Abstract: The study aimed to analyze the differences, correlations and changing rules of lactating performance of Chinese Holstein cows at different lactation periods and somatic cell levels by data analysis, so as to provide a theoretical basis for the feeding and management of dairy cows at different lactation periods, the control of milk somatic cell count and the improvement of milk quality. Based on a total of 285 045 production performance measurement reports (DHI, Dairy Herd Improvement) from 14 dairy farms in Beijing, selecting healthy and normal lactating cows, 4 lactation stages were divided according to 0-99 d, 100-199 d, 200-299 d and more than 300 d (including 300 d), the low SCC group (0-20×10⁴·mL^-1), medium SCC group (20×10⁴·mL^-1-50×10⁴·mL^-1), high SCC group (more than 50×10⁴·mL^-1) were divided. A total of 251 949 DHI reports were obtained from the raw data after pretreatment, and were analyzed by Duncan multiple comparison and Pearson correlation analysis. The results showed that: 1) There were significant differences in milk yield, milk fat percentage, milk protein percentage, F/P, lactose percentage, MUN and SCC of dairy cows in different lactation periods (P<0.01). Milk yield in different lactation periods had a significant negative correlation with milk fat percentage, milk protein percentage and SCC, and a significant positive correlation with lactose percentage, and a significant positive correlation between milk protein percentage and milk fat percentage (P<0.01). 2) There were significant differences in the days of lactation, milk yield, milk fat percentage, milk protein percentage, F/P and lactose percentage of dairy cows with different levels of SCC (P<0.01). The milk yield of dairy cows at low and high levels of SCC was significantly positively correlated with SCC (P<0.01), and the medium level of SCC was significantly negatively correlated (P<0.01). The milk yield at different levels of SCC was significantly negatively correlated with milk fat percentage and milk protein percentage (P<0.01), and significantly positively correlated with lactose percentage (P<0.01). 3) The curve of milk yield changing with the lactation days was: y=-3×10^-9x⁴+3×10^-6x³-1.5×10^-3x²+0.203 9×10^-1x+34.437(R²=0.975 9); The curve of lactose percentage changing with lactation days was: y =-1×10^-15x⁶+2×10^-12x⁵-2×10^-9x⁴+6×10^-7x³-1×10^-4x²+8.4×10^-3x+4.999 2(R²=0.984 7). The variation curve of SCC with lactation days was: y=-4×10^-12x⁵+6×10^-9x⁴-3×10^-6x³+8×10^-4x²-8.89×10^-2x+22.862(R²=0.782 9); The curve of milk protein percentage changing with lactation days was: y=4×10^-15x⁶-7×10^-12x⁵+5×10^-9x⁴-2×10^-6x³+3×10^-4x²-1.94×10^-2x+3.576(R²=0.943 7); The curve of milk fat percentage changing with lactation days was: y=-6×10^-13x⁵+9×10^-10x⁴-6×10^-7x³+2×10^-4x²-1.93×10^-2x+4.777 2(R²=0.975 5); The curve of F/P changing with lactation days was: y=2×10^-11x⁴-3×10^-8x³+2×10^-5x²-2.9×10^-3x+1.44(R²=0.863 6). The milk yield and milk composition of dairy cows with different lactation stages and SCC levels showed significant differences. The milk yield and lactose percentage of dairy cows with early lactation and low SCC level were the highest. With the increase of lactation days, the milk yield and lactose percentage showed the change rule of increasing first and then decreasing; SCC, milk fat percentage and milk protein percentage showed the change trend of decreasing first and then increasing, and the F/P showed the trend of decreasing and then gradually stabilizing. Grouping and planning the dairy cows reasonably according to the lactation stage and SCC level of the dairy cows, and optimizing the nutrition supply can achieve the optimal lactation performance of the dairy cows and improve the economic benefit of the pasture.

Key words: dairy cow, lactation period, somatic cell level, milk composition, correlation, change rule