The lactation cycle of the dairy cow induces large changes in body fat and protein pools, which can be monitored through loin backfat (BF) and longissimus dorsi (LD) measurements. Data from two experiments (exp) using Holstein-Friesian dairy cows (no. = 40 and 32 respectively) were used to study the association of body weight (BW), BF and LD depth with body condition score (BCS) for the last 6 weeks of the dry period (DP) and the first 8 weeks of lactation. Loin and tail BCS were manually assessed (0 to 5 scale) and BF and LD depth were measured by ultrasound at the fifth lumbar process. The BCS data ranged from 1.3 to 3.0 units in the DP, and from about 1.1 to 3.1 units during early lactation in both experiments. Data were analysed by two models: BW, LD or BF = exp + period (DP or lactation) + BCS + interactions + cow + error (model 1); and BCS = exp + period + LD + BF + LD2 + BF2 + exp × LD + exp × BF + exp × LD2 + exp BF2 + period × LD + period × BF + period × LD2 + period × BF2 BF2 + cow + error (model 2).
A first-order autoregressive (AR(1)) covariance structure was employed for the error terms to account for the correlation among repeated measures within cow. Regressions of BW and LD on BCS (model 1) found pre- to post-calving differences (P < 0.001) in intercept for BW and LD, and slope coefficients of 35 (DP) and 21 (lactation) kg BW, and 5.8 mm LD per BCS unit. Regression of BF on BCS (model 1) showed an exp × period interaction (P < 0.001), with 0.4 mm BF (exp 1; P < 0.05) and 2.0 mm BF (exp 2; P < 0.001) per BCS unit. Regression of BCS on LD and BF (model 2) showed intercepts not equal to 0 (P < 0.06), and differences (P < 0.001) between DP and lactation; BCS increased (P < 0.001) by 0.027 units per mm BF and 0.05 units per mm LD, but LD had a quadratic term −0.0004 (P = 0.02). It is concluded that at BCS lower than 3, LD contributes to BCS following a quadratic function, whereas BF causes BCS to increase linearly. Each unit of BCS equated to about 35 and 20 kg BW for DP and lactation periods respectively, to 5.8 mm LD, and to between 0.4 and 2.0 mm BF.