normal designs for dry period, pre-calving and after-calving (lactation diet). As components describing reproductive efficiency they measured the number of days to first after-calving ovulation, the number of days to first AI, the number of days to pregnancy and the number of pregnancies per AI. The results show that the number of days to first after-calving ovulation occurred sooner (35 days) for the shortened group than for the standard group (43 days). The percentage cows that were classified not ovulating by 70 days in milk were less than half in the shortened group (8%) than in the standard group (18%). The percentage of cows pregnant at 70 days in milk tended to be greater for the shortened group than for the standard group, whereas the number of days open tended to be fewer in the shortened group than the standard group. If the results from first and second service are combined, pregnancies were greater in the shortened group (32%) than in the standard group (24%). It was interesting to note that for young cows these differences between the shortened and standard groups were less convincing than for older cows. With respect to energy balance as measured by blood profiles, cows in the shortened group had more reserves than cows in the standard group.

The results showed that, at least for older cows, if the length of the dry period is shortened reproductive efficiency can be improved. This most probably is due to the fact that the length of the dry period influences after-calving energy balance of the cow, thereby altering reproductive performance.