T. Sanchez

Frequency of luteinizing hormone pulses and circulating 17β-oestradiol concentration in cows is related to concentration of progesterone in circulation when the progesterone comes from either an endogenous or exogenous source

Abstract The present study was designed to test whether progesterone from exogenous and endogenous sources modulates the frequency of luteinizing hormone (LH) pulses in a differential fashion. Our working hypotheses were: (1) the frequency of secretion of LH pulses and concentration of 17β-oestradiol in circulation would be directly related to circulating concentration of progesterone; (2) progesterone from an exogenous source would have a different effect on frequency of LH pulses and circulating concentration of 17β-oestradiol than progesterone from an endogenous source. Cows were treated with graded doses of PGF 2α or progesterone to result in varying concentrations of progesterone in circulation from endogenous and exogenous sources. Treatment with PGF 2α resulted in concentrations of circulating progesterone among animals in this group that ranged from 0.5 to 10.9 ng ml −1 of plasma. Treatment with progesterone resulted in circulating concentrations of progesterone among animals in this group that ranged from 0.8 to 11.8 ng ml −1 of plasma. Concentrations of progesterone in circulation and frequency of LH pulses were negatively correlated ( r = −0.59) in cows with functional corpora lutea and in cows treated with progesterone without corpora lutea ( r = −0.62). The frequency of LH pulses had the same relationship to the varying concentrations of progesterone in both treatment groups. Cows with higher progesterone levels had fewer LH pulses and cows with lower progesterone had a greater frequency of LH pulses. Mean circulating concentrations of 17β-oestradiol also had the same relationship with varying concentrations of progesterone in both treatment groups. Cows with higher progesterone levels had lower 17β-oestradiol and cows with lower progesterone had higher 17β-oestradiol levels. The results indicate that there is a direct relationship between circulating concentration of progesterone, frequency of LH pulses and circulating concentration of 17β-oestradiol. In addition, progesterone modulates the frequency of release of LH pulses from the pituitary and circulating concentration of 17β-oestradiol in a similar fashion when coming from either an endogenous or exogenous source.

Gonadotropin secretion and development of ovarian follicles during oestrous cycles in heifers treated with luteinizing hormone releasing hormone antagonist

The hypothesis tested was that reduced LHRH stimulation of the anterior pituitary would lead to attenuated development of ovarian follicles as a result of reduced gonadotropin secretion during oestrous cycles of cattle. Twenty heifers were randomly assigned to be treated ( n = 5/treatment) with an antagonist to LHRH (LHRH-Ant) 1) from Day 2 to 7 (Day 0 = behavioural oestrus), 2) Day 7 to 12, 3) Day 12 to 17, 4) or serve as untreated control animals. LHRH-Ant suppressed LH pulses of heifers in all treatment groups from treatment initiation through Day 17 as compared with untreated control heifers [Peters et al., 1994. Luteinizing hormone has a role in development of fully functional corpora lutea (CL) but is not required to maintain CL function in heifers. Biol. Reprod., 51 (1994) 1248-1254]. Circulating concentration of FSH from Day 8 to 10 of the oestrous cycle did not increase in heifers treated with LHRH-Ant from Day 2 to 7 or Day 7 to 12; however, there was increased (P < 0.05) FSH from Day 8 to 10 of the oestrous cycle in heifers treated with LHRH-Ant from Day 12 to 17 and control heifers. Compared with control heifers, heifers treated with LHRH-Ant from the Day 2 to 7 had suppressed (P < 0.05) size and persistence of the first and second dominant ovarian follicles. Heifers treated with LHRH-Ant from Day 7 to 12 had suppressed size (P < 0.05 and tended (P < 0.10) to have a shorter persistence of the second dominant ovarian follicle compared with control heifers. Heifers treated with LHRH-Ant from Day 12 to 17 had a similar (P > 0.10) size and persistence of dominant ovarian follicles but had reduced (P < 0.10) numbers of large follicles compared with control heifers. Heifers treated with LHRH-Ant from Day 2 to 7 had lower (P < 0.01) concentrations of 17 beta-oestradiol during the treatment period and tended (P < 0.10) to have lower concentrations of 17 beta-oestradiol from Day 7 to 12 of the oestrous cycle compared with control heifers. Heifers treated with LHRH-Ant from Day 7 to 12 or Day 12 to 17 had similar (P > 0.10) circulating LH concentrations of l7 beta-oestradiol compared with control heifers. Reduced LHRH stimulation of the pituitary from Day 2 to 12 of the oestrous cycle and the resulting modulation in circulating LH and FSH led to suppressed ovarian follicular development and oestradiol secretion. After Day 12 of the oestrous cycle, reduced LHRH stimulation of the anterior pituitary did not lead to altered ovarian follicular development to the extent as reduced LHRH stimulation before Day 12 although pulsatile release of LH was similarly suppressed by treatment with the LHRH-Ant.

Influence of fenceline bull exposure on duration of postpartum anoestrus and pregnancy rate in beef cows

Two experiments (1991 and 1992) were conducted to test the hypotheses that fenceline exposure of cows to bulls following parturition shortens duration of postpartum anoestrus and improves pregnancy rate to artificial insemination (AI). In Experiments 1 and 2, 75 and 125 postpartum crossbred beef cows, respectively, were stratified by calving date and while maintaining equivalent primiparous cows in each treatment group were randomly assigned to be exposed to bulls through the fenceline (BE) or not exposed to bulls (NE). In each experiment, twice weekly blood samples were collected for 13 weeks beginning 1 to 4 weeks postpartum. These samples were used to assess concentrations of progesterone and determine time of onset of ovarian luteal function following calving. In Experiment 2, a 23 day program of AI was included in the study in which signs of behavioral oestrus were detected twice daily and cows were artificially inseminated 12 h following detection of oestrus. Pregnancy rate to AI was determined by concentrations of progesterone, detection of pregnancy by rectal palpation, and confirmed by calving date. Primiparous cows in the BE group had shorter durations of postpartum anoestrus in each experiment than primiparous cows in the NE group (Experiment 1: BE = 78 days, NE = 92 days, P < 0.05; 2: BE = 109 days, NE = 117 days, P < 0.10). There was no influence of fenceline bull exposure on the duration of postpartum anoestrus of multiparous cows in either experiment. Pregnancy rate to AI in Experiment 2 was not improved by fenceline bull exposure. We conclude that fenceline bull exposure was effective in shortening length of postpartum anoestrus in primiparous cows but this improvement did not translate into improved pregnancy rates during the subsequent breeding season.

Concentrations of gonadotropins, estradiol and progesterone in sows selected on an index of ovulation rate and embryo survival

The objective of this study was to determine concentrations of follicle stimulating hormone (FSH), luteinizing hormone (LH), progesterone (P4) and 17beta-estradiol (E2) in sows from a line selected on an index which emphasized ovulation rate (Select) and from a control line. A further classification of the sows in each line was made according to the estimated number of ovulations during an estrous cycle. Sows in the Select line were ranked into a high (HI) or low group (LI) when their estimated number of ovulations were 25 or more and 14 to 15, respectively. Sows of the control line were classified into groups as high (HC) or low (LC) when the estimated values for ovulation rate were 14-15 and 8-9 ovulations, respectively. Blood samples were collected every 12 h during a complete estrous cycle and samples were analyzed for concentrations of FSH and LH. Samples collected every 24 h were assayed for P4 and E2. Mean concentrations of FSH, LH, P4 and E2 did not differ (P > 0.10) between lines or between HI and LI or HC and LC groups. Selection of pigs for ovulation rate and embryonal survival did not affect concentrations of FSH, LH, P4 and E2 in sows during the estrous cycle.

Development of persistent ovarian follicles during synchronization of estrus influences the superovulatory response to FSH treatment in cattle.

The synchronization of estrus with synthetic progestins or progesterone (P(4)) results in the development of a large, persistent ovarian follicle. The objectives of the present study were to determine if development of a persistent ovarian follicle during synchronization of estrus suppresses recruitment of additional follicles during FSH treatment. On Day 5 of the estrous cycle (estrus = Day 0), beef cows were treated with 0.5 or 2.0 P(4) releasing intravaginal devices (PRIDs) for 8 d (Experiment 1, n = 20), 5 or 2 d (Experiment 2, n = 44) before initiation of FSH treatment. Prostaglandin F(2alpha) (25 mg) was administered on Days 5 and 6. Superovulation was induced with 24 mg of recombinant bovine FSH (rbFSH, Experiment 1) or 28 mg of FSH-P (Experiment 2) over a 3- or 4-d period, respectively. The PRIDs were removed concurrently with the 5th injection of rbFSH or FSH-P. There was a treatment-by-day interaction (P < 0.001) for the concentration of 17beta-estradiol in cows treated for 8, 5 or 2 d before FSH treatment. In Experiment 1, FSH treatment initiated 8 d after insertion of a 0.5 PRID did not affect the number of CL (6.9 +/- 1.4 vs 6.7 +/- 1.6), ova/embryos (3.7 +/-1.3 vs 3.0 +/- 1.3) and transferable embryos (2.4 +/- 0.9 vs 3.0 +/- 0.9) compared with that of the 2.0 PRIDs. In Experiment 2, FSH treatment initiated 5 d after insertion of a 0.5 PRID decreased the number of CL (4.0 +/- 0.5 vs 8.3 +/- 0.8; P < 0.001), ova/embryos (3.0 +/- 0.6 vs 5.9 +/- 1.2; P < 0.03) and transferable embryos (2.3 +/- 0.6 vs 5.1 +/- 1.0; P < 0.03) compared with that of a 2.0 PRID, respectively. Initiation of FSH treatment 2 d after insertion of a 0.5 PRID compared with a 2.0 PRID had no affect on the number of CL (8.0 +/- 2.1 vs 8.7 +/- 1.2), total ova (4.8 +/- 1.4 vs 6.9 +/- 1.4) and transferable embryos (2.9 +/- 1.2 vs 6.1 +/- 1.7). In conclusion, treatment with low doses of P(4) (0.5 PRID) for 5 d but not for 2 or 8 d before initiation of FSH treatment results in the development of a dominant ovarian follicle, which reduces recruitment of ovarian follicles, and the number of CL, total ova and transferable embryos.