Logistic analysis of lactation stage influence on probability of milk fat depression in Iranian Holstein cows

To evaluate the effect of lactation stage on milk fat depression (MFD) a total of 938,047 test day records belonging to 108,077 first parity Iranian Holstein cows calving between 1995 and 2009 and distributed in 427 herds were used. Bassed on fat% to protein% ratio for each test day record, and the threshold point of -0.12, dependent variable was defined (codes 0 and 1 for the non- incidence and incidence of MFD, respectively). The data were analysed with a logistic generalized linear mixed model using GLIMMIX procedure of SAS software. In the model, fixed effects of stage of lactation, production season, sperm type used for dam insemination, age at first calving, animal type (grade or pure Holstein) and calving year, as well as random effect of sire were included. Mean probability of MFD increased from the beginning (0.1989) towards the middle (0.3192) of the lactation and afterwards it decreased. The maximum odds ratio (1.313) was obtained for MFD at the fifth month of lactation as compared to the last lactation month. With respect to the results, it could be therefore concluded that more economic return may be obtained through paying more attention to the nutrition regime of dairy cows.