Time of onset of estrus in gilts

An experiment was conducted to determine when, during a 24-h period, gilts show the onset of behavioral estrus. Beginning on Day 16 of their first estrous cycle, 42 crossbred gilts were observed at 0600, 1200, 1800, and 2400 h for the onset of their second estrus. Fifty-five percent of the gilts had shown onset of estrus by 0600 h. None of the gilts showed the onset of estrus from 0600 to 1200 h, whereas 24% and 21% of the gilts had shown onset of estrus by 1800 and 2400 h, respectively. Chi-square analysis demonstrated that more (P < 0.025) gilts had shown onset of estrus by 0600 than by 1200, 1800, and 2400 h. When the data were combined for estrous checks by 0600 and 1800 h, 76% of the gilts had their onset of estrus by 0600 h as compared to 24% of the gilts by 1800 h (P < 0.005). In conclusion, more gilts had shown onset of estrus by 0600 h than at any other 6-h period.