RESUMO
Two studies were conducted to investigate the effect of selection on predicted breeding value (PBV) for milk production, and its associated metabolic and endocrine milieu, on follicular development and ovulation in the postpartum period. A total of 71 cattle from lines selected to differ in their PBV were used in two consecutive years. In Study 1 the first ovulation and commencement of normal luteal function occurred significantly earlier in low (L) line cows than high (H) line cows. In Study 2 average daily milk production during the study period did not differ (P>0.10) between cows in either the H (=31.0+/-1.5kg/day) or L (=30.2+/-1.7kg/day) PBV lines although, 305 days milk production was significantly different (P<0.01; H=6880+/-164kg versus L=5795+/-317kg). As in Study 1, first ovulation postpartum in Study 2 occurred earlier (P<0.01) in the L (day 19) versus the H line (day 28). Circulating concentrations of glucose and insulin were significantly lower, whilst concentrations of GH and BOHB were higher in cows from the high PBV line. No differences in gonadotrophin concentrations were seen between lines. Both changes in body weight and patterns of follicle development did not differ between lines. By day 15 postpartum all cows had follicles of all three-size categories (small, medium-sized and large). Small (P<0.07) and medium-sized follicle numbers increased (P<0.01) with day postpartum. However, the inclusion of predicted changes in body weight as a covariate in the analysis, demonstrated that changes in number of small and medium-sized follicles were associated with changes in body weight. In conclusion, selection on PBV for milk production is associated with a longer interval from parturition to first ovulation, independent of changes in follicular development, milk production and body weight implicating other factor(s) associated with genetic selection.
