Estrus and ovulation relationships in beef cattle

A study involving 8o cows and heifers of the Angus and Hereford breeds was conducted in two separate years at the Knoxville Experiment Station of the University of Tennessee.

This study was conducted to determine the length of estrus and the time of ovulation in relationship to estrus and the effects certain environmental and genetic factors had on these phenomena.

The mean duration of 136 estrous periods was 9.51 ± .83 hours. Although parous females has a longer estrus than non parous cattle, the difference was not significant. It was also found that breed had no significant effect on the length of estrus. When both estrous periods of each trial were considered together, trial has no significant effect on length of estrus; but the length of the second estrous period was significantly (P < .05) shorter in Trial III than in Trial I or II.

The mean interval from beginning of estrus to time of ovulation was 24.8 ± 1.35 hours and the mean interval from end of estrus to time of ovulation was 15.5 ± 1.3 hours. Trial had a significant (P < .05) effect of time of ovulation. Trial represents the combined effects of differences in cattle, observers, season, and the effects of estrus synchronization in Trial III. The effect of breed and parity were confounded by an interaction between the two factors. Angus heifers had a significantly (P < .05) shorter interval from the end of estrus to time of ovulation than Hereford heifers or Angus cows, while Hereford cows had a significantly (P < .05) shorter interval than Angus cows or Hereford heifers.

Estrus occurred most frequently during the early morning and early afternoon hours, while ovulation occurred most frequently during the early afternoon and evening hours.