Effect of pretreatment with bovine somatotropin (bST) and/or gonadotropin-releasing hormone (GnRH) on conception rate of dairy cows with ovarian cysts subjected to synchronization of ovulation and timed insemination.

The objective of this study was to determine the effect of pretreatment with bovine somatotropin (bST) and/or gonadotropin-releasing hormone (GnRH) 7 days prior to initiation of a protocol for synchronization of ovulation and timed insemination (Ovsynch) on conception rate (CR) of cows with ovarian cysts. A total of 254 lactating dairy cows with ovarian cysts was divided into four groups (Day 0). On Day 0, cows in Group 1 (n = 61) were pretreated with 500 mg bST, s.q., and 100 microg GnRH, i.m.; cows in Group 2 (n = 73) were pretreated with 100 microg GnRH, i.m.; cows in Group 3 (n = 59) were pretreated with 500 mg bST, s.q.; and cows in Group 4 (n = 61) received no pretreatment. All cows were subjected to the Ovsynch protocol 7 days later. All cows previously received routine bST treatment every 14 days until milk production decreased to a minimum level established by the management of the herd. CR was assessed using logistic regression after adjusting for timing of concurrent bST treatment relative to Day 0, parity, season at time of insemination, and days in milk (DIM) on Day 0. CR for cows in Group 3 (12%) was significantly lower (P < 0.05) than that for cows in Group 4 (27%), and CR for cows in Group 1 (18%) and Group 2 (15%) tended to be lower (P < 0.10) than that for cows in Group 4 (27%). From the results of this study, it was concluded that bST pretreatment decreased CR, and pretreatment with GnRH, and GnRH with bST tended to decrease CR in lactating dairy cows with ovarian cysts concurrently treated with bST and subjected to the Ovsynch protocol.

The effect of interval from day of administration of bovine somatotropin (bST) to synchronization of ovulation and timed-insemination on conception rate of dairy cows with and without ovarian cysts.

The objective of this study is to evaluate the effect of the interval from the day of administration of bovine somatotropin (bST) to the day of initiation of synchronization of ovulation (Day 0 and 7) and timed-insemination (TAI) on conception rate (CR) of dairy cows with and without ovarian cysts, respectively. Lactating dairy cows (n = 359) were divided into two groups. Cows in Group 1 (n = 238, without ovarian cysts) were treated with 100 microg, i.m. GnRH on Day 0; 25 mg, i.m. PGF2a on Day 8. 100 mirog. i.m. GnRH on Day 10; and inseminated 16 h later without detection of estrus. Cows in Group 2 (n = 121, with ovarian cysts) were treated with 100 microg, i.m. GnRH on Day 0; 100 microg, i.m. GnRH on Day 7; 25 mg, i.m. PGF2alpha on Day 14, 100 ug, i.m. GnRH on Day 16; and inseminated 16 h later without detection of estrus. Between 60 and 63 days postpartum, all cows in the herd were given bST every 14 days for the duration of the study. Logistic regression was used to assess the risk of nonpregnancy associated with interval from bST treatment to Day 0 for cows without ovarian cysts. and both Day 0 and 7 for cows with ovarian cysts adjusting for parity and days in milk. The CR for cows in Group 1 was significantly higher when the interval from last treatment with bST to Day 0 was between 1 and 3 days (28%) compared to 4-6 days (14%). In addition, the risk of nonpregnancy was 2.19 times greater in cows 4-6 days after bST treatment compared to 1-3 days after adjusting for parity and days in milk. The CR for cows in Group 2 was not significantly different when the interval from last treatment with bST to both Day 0 and 7 was between 1 and 3, 4 and 6, and 7 and 14 days. In conclusion, the results of this study suggested bST treatment closer to Day 0 had a positive effect on CR of cows without ovarian cysts, but bST treatment closer to both Day 0 and 7 had no effect on CR of cows with ovarian cysts. This was interpreted to mean that bST had a beneficial effect on either, or both, the preovulatory follicle and the oocyte in dairy cows without ovarian cysts, but not in dairy cows with ovarian cysts.

Effect of constant light exposure on circulating gonadotrophin levels and hypothalamic gonadotrophin-releasing hormone (GnRH) content in the ovariectomized pony mare.

Melatonin is thought to play a role in relaying photic information to the central nervous system as part of the seasonal reproductive cycle of the mare. However, the mechanisms by which melatonin may act are unknown. Therefore, this study was designed to determine whether exposure to constant light would, by reducing circulating melatonin concentrations, have any effect on hypothalamic gonadotrophin-releasing hormone (GnRH) content and circulating levels of luteinizing hormone (LH) and follicle-stimulating hormone (FSH). Blood samples were collected for 12 h at 15-min intervals from 8 ovariectomized (OVX) pony mares under ambient light conditions (12 h light, 12 h dark, October 6). Animals were then placed under representative ambient light (12 h light, 12 h dark; control, n = 4) or constant light (24 h light, 0 h dark; treatment, n = 4) in light-controlled rooms. Blood samples were collected daily and on experimental Days 14, 21 and 28 samples were collected at 15-min intervals for 12 h for analysis of circulating LH and melatonin (Bleed 2, 3 and 4, respectively). All animals were killed on Day 28 (following Bleed 4) and the hypothalami were collected. Mares exposed to constant light had significantly higher (P less than 0.05) LH concentrations in daily blood samples and showed significantly (P less than 0.05) higher LH concentrations during frequent sampling periods on Days 14 and 21 (Bleeds 2 and 3) compared with control mares. FSH did not differ significantly among groups in the daily samples. GnRH content was 1.5994 +/- 0.325 and 0.9457 +/- 0.193 pg/mg protein (treatment and control respectively).(ABSTRACT TRUNCATED AT 250 WORDS)