Effect of Stallion on Embryo Recovery, Quality, and Stage, and on Pregnancy Rate after Transfer, in a Commercial Embryo Transfer Program
FL Riera,  JE Roldán, CC Love¹ and K Hinrichs¹
Centro de Reproduccion Equina DoZa Pilar, Lincoln, Argentina, and 
¹Department of Veterinary Physiology and Pharmacology, College of Veterinary Medicine
Texas A&M University, College Station, Texas, USA

            It is well known that there is an effect of stallion on pregnancy rates in mares.  However, since pregnancy is typically diagnosed more than 11 days after ovulation, pregnancy failure may be due to lack of fertilization or to reduced embryo viability.  We recorded data for 465 cycles over two breeding seasons in donor mares having no evidence of reproductive abnormalities, to examine the effect of stallion on embryo recovery rate, embryo quality and stage, pregnancy rate after transfer, and pregnancy loss between days 14 and 60.

Donor mares were inseminated before ovulation if possible; however some mares were first inseminated at the time ovulation was detected (within 12 hours after ovulation occurred). We have previously reported that post-ovulation breeding does not affect embryo grade, although it does lower the recovery rate and results in recovery of earlier-stage embryos. Therefore, embryos were recovered on Day 7 for mares inseminated before ovulation, and on Day 8 for mares inseminated after ovulation.  Embryos were graded for quality on a 1 to 4 scale, and were transferred transcervically to the uteri of recipient mares.  Ultrasound examination for pregnancy was performed between Days 14 and 21, and again between Days 40 and 60.

Twenty-three stallions were used, however only stallions which were used for 10 or more cycles were considered for analysis; thus 11 stallions were evaluated.  The embryo recovery rate for single-ovulation cycles in which mares were inseminated before ovulation varied among stallions from 26/37 (70%) to 14/15 (93%); there tended to be a difference between the lowest and highest performing stallions (P=0.07).  The recovery rate from double ovulating mares inseminated before ovulation was significantly different among stallions.  In the highest performing stallion, 10/13 (77%) cycles yielded 2 embryos; whereas in the lowest performing stallion only 3/13 (23%) cycles yielded 2 embryos.  This difference was significant (P = 0.02).

Nine stallions which produced 10 or more embryos each were evaluated for differences in embryo quality.  The proportions of embryos classified as Grade 1 over all stallions was 364/438 (83%); this varied significantly between the high and low performing stallions (60/67 (90%) vs. 59/78 (76%) respectively, P=0.02). However, pregnancy rates were similar between Grade 1 and Grade 2 embryos (229/364, 63% and 45/61, 74% respectively).  Only 12 embryos were categorized as Grade 3, however, the pregnancy rate was significantly lower for these embryos (2/12, 17%; P = 0.001). Considering only mares inseminated before ovulation, the distribution of embryo stages recovered (morula-early blastocyst, expanded blastocyst, or greatly expanded blastocyst) tended to vary among stallions (P = 0.07). 

The pregnancy rate after embryo transfer differed significantly among stallions.  The pregnancy rate for embryos from the high performing stallion (53/66, 80%) was significantly different from that for embryos from the low performing stallion (21/45, 47%; P < 0.001).  Because these two stallions were used exclusively in different seasons in which there were different overall pregnancy rates, we also evaluated the effect of stallion within one season.  Again, there was a significant effect of stallion on pregnancy rates between the low performing stallion (given above) and the high performing stallion for that season (pregnancy rate 53/79, 67%; P = 0.02).  The pregnancy loss between days 14 and 40-60 varied among stallions from 0-19%, but was not significantly different.  

We conclude that in our embryo transfer program, there was a significant effect of stallion on embryo recovery from double- but not single-ovulating mares.  There was an effect of stallion on embryo grade, and a tendency for an effect on embryo quality.  Interestingly, there was a significant effect of stallion on the viability of the embryo after embryo transfer.  This may be an effect related to embryo transfer, indicating that embryos from some stallions are more susceptible to damage during the recovery and transfer process.  Alternatively, this may indicate that a substantial proportion of the apparent conception failure noted in mares bred to specific stallions is due to embryo loss after day 7 rather than to fertilization failure.  Information about the proportion of viable embryos produced by a particular stallion would be of value when counseling clients as to the potential for success with embryo transfer.