Wednesday, October 2, 2013

Understanding Embryo Survival

New Zealand researchers evaluated the effect of age, weight, and sire on embryo and fetal survival in sheep. Data consisted of 11,369 records on ovulation rate and litter size.

Ovulation rate was lowest at age 2 and increased from age 2 to 6. In ewes that survived to year 6, the mean litter size was 1.87, 2.05, 2.01, 2.07, and 1.91 in ewes of age 2, 3, 4, 5, and 6, respectively. Litter size was less in ewes of age 2 and 6 compared to ewes of age 3, 4, and 5.

Two-year-old ewes had lower embryo survival than 3-yr-old ewes and the probability of embryo survival decreased after age 3. Thus, embryo survival contributes significantly to lower fertility in 2-year-old ewes. In ewes with high ovulation rates,  more balanced ovulations tended to be associated with increased embryo survival.

There was a quadratic effect of ewe weight on embryo survival, with decreased embryo survival for low and high ewe weights. There was a positive linear relationship between pre-breeding ewe weight and ovulation rate. The optimal ewe weight for embryo survival increased with ovulation rate, which is consistent with the requirement of greater body reserves for maintaining a larger number of fetuses during gestation.

A quadratic relationship between ewe weight and the probability that a ewe is able to maintain a pregnancy was also observed,  Pregnancy loss is due to failure of the embryo or fetus or failure of the dam to maintain the pregnancy. The sire of the embryo only influences the embryo, whereas the maternal grand sire influences both the ewe and the embryo. 

Genetically, the dam’s ability to maintain a pregnancy has 5.5 times the effect on pregnancy loss than the embryo’s ability to survive. Therefore, selection among dams based on the mean embryonic survival of their embryos will provide an effective way to improve embryonic survival.

Source:  Abstract, Journal of Animal Science. October 2013.

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