Days from weaning until estrus is influenced by such factors as length of lactation, parity, season and nutritional level, but should range from 4 to 7 days. Follicular growth accelerates about 3 days before estrus and is influenced by FSH or follicle stimulating hormone released from the pituitary gland, located at the base of the brain. A maturing ovum is held within each follicle. Granulosa cells within the follicle secrete estrogen, a hormone, which among other things is responsible for the typical signs of estrus. Ovulation occurs about 40 hours after the onset of estrus, but this interval is variable.
Several factors can influence ovulation rate or number of ova shed.
The white or maternal breeds generally have a higher ovulation rate. Crossbred females generally have a higher ovulation rate than either of the parent breeds. Therefore, use of an intact or a vasectomized boar is an important part of a regular heat detection program. Mature boars are superior to young boars in stimulating this response. Increased nervous activity.