Abstract: The purpose of this study was to estimate the correction coefficient and parity adjustment coefficient of standardized 305-day milk yield corresponding to days in milk (DIM) in different regions, by analyzing the test-day data from scaled dairy farms.Wood model (Incomplete Gamma Function) was used to fit the lactation curves of 683 160 test-day milk yield records from 11 749 Holstein dairy cows from 9 reference population in different temperature zones in China from 2010 to 2020.The parameters of lactation curve were estimated, and the correction coefficients corresponding to the lactation days from 1 to 305 days of primiparous and multiparous cows were calculated.The differences between the standardized 305-day milk yield and actual accumulated milk yield at 305 days of reference group and validation group were analyzed respectively.Mixed linear model in SAS 9.2 was used to estimate the parity effects for cows with at least the first five lactations standardized 305-day milk yield of 1-6 parities, and the multiplicative adjustment coefficients of 1-6 parities were calculated.The difference between the conventional adjustment coefficient and new adjustment coefficient were compared.The results showed that:①For primiparous and multiparous cows of reference population, the goodness of fit (R²) ranges of lactation curve equation were 0.4593 to 0.4913 and 0.5796 to 0.6341, respectively.The peak days of lactation were 79 to 85 and 53 d, the peak milk yield was 33.1 to 34.4 and 46.0 to 48.6 kg, respectively.②In the reference population, the adjustment coefficients were basically consistent when DIM more than 90 d of primiparous and DIM more than 30 d of multiparous cows.Standardized 305-day milk yield for DIM more than 60 d was close to the actual accumulated 305-day milk yield, the difference of milk yield was lower than 100 and 200 kg, for primiparous and multiparous cows, respectively.③For the standardized 305-day milk yield of validation population, adjustment coefficients of DIM more than 150 d of primiparous and DIM more than 180 d of multiparous cows yielded better fitness, the ratio of standardized 305-day milk yield and actual accumulated 305-day milk yield was higher than 79%.④Adjustment coefficients of 1-6 parities were 1.2121, 1.0380, 1.0063, 1.0000, 1.0220 and 1.0290, respectively.⑤Compared with the conventional adjustment coefficient, new adjustment coefficient of DIM derived from current study suited better to adjust the lactation effect, difference between 305-day milk yield standardized by new adjustment coefficient and the actual accumulated 305-day milk yield of primiparous and multiparous cows were within 900 and 700 kg, respectively.Using the new adjustment coefficient, milk production of 2-4 parity cows could be adjusted to 5th lactation to mature equivalent more accurately.The results indicated that updating the adjustment coefficients of DIM and parity for standardized 305-day milk yield regularly was necessary, milk yield of lactating cows with different DIM and parities could be accurately adjusted to the same benchmark.Thus, the comparison of milk performance among individual dairy cows would be more efficient and provided fair reference for management in dairy farm.

Key words: dairy cow; Wood model; days in milk; standardized 305-day milk yield; adjustment coefficient