Objective  To identify the effects of season change, parity and lactation stage on milk performance traits of Holstein dairy cows in South China.

Method  Dairy herd improvement (DHI) records of 10 450 Holstein cows were collected from a large dairy farm in Guangzhou throughout 2015. Information of milk performance traits including milk yield, milk fat percentage, milk protein percentage and fat-to-protein rate were extracted. The effects of season, parity and lactation stage on milk performance traits were evaluated. The correlations among factors and traits were analyzed.

Result  Season, parity and lactation stage all had highly significant effects on both milk yield and milk nutrient contents (P<0.001). Summer milk yield was highly significant lower compared with the other seasons (P<0.01). All indexes of milk constituents reached the lowest in summer and were highly significantly lower than those in spring or winter (P<0.01), and the spring values were significantly higher than winter values(P<0.01). Milk yield and milk protein percentage were the highest in December, but not in January which was the coldest month of the year. Peak milk yields of first-parity cows reached during the third month of lactation, but those of the other parity cows reached during the second month of lactation with higher peak values compared to first-parity cows. However, the decreasing rate of milk yield was slower in first-parity cows than in the other parity cows. Milk yield increased gradually from first to second parity, and then decreased with rising parity. For milk constituents, all indexes reached the maximum at first-parity, decreased gradually with rising parity and then increased from fourth to fifth parity. During lactation, fat-to-protein rate was higher at an early stage and lower at a later stage, and there was highly significant positive correlation between milk fat and protein percentages(r=0.63, P<0.01).

Conclusion  Lactation cows of South China are subjected not only to heat stress in summer, but also to cold stress in winter, which restricts the milk performance traits. High parity (above third-parity) cows account for a low percentage of the studied population, resulting in a low value of annual milk parity. Low dietary nutrition and feeding level are strongly restricting the production performance at a later lactation stage for high parity cows.