April 10, 2009
Information gathered helps determine whether a horse’s embryo is likely to survive.
By Dr. Patrick M. McCue
Embryos are routinely evaluated after collection and prior to transfer into a recipient mare. The evaluation usually takes only a few moments to perform and can provide valuable information as to the probability that a given embryo will survive after transfer.
A variety of embryo characteristics are examined including developmental stage, size, shape, color and the number and compactness of cells. An embryo is then given a numeric grade based on the observed characteristics.
Generally, an embryo’s development is directly related to its age. The development of an equine embryo increases each day after ovulation.
Breeding horses seems to get more technical by the moment. A copy of AQHA’s Horse Reproduction Report will help you sort through all the facts. Get the skinny on a variety of breeding methods plus prepare yourself for the costs involved with breeding and the basic procedures.
The size of an embryo is also typically related to the age of the embryo. A Day 6 equine embryo is approximately 0.2mm in diameter while day 7, 8 and 9 embryos are about 0.4, 1.1 and 2.2 mm in diameter, respectively.
Only small embryos of less than 0.3 mm in size are suitable for freezing, so it is important to flush the embryos before they reach Day 7.
Be sure to download AQHA’s Mare Care: Breeding Tips FREE report.
Grade refers to the quality score of an embryo. A scoring system has been developed that ranges from 1 to 5. A Grade 1 embryo is considered excellent. It has a spherical shape and cells of uniform size, color and texture. Conversely, a Grade 5 embryo would be totally degenerated or dead.
During assessment, embryos can sometimes be confused with unfertilized oocytes. Though oocytes typically stay within the oviduct, they are occasionally recovered during an embryo flush.
The easiest way to tell them apart is that an embryo is spherical and can roll across the bottom of a Petri dish while examined, while an unfertilized oocyte is flat and cannot roll.
Pregnancy rates in recipient mares are correlated with the grade or quality score of the embryo prior to transfer. Grade 1 and 2 embryos have approximately the same transfer success rate, whereas Grade 3 and 4 embryos have significantly lower initial pregnancy rates after transfer and a higher incidence of embryonic loss if a pregnancy is initially established.
Once pregnancy in your mare is established, whether through embryo transfer, artificial insemination, or live cover, you need AQHA’s Horse Reproduction Report. This four-part guide to pregnant mare care and breeding your horse is a necessary addition to any horse breeder’s library.
Multiple Foal Production Numbers from Registration
How many foals are mares typically having through the use of embryo transfer under the multiple foals rule?
According to AQHA registration records, there have been 6,213* sets of multiple foals born, all foaling years combined (a mare is counted for each set of multiple foals she produced).
In that number, there were:
- 4,830 sets of two foals
- 1,067 sets of three
- 253 sets of four
- 47 sets of five
- 11 sets of six
- 4 sets of seven
- 1 set of nine foals
Those 6,213 sets represent 14,251 registered foals, or an average of 2.29 foals per set. *These numbers are as of February 5, 2009.
Come to QuarterFest
It’s a three-day, education-packed celebration of the horse! Horse enthusiasts from around the world will gather in Murfreesboro, Tennessee, May 1-3 for fun, entertainment and festivities to honor the world’s most versatile horse – the American Quarter Horse. All horse lovers welcome – come for the fun and bring the family. Learn more about QuarterFest at www.aqha.com/quarterfest. Hurry, tickets are going fast!