Abstract: [Objective] By analyzing four body weights（BW,kg）in four phases of heifers'; life and milk production data from 2014 to 2024.The first objective was to analyze the changes of BW and average daily gain（ADG,kg/d） over the past decade,while the second was to examine the relationship between BW and ADG,as well as their correlation with primiparous milk yield. [Method] ADG calculation method 1：（weight at a designated phase-birth weight）÷age of days;ADG calculation method 2：（weight at designated phase-weight at previous phase）÷（weighing date at designated phase-weighing date at previous phase）,analyzed heifers'; growth and development status and their primiparous milking performance. [Result]（1）Weaning ADG increased from 0.80 kg/d in 2015 to 0.88 kg/d in 2023,first calving BW had increased,while the age at first calving had decreased.（2）There were significant differences in ADG at each phase.Weaning ADG ranged from 0.85 to 1.00 kg/d,maximum ADG was 0.95~1.10 kg/d which during transfer phase,first breeding ADG was 0.86~0.96 kg/d,and minimum ADG was 0.75~0.83 kg/d during first calving.（3）The larger first calving BW,the higher 305-d milk yield and peak milk yield.However,most cows'; first calving BW was concentrated in （550,600]kg（Group A） and （600,650]kg（Group B）（accounted for 70.5%,48.6% of them at the age of 660 d to 690 d）.（4）There was no significant difference in BW and ADG between Group A and Group B at weaning phase,but during the transfer and first breeding phase,the differences in BW gradually widened.First calving BW of Group B was about 45 kg higher than Group A';s.With the age increases,the difference in ADG between Group A and Group B became increasingly evident.First calving ADG of Group B was 0.06 kg/d higher than Group A's.（5）At the age of 660~690 d（22~23 m）,305-d milk yield of Group B was approximately 360 kg higher than Group A's,peak milk yield of Group B was approximately 1.3 kg higher;305-d milk yield of Group B was approximately 848 kg higher than Group C（500,550]kg,peak milk yield of Group B was approximately 3.3 kg higher. [Conclusion] To achieve first calving BW 600~650 kg（pre-calving BW 640~690 kg） for heifer at the age of 22~24 m,farm managers need to regularly monitor heifers'; BW and evaluate the diet to monitor their growth status and balance the relationships between age and BW.This will provide sufficient nutrient intake for heifer,to ensure maximizing their production potential under optimal developmental conditions.

Key words: heifer, body weight（BW）, average daily gain（ADG）, milk yield