An observational study of growth rate and body weight variance partition for United Kingdom dairy calves from birth to 20 weeks of age.

This study estimates the birth weight and growth rate for UK dairy calves and partitions unexplained variability in the weight of calves aged up to 20 wk. Raising replacements is a major contributor to the sustainability of dairying, but existing industry targets may not be applicable and achievable on all farms. For hierarchical data, variance partition coefficients (VPC) describe the proportion of unexplained variance residing at each level of the hierarchy due to clustering and indicate the potential to change outcomes through influencing variables acting at each level. This information could be used to inform the allocation of resources for further investigation. Calves were weighed according to farm-specific protocols. The data set analyzed contained records between June 5, 2014, and February 28, 2020, from 28 veterinary practices servicing 139 farms with 19,708 calves up to 20 wk of age, from which there were 59,588 weight recordings. Calves were Holstein-Friesian females. Calf weight was described using a multivariable mixed linear model with fixed effects for age. Mean birth weight was 41 kg. Mean growth rate increased from 0.59 kg/d at 1 d of age to 0.87 kg/d after 138 d. Cumulative mean growth rate up to 138 d of age was 0.73 kg/d. Birth weight and growth rate estimates are comparable with those made previously but are more precise due to larger sample size. Calf growth rate varied between farms, meaning that VPC at the calf, farm, and veterinary practice levels depended on calf age. Most unexplained variation in the weight of calves aged 66 d and over was due to differences between farms. At birth and 130 d of age, VPC at the farm level was 0.02 and 0.77, respectively. In contrast, most variation in neonatal calf weight was due to differences between calves. At birth and 130 d of age, VPC at the calf level was 0.84 and 0.20, respectively. The 0.025 to 0.975 quantile coverage of cluster-specific mean calf birth weight for combinations of veterinary practice and farm was 34 to 49 kg. The 0.025 to 0.975 quantile coverage of cluster-specific cumulative mean calf growth rate for combinations of veterinary practice and farm was 0.56 to 1.00 kg/d. Understanding reasons for these differences should be the basis of research into optimal calf management strategies to define economic targets for specific circumstances.