Although fertilization rate usually is very high when male fertility is normal, pregnancy rates are below expectations when defined by the birth of live offspring in response to first service. Factors that affect establishment and retention of pregnancy include 1) preovulatory influences on the follicle and oocyte, 2) early postovulatory uterine and luteal function, 3) concentrations of hormones associated with trophoblastic and endometrial function during maternal recognition of pregnancy, and 4) less-well understood factors during the peri-attachment period. For example, decreased progesterone during preovulatory follicular development leads to a persistent follicle, premature resumption of meiosis, and a high incidence of embryonic death between the 2- and 16-cell stages. Elevated PGF(2alpha) during d 4 to 9 of the estrous cycle not only caused luteolysis but also had a direct embryotoxic effect during the morula-to-blastocyst transition. Ideal conditions during placentation and attachment are not clearly defined. Late embryonic mortality might be increased after ovulation of persistent or immature follicles. Nominal increases in secretion of PGF(2alpha) between d 30 and 35 might be important for attachment and placentation. Lower survival of embryos from wk 5 to wk 7 to 9 of gestation in the cow was associated with lower circulating concentrations of progesterone on wk 5. To maximize embryonic survival in the cow, management must provide high progesterone before estrus, quality detection of estrus, and timely insemination. Luteolytic influences of estradiol-17beta or PGF(2alpha) must be minimized early after mating and during maternal recognition of pregnancy, and high progesterone is needed during the late embryonic/early fetal period.

Preovulatory, postovulatory, and postmaternal recognition effects of concentrations of progesterone on embryonic survival in the cow

Low doses of progestogens in regimens for synchronization of estrus result in persistent follicles (PF) and reduced conception rates in cattle. The effects of PF on recovery of oocytes/embryos, fertilization, and embryonic development were examined in beef cows. To produce PF, cows received a used Controlled Internal Drug Releasing device on Days 4 through 13 after pretreatment estrus (Day 0) and two injections of 20 mg prostaglandin (PG) F2 alpha, i.m., 12 h apart on Day 6 (PF; n = 20). Control cows received only PGF2 alpha on Day 6 so they would ovulate growing follicles (GF; n = 22). With this model, the largest (dominant) follicle from the first wave of follicular development was being ovulated in cows in each group. Cows were observed for estrus twice daily and were then mated to highly fertile bulls. Follicular development was monitored by ultrasonography on alternate days from Day 2 until estrus and then daily until apparent ovulation. Jugular blood samples were collected every other day from Day 2 through 6 days after mating at posttreatment estrus, when uteri were flushed nonsurgically. The ovulatory follicle, which was followed retrospectively and had been observed longer (14 +/- 1 vs. 8 +/- 1 days), was larger in PF (17.2 +/- 0.5 mm) than in GF (14.5 +/- 0.6 mm; p < 0.01). Mean estradiol-17 beta (E2) in jugular plasma during the 6 days before estrus was higher in PF than in GF (5.7 +/- 0.4 vs. 2.6 +/- 0.4 pg/ml; p < 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)

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Effect of persistent follicles on early embryonic losses in beef cows.

The development of ovarian follicular and luteal structures during the estrous cycle and early pregnancy in ewes was examined. Ewes were treated with prostaglandin F2 alpha to induce estrus; they were placed with either two fertile or two vasectomized rams and observed for estrus twice daily. Beginning at estrus (Day 0), ovaries were scanned for CL and for follicles > or = 2 mm by transrectal ultrasonography daily for the first 25 days of pregnancy (PREG, n = 17 ewes) or until Day 8 of the second estrous cycle (CYC, n = 15 ewes). Jugular blood samples were collected at ultrasonography and on Days 30, 35, and 40 of gestation for RIA of progesterone and estradiol-17 beta. Total follicles (17.6 +/- 1.3), number of follicles > or = 4 mm in diameter (11.3 +/- 0.6), and number of follicles that became the largest during the first 16 days of pregnancy or the first estrous cycle (4.9 +/- 0.3) did not differ between PREG and CYC ewes. Number of newly detected follicles each day and difference in mean diameter between the largest two follicles and all other follicles were similar between PREG and CYC ewes until luteal regression. At that time, difference in diameter between the largest two follicles and all other follicles increased in CYC ewes in parallel with estradiol-17 beta in serum. Mean concentrations of progesterone in serum were higher in PREG than in CYC ewes (2.4 +/- 0.1 vs. 2.0 +/- 0.1 ng/ml; p < 0.05) before onset of luteal regression, but mean areas of CL did not differ.(ABSTRACT TRUNCATED AT 250 WORDS)

Ovarian structures during the estrous cycle and early pregnancy in ewes.

Factors affecting retention of early pregnancy in dairy cattle

Studies in cattle assessing changes in number and size of antral follicles, concentrations of estradiol, androgens and progesterone in serum and follicular fluid, and numbers of gonadotropin receptors per follicle during repetitive estrous cycles and postpartum anestrus are reviewed. The rate of growth of small follicles (1 to 3 mm) into larger follicles increases as the estrous cycle progresses from d 1 to 18 (d 0 = estrus). Size of the largest antral follicle present on the ovary also increases with advancement of the estrous cycle. Most large follicles (greater than 10 mm) persist on the ovarian surface for 5 d or more between d 3 and 13 of the bovine estrous cycle. After d 13, most of these large follicles are replaced more frequently by new growing follicles (turnover) with an increased probability for recruitment of the ovulatory follicle after d 18. More research is needed to determine the time required for growth of bovine follicles from small to large antral size and evoke recruitment of the ovulatory follicle. Factors that regulate selection of the ovulatory follicle are unknown but may involve increased frequency of LH pulses in blood, altered blood flow and(or) changes in intrafollicular steroids and proteins. Quantitative evaluation of ovarian follicles indicated occurrence of consistent short-term changes in fluid estradiol and numbers of luteinizing hormone receptors in cells of large follicles only during the pre-ovulatory period. Presumably, low concentrations of follicular estradiol found during most of the estrous cycle are not due to a lack of aromatizable precursor or follicle-stimulating hormone receptors. Follicular fluid concentrations of progesterone increase only near the time of ovulation. Little is known about changes in follicular growth, turnover and function during postpartum anestrus in cattle. However, preliminary data suggest that the steroidogenic capacity of large follicles changes markedly during the postpartum period.

Ovarian follicular growth, function and turnover in cattle: a review.

Seventy-three multiparous beef cows (21 to 35 d postpartum) were used in an experiment with a 2 X 4 factorial design to test the effects of pretreatment with norgestomet and preovulatory gonadotropic stimulation on the life span of corpora lutea induced by human chorionic gonadotropin (hCG) Cows were implanted subcutaneously for 9 d with norgestomet (3 mg; 35 cows) or sham-implanted (38 cows) HCG (1,000 IU) was administered in to all cows 36 h (time = 0) after implant removal Each cow received one of the following treatments: (1) pregnant mare's serum gonadotropin (PMSG; 750 IU) in a single subcutaneous injection 48 h prior to hCG (-48 h); (2) follicle stimulation hormone (FHS-P) in saline (10 mg) in a series of four subcutaneous injections (25 mg/injection) at -48, -36, -24 and -12 h; (3) FSH-P in 5% beeswax-sesame oil (10 mg) administered as in 2; or (4) saline, administered as in 2 Samples of serum taken at the time of implantation, at the time of implant removal and daily on d 5 to 20 after administration of hCG were assayed for progesterone Samples taken at 12-h intervals from -48 to +12 h were assayed for estradiol-17 beta Cows were observed for estrus at 6-h intervals from 24 to 60 h after implant removal and twice daily until d 22 Seventy-one percent of the cows (25/35) were in standing estrus from 29 to 60 h after removal of a norgestomet implant, compared to 16% of the sham-implanted cows (six of 38) As determined by progesterone in serum and by palpation of the ovaries, 88% of the cows (64/73) formed corpora lutea in response to hCG, with a tendency (P less than or equal to 10) for a positive effect of pretreatment with norgestomet (94 vs 82%) The life span of corpora lutea was longer (P less than or equal to 01) in cows treated with norgestomet (196 d) than in sham-implanted cows (134 d) Level of function of the induced corpora lutea as indicated by the pattern of daily concentrations of progesterone in serum was increased by pretreatment with either norgestomet (P less than 01) or gonadotropin (P less than 05), but there was no interaction of these treatments The mean level of estrogen before ovulation was increased (P less than 01) by pretreatment with norgestomet or any regimen of gonadotropin, and, again, there was no significant interaction Pretreatment with progestogen was associated with a higher pattern of concentration of estrogen over time (P less than 01)

C. E. Sheffel

Papers

Induced corpora lutea in the postpartum beef cow. II. Effects of treatment with progestogen and gonadotropins.