Why is reproduction so important?
Profit for both dairy and beef depends upon successful breeding. Neither milk nor meat can be produced without pregnant cows. In theory, the process is simple – watch the cows and breed at standing heat - but in these days of large herds, artificial inseminations and busy employees, the reality is that getting cows pregnant can be quite a challenge. Thus numerous approaches have been developed to increase the likelihood of and opportunities for successful breeding of cows in heat.
How can reproduction be evaluated?
One of the most succinct ways to measure a herd’s reproductive performance is to look at its pregnancy rate (Ferguson). It describes the speed at which cows are becoming pregnant in a herd. In mathematical terms, pregnancy rate (PR) is the product of the heat detection rate (HDR) and conception rate (CR):
PR = CR x HDR
Calculating the PR, CR and HDR begins with farm records. Some dairy management software programs will be able to generate these parameters directly in a report; make sure you are familiar with that report’s design in order to interpret the results properly. If the farm is on DHIA, monthly sheets will supply much of the needed information.
CR – Conception rate is a measure of a cow’s fertility at service. It is calculated by dividing the number of pregnant cows by the total number of inseminations. Respectable rates are 50 – 55%, and should be highest at first service. The CR at first service is also the best indicator of the overall CR because all cows are in this calculation, including problem breeders culled later in their lactation. Conception rates are confounded by such factors as the physiologic fertility of the cow, semen quality, and semen handling and insemination techniques. It is not an easy variable to improve, other than to make sure good semen is being properly inserted into a healthy uterus.
CR = 1/ Services per conception (all cows)
If the overall conception rate is low, specific problem areas can be identified by comparing rates within groups, such as lactation number, inseminator, season bred or service number.
HDR – This is the rate at which estrus is detected on a farm. Heat detection is the easier variable to manipulate in order to improve pregnancy rate. The simplest method of heat detection is visual observation, and the more the better. The accepted “optimum time” is four evenly spaced periods of 15 to 20 minutes per period. However, a study in 1998 (Dransfield et al) reported an average of 8.5 (+/- 6.6) standing events per cow. If most mounts last about 3.27 seconds (Nebel), then the average cow exhibits standing behavior for about 28 seconds every 21 days. Easy to miss! Heat detection aids such as tail chalk or paint, Kmar detectors, pedometers, electronic pressure sensors, androgenized cows or vasectomized bulls, and hormone programs (discussed below) all increase the likelihood of identifying standing cows and/or breeding them at the appropriate time.
An overall heat detection rate is calculated as follows:
|(21 x Average services per pregnancy)
(Average days open – VWP +11)
|X 100 = HDR
Average services per pregnancy and average days open are provided by farm records; the producer will know the voluntary waiting period (VWP). Rates should be greater than or equal to 50%; excellent farms can achieve 80 to 90% with vigilant observation and other aids.
For a complete analysis of HD, there are two time intervals that should be evaluated: pre-service heat detection efficiency and post-service heat detection efficiency. Pre-service HD efficiency examines the period between calving and first insemination and tells how quickly cows are bred after the voluntary waiting period (VWP):
(Days to 1st service – VWP) + 11
|X 100 = Pre-service HD efficiency (%)
Low rates may indicate poor heat detection methods and/or non-cycling cows. Post -service heat detection efficiency examines the time between first service and when cows become pregnant. It relates to how quickly cows are rebred if they do not become pregnant:
| Services per pregnancy – 1
Average days open –Days to 1st service
|X 21 X 100 = Post-service HD efficiency (%)
Days between inseminations can also be grouped by 21-day intervals to identify problems with accurate heat identification, early embryonic deaths and missed heats (Example Calculation).
PR - Once the pregnancy rate is calculated, what does it mean? The economic returns will be optimum from reproduction at a pregnancy rate of 35% (Ferguson), but this is rarely achieved in the field. At a PR of 25%, it takes 42 days for 50% of the herd to become pregnant. Based on summarized DHIA data, the PR in the state of PA is actually only 13%, so it takes an average of 81 days for half of a Pennsylvania herd to become pregnant. If, for example, the average days to first service is 90, then the average days open for a herd is 171. If a cow gets pregnant at 171 days in milk, then her eventual calving interval will be almost 15 months, meaning that she will spend more time in the later part of her lactation, contributing less milk and thus less profit. And that’s just the average cow in the herd! Poor pregnancy rates represent lost economic opportunity and income.
It is important to know a herd’s culling rate when examining records. This rate can influence other reproductive indices, often falsely improving them by removing problem breeders from the denominator. At the very least, it is important to be aware of whether or not cull cows are included in your figures, and then interpret the data accordingly.