Pregnancy Rate

Ray L. Nebel
Extension Dairy Scientist,
Reproductive Management
(540) 231-4432
email: rnebel@vt.edu

Many different measurements have been employed to assess reproductive performance. Some commonly used measurements include average days open, first service conception rate, annual services per conception and calving interval. Unfortunately, these measures can fail to detect changes in performance on a timely basis. Ultimately, the question of interest to dairy producers is "How many of the cows eligible to become pregnant actually became pregnant in a given time frame?" Since the value is in the pregnancy, a measurement is needed to detect the rate that pregnancies are occurring in eligible cows. Recently, there have appeared articles in the popular press about using pregnancy rate to assess reproductive performance. Pregnancy rate can be defined as the percentage of cows eligible to become pregnant, in a given time frame that actually do become pregnant. A logical time frame would be 21 days, the typical length of an estrous cycle. The pregnancy rate calculation allows an assessment to not only determine how well cows are conceiving but also how quickly they are conceiving. Furthermore, by subdividing the breeding program into twenty-one day intervals, it can determine the effect of any recent changes on the breeding program. Pregnancy rate has often been defined as heat detection rate multiplied by conception rate. In many instances this will give a reasonable approximation of the actual pregnancy rate. The goal of any program should be to have 100% of the cows inseminated within 24 days of the voluntary waiting period. Pregnancy rate is the benchmark that incorporates service rate and conception rate in a timely fashion and should be the cornerstone of performance monitoring. DRMS calculates 21 day pregnancy rates and various reports using pregnancy rate are available with DART report 126. Reproductive performance is a function of certain management policies and how well these policies are implemented in the day-to-day management of the herd. It has long been known that there is an important economic advantage to be gained by efficient reproduction in dairy herds. The table below list current levels of five reproductive management benchmarks for Virginia DHI herds consisting of Holstein cows that have a rolling herd average for milk greater than 22,000 lbs. My goal and a level where I recommend a management change or intervention is also listed.

Parameter	Goal	Intervention level	1VA DHI herds >22,000 RHA milk
Days Open	130	175	156
Calving interval, months	13.3	15	14.4
Days to first service	75	100	91
Conception rate, first service, %	45	30	39
Heat detection rate, %	60	40	48
Pregnancy Rate	>22	12	16
1DairyMetrics report was generated on November 26, 2002 using current DHI information for Virginia Holstein herds that have greater than 22,000 lb rolling herd average for milk (n=156 herds).

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