The aim of the present study was to evaluate the control of ovulation by the administration of human chorionic gonadotropin (hCG) or gonadotropin-releasing hormone (GnRH) at the onset of estrus. Thirty-three multiparous sows housed under tropical conditions and showing standing estrus within 5 days after weaning were included. The sows were allocated to three groups, spontaneous ovulation (control group, n = 10), induced ovulation using 750 IU hCG (hCG group, n = 10), and induced ovulation using 50 μg GnRH (GnRH group, n = 13). The hormones were given at the onset of estrus and the occurrence of ovulation was monitored every 6 h by transrectal ultrasonography. Data for weaning-to-estrus interval, onset of estrus-to-ovulation interval (EOI), and the length of estrus were recorded. All sows in the control and hCG groups ovulated, while 3 out of 13 sows treated with GnRH developed cystic ovaries (did not ovulate). Of those sows ovulating, the EOI of the hCG (40.2 ± 1.7 h) and GnRH (37.5 ± 3.3 h) groups were shorter than that of the control group (63.6 ± 9.6 h; P < 0.05). In conclusion, the administration of either hCG or GnRH at the onset of estrus can control time of ovulation but, at the dose employed, sows receiving GnRH may develop ovarian cysts.

The reproductive performance of sows is influenced by numerous factors, including environment. The time of ovulation in response to hormonal treatment might vary depending on climate. Because control of ovulation in pigs using hormonal treatments has not been studied in the tropical climate, the effect of hCG or GnRH analog (buserelin) on the time of ovulation in weaned sows was investigated. Thirty-three multiparous Landrace (L; n = 14) and Yorkshire (Y; n = 19) sows housed in the mating and gestation unit on a commercial swine farm in Thailand were used. The average of maximum and minimum daily temperature and daily humidity during experiment was 35°C and 25°C and 31%, respectively. Estrus detection by back pressure test and presence of a mature boar was performed every 6 h. Only sows with weaning to estrus interval (WOI) of ≤7 days were studied. Estrous sows were randomly allocated to 3 groups: a control group (4 Y and 6 L), which was given no treatment; a group (5 Y and 5 L) given 750 IU of hCG i.m. at the beginning of estrus (hCG group); and a group (5 Y and 8 L) given 10 μg of GnRH analog i.m. at the beginning of estrus (GnRH group). The mean time of ovulation was monitored by transrectal ultrasonography every 6 h from the onset of estrus. Interval from onset of estrus to mean time of ovulation (EOI) was analyzed using general linear model procedures of SAS (version 9.0; SAS Institute, Cary, NC, USA). Least squares means and standard deviations of EOI were compared using ANOVA. The proportion of sows ovulating within 45 h after onset of estrus was compared using Fisher’s exact test. Differences with P < 0.05 were considered significant. All of the sows in the control and hCG groups ovulated within 5 days after onset of estrus, but 3 out of 13 (23%) sows in the GnRH group developed cystic follicles; these 3 sows were excluded from the analyses. Overall, WOI was 3.8 ± 0.9 days and did not differ among the groups (P ± 0.05). The breed of sow had no effect on the EOI (P ± 0.05). Although the EOI did not differ among the control (43.0 ± 19.2 h), hCG (40.2 ± 5.5 h), and GnRH (37.5 ± 10.3 h) groups (P ± 0.05), variation was less (P = 0.001) after administration of hCG or GnRH. In addition, the proportions of sows that ovulated within 45 h after onset of estrus in the hCG (9/10 sows) and GnRH (8/10 sows) groups tended to be higher than in the control group (5/10 sows; P = 0.10). In conclusion, results indicate that both hCG and GnRH are efficacious in inducing ovulation at a predictable time in weaned, spontaneously estrous sows. The occurrence of follicular cysts in GnRH-treated sows requires further investigation.