FACTORS AFFECTING HERD MILK-COMPOSITION AND MILK PLASMIN AT 4 LEVELS OF SOMATIC-CELL COUNTS

A longitudinal study was carried out on samples of bulk tank milk coll ected from 200 farms for 12 mo to evaluate SCC, plasmin and plasminoge n activities, psychrotrophic bacteria count, and protein quality in mi lk and to examine the proteolytic effects of plasmin on milk proteins. Herds were selected randomly from client lists of two dairies to crea te four groups based on milk SCC of the month before the study; herds were reassigned monthly to one of four groups based on SCC for that mo nth. Overall means were 3.73% fat, 3.13% protein by infrared analysis, 3.16% protein by Kjeldahl analysis, 2.42% casein percentage, 4.65% la ctose, 5.43 cells/ml of log SCC, 1.13 U of plasmin, 45.6 U of plasmino gen, and log 2.86 cfu/ml of psychrotrophic bacteria. The ANOVA showed a significant effect of month on all factors except SCC, which was fix ed by the experimental design. Plasmin and plasminogen activities were high from December to May. Plasmin activities and psychrotroph counts were significantly higher for the high SCC group. Casein percentage a nd number were significantly higher for the low SCC group.