Annual energy intake and the metabolic and reproductive performance of beef cows differing in body size and milk potential.

The metabolic status and reproductive performance of four pure breeds of beef cow (small size and low milk potential, Aberdeen Angus; small size and high milk potential, Welsh Black; large size and low milk potential, Charolais; and large size and high milk potential, Simmental) were monitored during their first two parities. Heifers from each breed were allocated to one of two levels of annual energy intake relative to metabolic body weight (M075) (mean daily intakes equivalent to 705 and 820 kj/kg M0·75) in a factorial design. In the 1st year 64 heifers (eight per treatment) which had calved as a consequence of first or second service were selected for the experiment. In the 2nd year 40 of these animals (five per treatment) which held to first or second service remained on experiment. Animals were housed all year round and given diets designed to represent energy intakes while grazing during the summer and conserved feeding during the winter. Pregnancy was established in late July of each year using a combination of oestrous synchronization and artificial insemination. Blood samples were collected at monthly intervals from weaning in November until calving in May; three times weekly from calving until oestrous synchronization (11 weeks later); and fortnightly thereafter until weaning.For the annual levels of dietary energy intake offered, two breeds (Welsh Black and Charolais) exhibited relatively high growth rates and had the longest post-partum anovulatory periods; and Charolais cows also had the poorest conception rates of all breeds. Relatively lean cows at calving (body condition score 2·5 units) were not. The data suggest: (i) that mechanisms controlling the anabolic processes governing maternal growth are antagonistic towards those that control reproduction; and (ii) the catabolism of lean tissue rather than fat tissue during the early post-partum period is also antagonistic towards the mechanisms that govern reproductive function in cows.