Influence of postpartum weight and body condition change on duration of anestrus by undernourished suckled beef heifers.

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.