Association of body weight, loin longissimus dorsi and backfat with body condition score in dry and lactating holstein dairy cows

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 fi rst 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 fi fth 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 + LD 2 + BF 2 + exp ✕ LD + exp ✕ BF + exp ✕ LD 2 + exp BF 2 + period ✕ LD + period ✕ BF + period ✕ LD 2 + period ✕ BF2 + cow + error (model 2). A fi rst-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