A total of 102,098 individual monthly-test-day records of Holstein cows from 189 herds of the state of Minas Gerais from January 1999 to December 2001 were used to evaluate the effects of environmental factors on the composition and somatic cell counts (CCSs) of milk. The CCSs were transformed to somatic cell scores (SCSs). Unadjusted means and standard deviations for milk yield, fat and protein percentages, and SCSs were, respectively, 22.87 and 7.35kg, 3.56 and 0.72%, 3.15 and 0.37%, 3.07 and 2.33 for first parity and 24.99 and 9.33kg, 3.57 and 0.76%, 3.14 and 0.38%, 4.23 and 2.3 for later parities. Least squares methodologies were used and the analyses were separated within lactation age group as first and later parities. Effects of herd-year, month of calving, days in milk, season of calving, age at calving and breed group were considered in order to explain total variations. In both data sets, except for protein percentage, for which days in milk was the most contributing effect for the variation, herd-year was responsible for most of the variation, followed by days in milk. The effect of herd-year was also the most important to explain the variation of SCSs, respectively, 19.0 and 34.6% for first and later parities, followed in importance by days in milk, respectively, 4.4 and 4.0% of the total variation. Milk fat and protein percentages were high early in the lactation, decreased to a minimum near 50 days followed by a steady increase to the end of the lactation while milk yield decreased. Fat and protein percentages were highest in winter and lowest in summer months. Seasonal variations of these components followed opposed trends of milk yield. The SCSs were high in the first 10 days of lactation decreasing to a minimum value respectively, from 50 to 60, and from 30 to 40 days for first and later parities and then increasing with similar rates to the end of the lactation. The variation of SCSs with month was similar for first and later parities. SCSs increased from February to June, and then decreased to a minimum in October. The increase of SCSs with cow age was curvilinear in first and linear in later lactations.
dairy cattle; Holstein; somatic cell; milk composition
