The study was designed to evaluate the effect of somatic cell count(SCC) on milk composition. In total, 127 dairy farms bulk milk wereused during 2007 and 2008. The farms located in east northern ofIran. Somatic cell count and milk composition were measured in thebulk milk. The SCC data was divided into five categories based on theCalifornia Mastitis Test (CMT) scores and these categories, rather thanthe continuous variable from which they were derived, were used forstudying the effect of SCC on milk composition. The range of leukocytelevels in different CMT scores were: N, 0-200000; T, 200000-400000; 1,400000-1200000; 2, 1200000-5000000; and 3, >5000000 per ml. Due to low number of records for the last score the data were excluded beforefurther analysis. The model for analyzing the effect of CMT scores onmilk composition also included year and season. The data were analyzedusing General Linear Models. The SCC averaged 590730 ± 12353 leukocytesper ml. The median was 454000 cells and 25 and 75% quartileswere 322000 and 713000 leukocytes per ml, respectively. The distributionanalysis showed that 44.3% of the data were between 250000 and499000 leukocytes per ml. The SCC was numerically reduced in year 2008 compare with 2007 which shows an improvement in milk quality(p=0.23; 616614±60316 and 575043±54308 cells/ml, respectivelyfor 2007 and 2008). The effect of season was significant and SCC wasgreatest in winter compare with summer and autumn (p<0.01). Milk fat(p<0.01; 3.46, 3.54, 3.56, and 3.63 ± 0.04%, respectively for N, T, 1, and2) and protein (p=0.04; 3.08, 3.06, 3.05, and 3.07 ± 0.01%, respectivelyfor N, T, 1, and 2) contents were affected by the CMT score groups. Theeffect of season was significant (p<0.01). Milk lactose and solids-notfat notfatcontents were similar among the groups. The results demonstratedthat milk fat and protein contents were affected by SCC. Results alsopresented a trend in milk quality improvement over the years.

Keywords