Functional traits related to costs are currently of interest for selection and management of dairy cattle. The present study was aimed to estimate heritability for body condition score (BCS) and heart girth (HG), to investigate the genetic relationships between BCS, HG and milk-yield traits using a test-day model and to analyse the consistency of the estimates in different lactation stages. Cows from 25 dairy herds were scored for BCS and measured for HG at 3-month intervals for 2 years. Approximately 5000 test-day observations on BCS, HG and milk fat and protein yield from 1429 Italian Friesian cows were analysed using two approaches: (1) repeated observations were treated as repeated measurements of the same trait, both within and across lactations; (2) observations collected in different stages of lactation (dry period, 1 to 75 days in milk (DIM), 76 to 130 DIM, 131 to 210 DIM, 211 to 300 DIM) were treated as different traits. (Co)variance components and related parameters were estimated using REML multiple-trait procedures and unequal design animal models.
Heritability estimates (approach 1) for fat and protein test-day yield, BCS and HG were 0.22, 0.18, 0.29 and 0.33, respectively. BCS was negatively correlated with yield traits (-0.43 and -0.48 for fat and protein yield, respectively) but positively correlated (0.33) with HG. Genetic relationships between HG and milk-yield traits were negligible. Heritability estimates (approach 2) were 0.28 and 0.27 for BCS recorded in the first half of lactation (1 to 75 and 76 to 130 DIM, respectively), 0.36 for BCS measured on cows in the second half of lactation and 0.32 for BCS recorded on dry cows. Heritability estimates for HG in different lactation stages ranged from 0.31 to 0.40. Genetic correlations between BCS measured in different lactation stages were generally high (0.85 or more), with the exception of the correlation between the first and the last stage of lactation (0.74) and of the relationships between the beginning of lactation and the dry period (0.7). Genetic correlations between HG measured in different lactation stages were mostly higher than 0.80.