A study on the effect of production season on incidence of milk fat depression metabolic disorder in Iranian Holsteins cows using logistic analysis

To study the effect of production season on probability of milk fat depression (MFD), a number of 938,047 test day records belonging to 108,077 first-parity Holstein cows belonging 427 herds and calved during 1995-2009 were used. Logistic generalized linear model using GLIMMIX procedure of SAS software was fit to the data. In the model, fixed effects of production season in four levels (spring, summer, autumn and winter), sperm type (internal and external) used for dam insemination, first calving age, herd production level, animal genotype (grade and pure Holstein), calving year, stage of lactation as well as random effect of animal’s sire were included. Based upon fat to protein ratio for each test day record and with respect to threshold point of -0.12, dependent variable was defined as a binary (zero code for lack of MFD and unit code for incidence of MFD) character. The results indicated that all factors included in the model (except sperm type) had significant effects on probability of MFD. Estimates of odds ratio (OR) for spring season was found to be 0.832, 0.835 and 1.248 relative to the summer, autumn and winter seasons, respectively, and for the summer season were 1.004 and 1.494 relative to the autumn and winter seasons, respectively, and for the autumn season was 1.493 relative to the winter season. MFD probability in the spring as compared to the summer and autumn seasons were lower by 16.8% and 16.5%, respectively while it was greater by 24.8% as compared to the winter season. MFD probability in summer season was greater than that of the autumn and winter seasons by 0.04% and 49.9%, respectively. OR estimate indicated that MFD probability was greater in the autumn season by 49.3% as compared to the winter season. With respect to a higher probability of MFD in summer season, more attention to roughage feeding is essential.