Responsiveness to boar stimuli and change in vulvar reddening in relation to ovulation in weaned sows

In 117 weaned sows, changes in estrous behavior and vulvar reddening were related to timing of ovulation. Detection of estrus was performed every 8 h with four levels of boar stimuli to record the change in responsiveness to these stimuli. This resulted in four overlapping phases of estrus, during which a standing response could be evoked: 1) man estrus (standing response to a back pressure test, in the absence of a boar), 2) spontaneous estrus (standing response in the presence of a boar, no back pressure test), 3) boar estrus (standing response to boar + back pressure test), and 4) detection-mating-area estrus (back pressure test in the presence of four boars). In addition to the detection of estrus, the change in reddening of the inner vulvar mucosa was recorded. Manifestation of estrus in response to the four stimuli occurred in 46, 56, 90, and 97% of the sows, respectively. Onset of the four phases occurred 24 h (SD 13 h), 23 h (SD 15 h), 34 h (SD 13 h), and 41 h (SD 12 h) before ovulation. The duration of the intervals between the various phases of estrus explained 10 to 50% of the variation in the timing of ovulation relative to the onset of the phases. However, these intervals could not be calculated for all sows because estrus was not expressed at every stimulus level by each sow. The end of vulvar reddening occurred, on average, 21 h (SD 14 h) before ovulation. Except for five sows that ceased to show vulvar reddening within 5 h after ovulation, the end of vulvar reddening occurred before ovulation, within a 70-h range. Of the sows showing boar estrus, 90% also showed vulvar reddening. For sows that showed vulvar reddening until after the onset of boar estrus (two-thirds of the sows), the end of reddening occurred within a much smaller range: from 36 h before, until 2 h after, ovulation. Onset of estrus, regardless at which stimulus level it is detected, appears too variable relative to timing of ovulation to be used as a predictor for ovulation. Duration of the different stages of responsiveness explains only some of this variation and cannot be obtained on all sows. Combining information on vulvar reddening and boar estrus can predict ovulation within a reasonable range for two-thirds of the sows.