Logistic analysis of some factors affecting on incidence probability of milk fat depression in early lactation of Iranian Holstein cows

The objective of the present research was to apply logistic analysis for investigating some factors affecting incidence probability of milk fat depression (MFD) in early lactation of Iranian Holstein cows. The data set was comprised 80381 test day records (milk، fat and protein traits) belonging to 80254 first parity cows in 427 herds with calving year between 1995 and 2009. Based on fat to protein percentages for the first month of the lactation and also with respect to threshold point of -0. 12، dependent variable was defined as a dichotomous (zero code for lack of MFD and unit code for MFD) variable. A logistic mixed generalized linear model was fitted on the data using GLIMMIX procedure of SAS software. In the model، fixed effects of sperm type (local or exotic)، production season، first calving age، cow type (grade or pure Holstein) and random effect of animal’s sire were included. Cow type had no significant affect on MFD. Estimation of odds ratio for comparing different ages was found to be 1. 112 indicating that cows calving at lower age of 25 months are expected to show MFD by 11. 2 percent smore than the cows calving at the greater ages. The estimate of odds ratio for comparing of spring with summer، autumn and winter seasons were 0. 73، 0. 722 and 1. 182، respectively. The corresponding figures for summer as compared to autumn and winter seasons were 0. 987 and 1. 615، respectively، and autumn as compared to winter was found to be 1. 636. Daughters of the local bulls are expected to show MFD by 10. 8 percent more than that of the daughters of the foreign bulls.