Inter-service Interval a Major Gap Contributing to Low 21-Day Pregnancy Rates

Posted: August 13, 2008

With the significant increase in adoption of resynchronization programs more routine determination of pregnancy status is being used.

Much has been written about 21-day pregnancy rate (PR), the new and more accurate key indicator of reproductive performance. It is defined as the percent of cows that are eligible to become pregnant that become pregnant during each 21 day period commencing at the voluntary waiting period (VWP). It combines the two major components of reproductive management, heat detection rate (HDR) and conception rate (CR). The average PR in Pennsylvania is 15%, considerably lower than the benchmark of 20%.

Heat detection rate is further subdivided into pre-breeding and post-breeding heat detection rates. Pre-breeding HDR refers to the efficiency of detecting heats prior to first insemination whereas post-breeding HDR is an indicator of efficiency of heat detection rate among cows which fail to conceive to first or subsequent inseminations. Several studies have clearly documented that the inaccuracy of detecting estrus is between 5-30%, that is, insemination of cows not in true estrus. The major challenge to most dairy producers has been to improve HDR. Alternatively, with the use of timed insemination programs the goal is to improve the submission rate to insemination.

Research studies consistently show that the duration and intensity of expression of estrus are reduced in higher producing cows. Management and environmental factors also contribute to lower HDR. As a result many producers have adopted synchronization or timed-AI programs to achieve more timely first inseminations. One survey of management practices used by large herds across the US (Caraviello et al. 2006 Journal of Dairy Science: 4723) documented that 87% of the herds utilized some method of estrous synchronization. Although pre-breeding HDR is not calculated by DHIA, an indirect indicator of pre-breeding HDR is average days to first service. Using national DHIA data, research published in 2007 (Miller et al. JDS 90:1594) reported that herds using timed insemination programs had 17 days fewer days to first service than herds not using this management tool. Consequently, we can assume that pre-breeding HDR (submission rate) is increasing.

Unfortunately, it is estimated that the average inter-service interval is approximately 50 days. This is a large gap between inseminations and identifies a significant opportunity for improvement. Thus on many farms post-breeding HDR or resubmission rate becomes the major bottleneck to achieving a 20% PR. Additionally, one potential disadvantage to using timed insemination programs is that the effectiveness of detecting natural heats for cows that fail to conceive may be low due to the fact that herd personnel have a tendency not to devote time to routine observation for estrus. Again, inaccurate heat detection may also be a factor when attempting to identify open cows based upon visual observation. All these factors have contributed to the adoption of resynchronization programs. Such programs are designed to resubmit open cows to timely insemination with acceptable conception rates. The effectiveness of such programs depends upon early and accurate identification of the open cows.

With the significant increase in adoption of resynchronization programs more routine determination of pregnancy status is being used. In contrast to rectal palpation, ultrasonography has been used more frequently for earlier determination of pregnancy status. Another method for early identification of open cows is BioPRYN® which is a blood test for pregnancy-specific protein B. This protein is produced by specific cells of the placenta and can be quantified at 30 days after insemination. BioPRYN® is reported to be 99% accurate in identifying open cows. Since the dairy producer collects the blood sample, this tool can be easily adapted to the reproductive management program for each dairy. Correct use of this test should narrow the number of days since insemination when pregnancy status is determined among a group of cows. Eligible cows (30 days after insemination) could be tested on a weekly basis without waiting for a veterinarian to palpate or use ultrasound. No matter which method is used for determining pregnancy status the objective should be to develop a system that incorporates early detection of failed inseminations and rapid resubmission for insemination. This approach will reduce the inter-service interval and should improve 21-day PR.

Michael O’Connor, Dairy and Animal Science Extension