Effect of preovulatory estradiol and(or) postovulatory progesterone on the establishment of pregnancy in beef cattle
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Preovulatory secretion of estradiol and postovulatory secretion of progesterone are essential for preparing the uterine environment for pregnancy in cattle. In a previous study, circulating concentrations of preovulatory estradiol (E2) at gonadotropin releasing hormone (GnRH)-induced ovulation (day [d] 0) and postovulatory progesterone (P4) on d 7 were the two most important factors affecting establishment of pregnancy in postpartum beef cows (Atkins et al., 2013). Therefore, two studies were conducted to further examine the effect of circulating concentration of estradiol at GnRH-induced ovulation (d 0; Chapter 3) and the effect of supplementing progesterone (d 4 to d 11) on pregnancy rate in postpartum beef cows (Chapter 4). Experiment (Exp) 1, ovulation was synchronized in cows (n = 1,015) as follows: on d -9 GnRH and an intravaginal progesterone implant (CIDR) were administered, on d -2 the CIDR was removed and prostaglandin F[subscript 2a] (PGF) injected, and on d 0 GnRH was administered. Estrus detection patches (Estrotect) were placed on cows on d -2, and any cow with an activated patch on d 0 was considered to have exhibited estrus and removed from the study (n = 424). For the remaining cows (n = 591), blood samples were collected on d -2 and 0, and plasma analyzed for estradiol by radioimmunoassay (RIA). Recipient cows were classified based on their d 0 (mean [plus or minus] SEM) concentration of estradiol as follows: Low (2.45 [plus or minus] 0.05pg/mla; n = 227), Medium (3.97 [plus or minus] 0.06pg/ml[b]; n = 156), or High (5.81 [plus or minus] 0.05pg/ml[c]; n = 208) estradiol groups ([superscript abc]P < 0.0001). Each cow received an in vivo produced embryo on d 7 and pregnancy diagnosis occurred between d 29 and 34. Pregnancy rate on d 30 to 34 for the Low, Medium, and High estradiol groups was 32%a, 46%b, and 52% [ab]P< 0.001). In Exp 2, cows were assigned to High or Low estradiol on d 0 as in Exp 1, and then were randomly assigned to Low or Normal P4 treatment groups. Final treatment groups were as follows: Low E2-Low P4 (LL; n = 71), Low E2-Normal P4 (LN; n = 69), High E2-Low P4 (HL; n = 74), and High E2-Normal P4 (HN; n =73). Cows that were assigned to the Low P4 group received three injections of PGF on d 3, 3.5, and 4, which reduced circulating concentrations of P4 on d 7 in HL compared to HN, and LL compared to LN treatment groups. Pregnancy diagnosis was performed on d 34 via ultrasonography, pregnancy rates for LL, LN, HL, and HN were 22%[superscript a], 30%[superscript a], 47%[superscript b], and 58%[superscript b] (P < 0.001), respectively. In the experiment described in Chapter 4, ovulation was synchronized as described for the preceding experiment; however, cows (n = 246) were artificially inseminated to a single bull at 66 hr following PGF-induced luteolysis and CIDR removal. Cows were allotted to a control (n = 125) or treatment (n = 121) group by breed, age, body condition score, and days postpartum. Treatment included insertion of a CIDR on d 4 and removal on d 11 to increase circulating concentrations of progesterone. Pregnancy rate on d 43 to 49 for the control and CIDR groups were 68% and 71%, respectively (P = 0.87). There was a main effect of estrous expression on pregnancy rate with cows in estrus by the time of FTAI having a higher (P = 0.07) pregnancy rate. Within the CIDR treatment group, cows that expressed estrus had a higher pregnancy rate to FTAI than CIDR treated cows that did not express estrus, 75% and 57%, respectively (P < 0.05). However, there was no difference (P = 0.64) in pregnancy rate to FTAI for cows that did or did not express estrus in the control group. In summary, elevated preovulatory estradiol on d 0, and increased change in estradiol concentration from d-2 to 0, increased pregnancy rate after embryo transfer; however, decreasing circulating postovulatory concentrations of progesterone had no effect of pregnancy rate. Furthermore, supplemental progesterone (CIDR from d 4 to 11) had no effect on pregnancy rate in beef cows.
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