A study of the relationships between the time of ovulation and insemination on litter size in pig herds

The aim of this work was to determine the time of ovulation in 5 breeding herds and to compare these observations with litter size after A.I.. The experiment was carried out with 222 crossbred sows. The herds were organised with group farrowing every 3 weeks. One to three groups per herd were studied; each group included weaned sows, gilts and non-gestating sows (confirmed by ultrasonography). Œstrus detection was performed by a boar three times a day. Sows were inseminated at least twice (between two and five times with commercially available semen). Blood samples were collected twice daily from the 3rd day of weaning until the day following the end of œstrus . The time of ovulation was determined by the measurement of progesterone levels and was defined as the first time when the concentration increased by one standard deviation above basal levels. The onset of the LH peak was defined using the same principle. For each sow fertility (farrowing or absence of farrowing) and the number of piglets born were recorded. A total of 92% of sows exhibited œstrus over 7 days, fertility was 88% and the sows produced on average 12.4 ± 0.2 piglets (mean ± s.d.) at farrowing. The duration of œstrus , the interval between the beginning of œstrus and the time of ovulation, the onset of the LH surge were variable between herds. The interval between the beginning of œstrus and time of ovulation ranged from 6 to 88 h and for the onset of the LH surge from 23 h before to 32 h after the beginning of œstrus. The duration of œstrus and the time of ovulation varied according to the physiological state of the sow. The interval between weaning and the onset of œstrus had no influence on the duration of œstrus and on the time of ovulation (p=0.09 and p=0.28, respectively). Litter size increased on average when AI was performed near the time of ovulation (p=0.10).