Quantifying Variation in the Abdominal Volume and Morphology of Dairy Heifers Using Three-Dimensional Imaging.

Accurate evaluation of body growth is essential for heifer management, and three-dimensional (3D) imaging offers a promising approach for precise morphological assessment. This study aimed to quantify variations in the abdominal morphology of dairy heifers using 3D imaging and to evaluate the relationship between abdominal volume and body weight. Twenty-seven records from 19 Holstein heifers aged 3-18 months were imaged using a three-camera 3D system. The abdomen was subdivided into 12 anatomical subregions to assess regional width variations. Age-related changes were evaluated using linear regression models, and age-adjusted residuals from the models were analyzed using principal component analysis (PCA) to characterize morphological differences. Abdominal volume increased linearly with age and was strongly associated with body weight (R2 = 0.96). Despite similar abdominal volumes, substantial variation in regional morphology was observed. PCA of the age-adjusted residuals indicated that overall abdominal width, proportional differences between cranial and caudal regions, and dorsoventral distribution patterns were the primary contributors to morphological variations. These findings demonstrate that 3D-derived abdominal volume strongly reflects body weight in growing dairy heifers, while regional morphological analysis reveals pronounced animal-level variations, highlighting the potential of 3D imaging for characterizing developmental differences in dairy heifers.