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

Abstract

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 an incredible work ethic ...

Effect of service sire on late embryonic and fetal loss in sheep

“…Oocyte quality and persistent follicles. Higher fertilization failure and embryonic mortality were observed in cattle and sheep when estrus was synchronized, especially with low dosages of progesterone (Lunstra and Christenson, 1981;Ahmad et al, 1995;Inskeep, 1995;Santos et al, 2004;Wurst, 2007). Synchronization protocols that induced estrus after allowing the dominant follicle to continue to grow might compromise the oocyte ovulated from a persistent follicle (Lunstra and Christenson, 1981;Ahmad et al, 1995;Santos et al, 2004).…”

“…Exposure of the oocyte to high peak frequency of LH induced premature resumption of meiosis (Revah and Butler, 1996;Mihm et al, 1999), and the biochemical and morphological changes in the oocyte in persistent follicles led to reduced fertility in cattle (Mihm et al, 1994;Inskeep, 1995) due to embryo mortality before the 16-cell stage (Ahmad et al, 1995). In contrast, Wurst (2007) did not find age of the oocyte from penultimate as compared to final follicular wave as a factor of embryonic or fetal losses in sheep.…”

“…If tissue density changes over time, it can be detected by pixel analysis (Pierson and Adams, 1995). This approach can provide more information about developmental changes in organs such as the ovary and uterus (Dixon et al, 2007;Wurst, 2007). However, good quality images are required for successful image analysis.…”

“…Ultrasonography use is well described for evaluation of the ovary and uterus in female sheep, cattle and horses (Buckrell et al, 1986(Buckrell et al, , 1988Pierson et al, 1988;Gearhart et al, 1988;Schrick and Inskeep, 1993). Ultrasonography is noninvasive and does not subject the animal to injury (Buckrell, 1988;Padilla-Rivas et al, 2005;Wurst, 2007). Its use in pregnancy determination was initially in the early fetal stage for diagnosis of pregnancy and the number of viable fetuses (Lindahl, 1971;Deas, 1977;Horvarth et al, 1978;Schrick and Inskeep, 1993).…”

“…Low progesterone portends loss, but the conceptus usually dies before luteal regression. Low concentrations of progesterone lead to excessive concentrations of other hormones that may cause embryonic death (Stronge et al, 2005;Diskin et al, 1989Diskin et al, , 2006Wurst, 2007).…”