Effects of ovulation of aged follicles, pregnancy diagnosis by ultrasonography, and treatments during lactation on reproduction in ewes

Effects of Ovulation of Aged Follicles, Pregnancy Diagnosis by Ultrasonography, and Treatments During Lactation on Reproduction in Ewes Aimee K. Wurst Ewes are polyestrous and litter-bearing seasonal breeders. Although average ovulation rates are sufficient, many ovulations do not result in live offspring and ewes normally breed only once per year. As ovulation rate increases, so does the number of ovulations not represented by live births. When ultrasonographic analysis is limited to ovarian follicles that grow from 3 to 5 mm, follicular waves can be identified in sheep. In most cases, ovulatory follicle(s) develop from the ultimate follicular wave; however, ovulatory follicles can be derived from penultimate or earlier follicular waves. The relationship between lifespan of large follicles and fertility is controversial in sheep. In experiment 1, three approaches to create a model to compare fertility when ovulatory follicles developed from only the ultimate follicular wave or both ultimate and penultimate waves were tested. Ewes were treated on d 10 or 11 with 50 μg of GnRH i.m. , the largest follicle(s) (≥ 4 mm) on each ovary were electrocauterized, or ewes were treated with 50 μg estradiol benzoate and 50 mg progesterone or 100 μg estradiol benzoate and 50 mg progesterone in attempts to induce follicular regression or luteinization. Corpora lutea were counted by ultrasound 7 days following estrus in order to verify ovulation of follicles that disappeared post estrus. Pregnancy was determined and embryos were counted 20 to 30 days following estrus. Neither treatment with GnRH nor estradiol and progesterone consistently caused regression or luteinization of ovulatory-sized follicles. Follicular ablation was successful in increasing the proportion of ovulations originating from the ultimate follicular wave in Dorset/Suffolk cross ewes, but did not alter the proportions of ovulations fom different waves in Polypay ewes. Average age of the ovulatory follicle was affected only in Dorset/Suffolk cross ewes. Due to lack of treatment effects, ovulatory follicles were evaluated regardless of treatment for effects on embryonic or fetal losses. Although loss tended to differ among follicular age combinations and wave sources of ovulatory follicles within ewes, older follicles did not have worse outcomes than younger follicles. Lactating ewes have not responded to treatments for the induction of estrus that are effective in dry ewes during the anestrous season. In experiment 2, lactating anestrous ewes 26 to 71 days postpartum were treated with either 25 or 50 mg progesterone/day for five days before ram introduction (RI). Twelve hours after RI, lambs were removed from half of the ewes in each group for 72 hours. Estrous response was recorded and ewes were scanned by transrectal ultrasonography on days 8 and 35 to 45 to determine ovulation rate, and pregnancy rate and number of embryos/fetuses, respectively. Number of lambs born was recorded at lambing. No effect of lamb removal was found for any variable examined. However, the lower dosage of progesterone reduced the mean time to estrus, increased the proportion of ewes ovulating, and decreased pregnancy loss. Overall, temporary lamb removal did not improve the response of lactating ewes to induction of estrus in the anestrous season. Whether ultrasonography itself causes embryonic or fetal loss was addressed in experiment 3. Ten flocks bred in either the estrous or anestrous season were randomized into control or examined groups within flock. Examined ewes were placed in a tilting squeeze chute and scanned for pregnancy by transrectal ultrasonography once between days 25 and 100 post-breeding. Control ewes were not subjected to handling in the squeeze chute or pregnancy diagnosis. Lambing rate and numbers of lambs born were recorded for all ewes. Examined ewes had similar pregnancy and lambing rates to control ewes and slightly greater prolificacy in the same flocks. Based on these data, the combination of transrectal ultrasonography with restraint is safe for pregnancy diagnosis in ewes. Cumulatively, these experiments help to address the question of what factors are associated with embryonic and fetal losses in ewes and how sheep producers may increase the number of lambs born per ewe by increasing either ovulation rate or the number of pregnancies per ewe per year.