RESUMO
Two experiments were conducted to determine the relationship between change in weight and body condition on postpartum interval of thin first-calf beef heifers. Twenty-nine (Exp. I) and 36 (Exp. II) Angus and Angus-sired crossbred heifers were limit-fed to reduce their condition score (BCS) to 4 (scale of 1 to 9) at calving. After calving, heifers were assigned to one of four daily ME intakes (range of 198 to 305 kcal ME/kg BW.75) designed to create a range of BCS changes. Each unit change in BCS was associated (R2 = .72; P < .0001) with a 33-kg weight change. Mean postpartum interval (PPI) from calving to initiation of luteal activity was 121 +/- 6.0 d. As dietary energy density increased, PPI decreased (P < .01). Condition score at calving (CCS) accounted for a greater proportion of the variation (R2 = .37) associated with PPI than did condition score change from calving to d 90 postpartum (CSCH90; R2 = .27) or weight change from calving to d 90 postpartum (WTCH90; R2 = .17). The "best fit" equation (R2 = .56) for predicting PPI included CCS and CSCH90: PPI = 244.5 - 28.28(CCS) - 23.07(CSCH90) + 6.36(CSCH90(2)). Maximum reduction in PPI occurred when heifers gained 1.8 units of BCS during the first 90 d of lactation. Gaining BCS at a faster rate was of no additional benefit.