Nutrition and folliculogenesis in post-partum beef cattle
The cumulative effects of different levels of annual energy intake over the first two parities on two breeds of beef cow (Aberdeen Angus: small size and low milk potential and Simmental: large size and high milk potential) were assessed for productive and reproductive traits during the early post-partum period. Heifers from each breed were allocated to one of two levels of annual energy intake (mean daily intakes equivalent to 705 and 820 kJ/kg M0.75 metabolisable energy) in a 2 (genotype) x 2 (feeding level) x 4 animals (replicates) factorial design each year. Animals were housed and individually fed throughout the year. The diets were designed to represent energy intakes while grazing during the summer and conserved feeding during the winter. Body condition, live weight, milk yield, blood metabolite and gonadotrophin concentrations, and follicular development were measured during the post-partum period. LH release was measured (15 min intervals for 8 h) in relation to dominant follicle development. Simmental cows gave higher milk yields and lost more body condition score during the post-partum period. Low body condition scores at calving and high losses of body condition during the post-partum period were associated with decreased LH pulse frequency and size of dominant follicle, and longer intervals from calving to first ovulation. LH pulse frequency increased towards ovulation but mean LH concentration and LH pulse amplitude did not . LH pulse frequency appeared to be higher during the dominance phase although this observation was not statistically significant. LH pulse frequency was lower and dominant follicle size smaller during the second than first parity. In conclusion, the data indicate that, during the early post-partum period, reproductive efficiency of Simmental cows appears to be lower than Aberdeen Angus cows when offered similar amounts of metabolisable energy per kg metabolic live weight.