Fig. 1

Experimental design of heifers from birth to parturition. The heifers in the experiment were fed a TMR throughout the study period, concentrate up to 300 days of age, milk replacer up to 90 days of age (days to weaning), and hay from weaning to 300 days of age. Following the collection of fecal samples, correlation analysis and machine learning were conducted based on the bacterial population data. Subsequently, causal inference was performed. Artificial insemination (AI) during gestation period was performed only on heifers meeting the AI physique criteria: body weight ≥ 270 kg and body height ≥ 116 cm [10]