ESTROUS EXPRESSION IN DAIRY-COWS AND HEIFERS (BOS-TAURUS) FOLLOWING REPEATED PGF(2-ALPHA) INJECTION AND CHOICE OF SELECTING A MOUNTING PARTNER

Research findings on the behavioral patterns of animals in estrus have indicated a strong relationship between social structure of the herd and estrous expression. It was therefore of interest to know if the li aisons established during a previous heat are maintained through subse quent estrous periods, particularly with respect to the choice of moun ting partners. Fourteen Holstein cows and ten heifers were synchronize d on six occasions. Every 11 days, each animal received an i.m. inject ion of 25 mg PGF(2 alpha), except for the fifth and sixth trials, when only half of the animals were treated. Sexual behavior of both groups was observed continuously from 40 to 100 h following injection. The ' consistency value' representing the repeatability of mounting behavior during estrus was higher in cows than in heifers (0.18 +/- 0.1 vs. 0. 13 +/- 0.01; P < 0.05). Cows in estrus formed a greater number of sexu ally active groups (3.5 +/- 1.2 vs. 2.0 +/- 0.6; P < 0.05), with fewer members (2.65 +/- 1.52 vs. 3.30 +/- 1.36; P < 0.05) than heifers. The length of the estrous periods were 17.6 +/- 1.96 h in cows and 20.6 /- 2.54 h in heifers (P < 0.05). Mounting behavior was equally distrib uted between day and night, Sexual activities showed marked contrasts between groups and among trials. It was concluded that cows form more stable social groups than heifers, as shown by a greater consistency i n their choice of mounting partners.