Social and breed effects on the expression of a PGF2alpha induced oestrus in beef cows.

Social organization and breed effects following PGF2alpha were studied in mature Angus, Brahman and Senepol cows allocated into two groups (each A = 5, B = 5 and S = 5). Variables including interval to oestrus onset (IEO), oestrous duration (DE), total mounts received (TMR), and oestrous intensity (IE) were derived via HeatWatch. Breed-type influenced IEO (B = 42.6 +/- 6.7 h; S = 54.6 +/- 6.0 h; and A = 27.8 +/- 5.8 h; p < 0.003). Within breeds, dominant B (69.4 +/- 13.3 h) and S (65.5 +/- 7.4 h) cows were slower (p < 0.05) to be detected in oestrus than subordinate (38.1 +/- 4.4 h) and intermediate (40.6 +/- 6.0 h). However, within A, dominant cows (16.4 +/- 12.5 h) were detected in oestrus earlier (p < 0.05) than intermediate (44.3 +/- 9.2 h) and subordinates (32.7 +/- 5.1 h). Angus (21.5 +/- 2.4 h) and B (22.1 +/- 3.0 h) cows had longer (p < 0.01) DE than S (9.1 +/- 2.8 h). Dominants (20.4 +/- 3.0) and intermediates (20.2 +/- 2.3 h) cows had longer DE (p < 0.04) than subordinates (12.1 +/- 2.1 h) although the interaction breed x social order showed that dominant S had shorter DE than dominant A and B (10.1 +/- 3.3; 34.8 +/- 6.0 h; and 20.0 +/- 6.4 h, respectively; p < 0.001). Angus cows had less TMR than B (p < 0.02) and tended to be less than S cows (p < 0.06). Overall, greatest (p < 0.008) IE occurred in the first 9 h after onset of oestrus with no breed effect (p > 0.05). Dominant cows tended (p < 0.10) to have less TMR (3.2 +/- 0.7 mounts) than subordinate (4.1 +/- 0.4 mounts) and intermediate (4.7 +/- 0.6 mounts) throughout, especially 3-6 h after oestrus onset (p < 0.07). Breed and social order both influence PGF2alpha-induced oestrus behaviour.