A study on the Effects of some Environmental Factors on Milk Production Characteristics Estimated Based Upon Gompertz Nonlinear Function in Holstein Cows of Mashhad

In order to analyse the effects of some environmental factors on milk production characteristics of Holstein cows in dairy farms of Mashhad، a total of 46،420 test day milk records belonging to 5،323 first lactation cows calved between 1994 and 2007 were utilised. First، Gompertz non-linear function was fitted to test day milk records of individual cows using SAS software. Then، based upon the estimated parameters of the function، milk porduction characteristics including time to peak yield (TM)، milk yield at peak time (YM)، lactation milk yield (YL) and initial milk yield (Y0) were calculated. Analysis of the environmental factors for the traits was carried out through a set of linear mixed models. The fixed effects of herd، year، season of calving and sperm type were significant on all the traits. Holstein gene percentage had significant influence on YL، PT and Y0. Calving age had only significant affect on YM and Y0. The effect of days open was significant for YL، YM and TM. There were positive and significant annual phenotypic trends for YL (10. 27 kg)، YM (0. 43 kg) and TM (2. 84 d).