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 pot ential, Charolais; and large size and high milk potential, Simmental) were monitored during their first two parities. Heifers from each bree d were allocated to one of two levels of annual energy intake relative to metabolic body weight (M-0.75) (mean daily intakes equivalent to 7 05 and 820 kJ/kg M-0.75) in a factorial design. In the 1st year 64 hei fers (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 s ervice 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 esta blished in late July of each year using a combination of oestrous sync hronization and artificial insemination. Blood samples were collected at monthly intervals from weaning in November until calving in A May; three times weekly from calving until oestrous synchronization (11 wee ks later); and fortnightly thereafter until weaning. For the annual le vels of dietary energy intake offered, two breeds (Welsh Black and Cha rolais) exhibited relatively high growth rates and had the longest pos t-partum anovulatory periods; al and Charolais cotes also had the poor est conception rates of all breeds. Relatively lean cows at calving (b ody condition score less than or equal to 2.5 units) were sensitive, i n terms of the duration of the anovulatory period, to live-weight loss during the early post-partum period, particularly when blood glucose levels were low, whereas relatively fat cows at calving (body conditio n score >2.5 units) were not. The data suggest: (i) that mechanisms co ntrolling the anabolic processes governing maternal growth are antagon istic towards those that control rc production; and (ii) the catabolis m of lean tissue rather than fat tissue during the early post-partum p eriod is also antagonistic towards the mechanisms that govern reproduc tive function in cows.