Abstract: [Objective] To investigate the changes in progesterone and milk composition traits in dual-purpose cattle during estrus, reveal their regulatory mechanisms, and provide a scientific basis for optimizing breeding management.[Method] Sixty-five healthy dual-purpose cattle were selected. Progesterone concentrations during the estrous cycle were measured using enzyme-linked immunosorbent assay （ELISA）,while milk fat rate,milk protein rate, and lactose content were determined with a fully automatic milk composition analyzer.The associations between progesterone and milk components were analyzed via linear mixed models （LMMs）, multiple regression, and analysis of covariance （ANCOVA）, with parity and lactation stage evaluated as regulatory factors.[Result] Progesterone exhibited significant dynamic changes during estrus, peaking at 48 hours pre-estrus and sharply declining on the day of estrus. Milk fat and protein rates fluctuated synchronously, showing strong positive correlations with progesterone （r>0.30,p< 0.05）.Lactose content varied minimally, with no significant association to progesterone （r < 0.1,p> 0.05）. The progesterone-milk fat correlation was 23.60% higher in second-parity cows than in multiparous cows, with the strongest correlation observed during mid-lactation（r=0.42,p< 0.01）.[Conclusion] Progesterone significantly influences milk fat and protein synthesis by regulating mammary gland metabolic activity, with effects modulated by parity and lactation stage. These findings provided a critical time window and scientific basis for targeted nutritional interventions and reproductive management.

Key words: dual-purpose cattle, progesterone dynamics, milk composition, estrus period