Genomic Evaluations for Body Maintenance Requirements in Canadian Holsteins

Abstract

A genomic evaluation was developed for Body Maintenance Requirements (BMR) in Canadian Holsteins, with the first official publication in April 2023. The BMR index characterizes feed requirements for maintenance based on the metabolic body weight of the animal. Body weights of lactating cows recorded through feed advisory services in Quebec since 2002 are used in the genetic evaluation. Metabolic body weight (MBW), calculated as body weight^0.75, in first, second, and third lactations are analyzed in a three-trait linear animal model as separate but correlated traits with repeated records within a lactation. Genetic parameters were estimated by MC EM REML method using a subset of the data including 373 219 records from 195 198 cows. Heritabilities for MBW in first, second, and third lactation were 0.34, 0.43, and 0.47, respectively, and repeatabilities were 0.53, 0.61, and 0.64, respectively. Genetic correlations between different lactations were strong, ranging from 0.78 to 0.86. A Single-Step genomic evaluation was implemented using the MiX99 software. The April 2023 official evaluation run had records from 540 619 cows of which 28 263 were genotyped and a total of 47 967 genotyped animals in the model. The BMR index combines GEBVs for MBW in the three lactations at equal weightings. This index is published as a relative breeding value, with a mean of 100 and standard deviation of 5 for base bulls, where the sign is reversed such that higher values represent a lower MBW and thus lower body maintenance requirements. The average reliability of BMR for young, genotyped bulls was approximately 68%. Observed phenotypic and genetic trends demonstrated that animal size has been steadily increasing over time. The BMR evaluations can be considered by producers looking to reduce or maintain cow body size in their herd as another way to reduce feed costs.