Measuring Reproductive Efficiency

July 25, 2012

Keeping good breeding records is one of the cornerstone principles of sound reproductive management. For many of you the breeding season is now coming to a close, whether you are a stallion owner reviewing the conception rates for the mares that were bred to your stallion, or a breeder reviewing the reproductive performance of your mare herd, now is a great time to start compiling the data. Best to do it now whilst your experiences are still fresh in your mind and you can readily lay your hands on the paperwork. Plus as a stallion owner it may take some time to get the feedback you need from your mare owners. In this blog article we discuss the importance of keeping detailed accurate breeding records and what parameters you should calculate and follow.

Some of the reasons for keeping good breeding records are to:

  • determine reproductive efficiency of stallions, mares and management practices
  • compare the results of this year to previous years and to expected values 
  • identify trends and tendencies related to age, season, etc.
  • pinpoint problem areas
  • implement management changes based on objectively gathered data
  • set attainable goals for the following season

Creating a simple spreadsheet to track those indicators of reproductive performance that you are most interested in monitoring will make it easier to be efficient and organized as the data is collected throughout the breeding season. Inserting formulas into an Excel spreadsheet to calculate averages and percentages in real time can provide you with an informative status update at the end of each month. Regular examination of such data during the course of the breeding season may identify a problem that can be corrected before it negatively affects reproductive efficiency, client confidence and your stallion's reputation, plus it may save time and money in the long-term.

Semen Quality Data

Tracking semen collection and quality data is a key responsibility of the stallion manager. If your stallion’s mares are not getting pregnant, the first place you’ll look for answers is his semen quality. So it is important to have up to date and accurate records of his collection history, semen quality and longevity. Include notes about the overall health status of your stallion, for example if he runs a fever, it may be 2 months before the effect of elevated testicular temperature may be evident in his semen quality. If your stallion is collected offsite at a veterinary clinic or breeding farm, discuss with them the data they are recording and request copies for your own records.

Having these records to refer to can provide valuable information beyond trouble shooting poor conception rates, it can be used to answer questions and concerns from mare owners about semen quality, evaluate changes in the stallion’s environment, monitor new semen processing techniques or technicians and guide decisions related to stallion management. For example, as the stallion gets older a review of these records could reflect a decline in semen quality and longevity, which may prompt a change in semen processing or stallion management - maybe the breeding dose should be increased to ensure there are still 500 million progressively motile sperm at 24hr and this may ultimately necessitate reducing his book of mares.

Semen collection and quality parameters to monitor:

  • # mounts
  • seminal volume 
  • initial sperm concentration
  • total number of sperm per ejaculate
  • total and progressive motility
  • periodic sperm morphology (recommend once per month)
  • periodic bacteriology of raw and extended semen (recommend once per month)

Additionally for cooled semen programs:

  • volume and concentration of the breeding dose
  • total and progressive motility at 24 and 48 hours post collection
  • number of progressively motile sperm per breeding dose at 24 and 48 hours
  • # doses shipped and # shipments per mare

Fertility Data

Pregnancy Rate
Pregnancy rate is the number of mares diagnosed pregnant divided by the number of mares bred and then multiplied by 100. Pregnancy rates can be influenced by the mare, the stallion or by the expertise of the veterinarian or breeding manager coordinating the insemination. With respect to the mare, pregnancy rates can be affected by the reproductive status of mares, the age of mares and the time of breeding in relation to the estrous season. For example, early season pregnancy rates may be lower because of transitional estrus irregularities in some mares and pregnancy rates may decline in a population of older, barren mares. The stallion can also influence pregnancy rate outside of his inherent fertility, for example if he is managed poorly with respect to the number of mares bred, collection frequency and in semen processing techniques.

Pregnancy rate can be calculated a number of different ways:

  • End of season pregnancy rate - The percentage of mares pregnant at the end of the breeding season.
  • Per cycle pregnancy rate - The number of cycles resulting in a pregnancy, divided by the total number of cycles bred.
  • First cycle pregnancy rate - The percentage of mares pregnant after the first breeding cycle.

When reviewing the fertility of a stallion it can be misleading to look at only the percentage of mares in foal at the end of the breeding season. For example two stallions with the same overall pregnancy rate of 85% are not necessarily of equal fertility. If one stallion has a 70% per cycle pregnancy rate, fewer than 2 cycles per mare are required to attain his overall pregnancy rate of 85%. On the other hand, if the second stallion has a per cycle pregnancy rate of 35%, it requires more than 4 cycles per mare on average to get 85% percent of his mares in foal. Obviously, under similar management conditions, the first stallion is more ‘fertile’ than the second. A great deal more effort (labor, time, veterinary costs, mare board, etc) is required for the second stallion. The difference in per cycle pregnancy rates between these two stallions quickly separates them in terms of ‘reproductive efficiency’.

This concept of reproductive efficiency can also be applied to mares. Mares with a poor reproductive history (low-grade uterine biopsy, for example) have a lower inherent fertility which, while they may ultimately conceive, will require more effort to achieve a pregnancy.

If you offer your stallion by different breeding methods, e.g both cooled and frozen semen, calculate the pregnancy rate for each of the methods separately. If the frozen semen was processed at the same time, i.e. within the same freezing session or time-period, the pregnancy rates can be accumulated over time. Thus although your stallion may only breed a handful of mares by frozen semen each year, it is the same frozen semen you are using each year, so the pregnancy rate applies to that batch of frozen semen. Bearing in mind that pregnancy rates for frozen semen may change for semen frozen when the stallion was 4yrs old compared to when the stallion was 20yrs old. Additionally, if you breed mares on the farm as well as ship semen, calculate pregnancy rates separately for on farm mares compared to those receiving shipped semen.

Other related parameters that you may wish to calculate and monitor:
• number of cycles per pregnancy
• average number of shipments per pregnancy for cooled semen
• average number of doses per pregnancy for frozen semen
• pregnancy rate per cycle for maiden vs. foaling vs. barren mares

Foaling Rate
The ultimate goal of any breeding program is to put a live foal on the ground. To calculate foaling rate divide the number of mares foaled by the number of mares bred and multiply by 100. You may also wish to track the numbers of colts or fillies, and color if you breed for color based traits. Set reminders in your calendar to call mare owners to determine the foaling status of their mare(s).

Pregnancy Loss
The difference between foaling rate and pregnancy rate (from the first pregnancy check) is pregnancy loss. Pregnancy loss could be due to misdiagnosis of pregnancy, early embryonic loss or abortion. To better characterize pregnancy loss you may wish to split out pregnancy rate at day 14 and day 60. Embryonic loss would be those pregnancies lost between day 14 and day 60, fetal loss would be those pregnancies lost between day 60 and term and the still birth rate would be the fetal loss rate near term (e.g. after 300 day).

A note about numbers
When inquiring about the pregnancy rate for a particular stallion it is important to give these values some reference with regard to the number of mares bred. We’ve all heard stallion owners claim their stallion has a 100% conception rate, when he has bred only 3 or 4 mares. Then there is the mare owner whose mare always conceives on the first cycle, utterly distressed that the 3rd year she’s bred it takes 3 cycles. The data from a small number of individuals can easily be skewed in one direction or another by just luck - good or bad. Of course its still good information to have, but it must be placed in context with the size of the population upon which it is based and take into account any extenuating circumstances that could have easily influenced the result.


The goal of most horse breeding operations is to increase the efficiency and decrease the cost of producing live foals. Since the overall performance of a breeding program is determined by the interrelated reproductive efficiency of mares and stallions and the effect of breeding method and management, all of these factors should be carefully considered before breeding decisions are made. Once reproductive success has been calculated, the easier it is to determine if your success is better or worse than normal, and perhaps more importantly, why. When we understand why we are getting the results that we are, we can make better decisions and modification of management practices to improve our success. However, sometimes we must just learn how best to manage the horses we have and be realistic in our expectations. Even stallions with poor fertility can obtain excellent results with the application of proper management. When you know that one part of the fertility equation is not optimum, make sure that the others are.

Stay tuned for our topic next month - What is normal reproductive performance?