AbstractInformation from 186 herds enrolled in the Puerto Rico Dairy Herd Improvement Program (PRDHIP) was analyzed to evaluate the effect of milk so- matic ceil count on milk production in complete lactations. The data set used consisted of 9,507 lactations from June 1981 to August 1983. The average somatic cell count over the whole lactation was used either as the original count or as the average of the natural logarithm thereof. Simple correlation coefficients between average logarithm somatic cell counts and daily and 305 days' milk production indexes within lactation were low but significant (P < 0.01), -0.04 and -0.03, respectively.The partial regression coefficient for the average logarithm of the somatic cell count on milk yield was highly significant (P < 0.01). The regression model included as independent variables the effects of herds, breed of cow, year of calving, month of calving nested within year, number of days dry, number of days open and number of days in milk. Decrease of 93 ± 10 kg of milk per lactation was found with each unit increase in the average natural logarithm of the somatic cell counts. This result suggests a higher loss in milk production per unit increase in the average natural logarithm at lower cell counts.The variability of the somatic cell counts within lactation did not significantly affect the milk production in 305 days.
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