RELATIONSHIP OF SOMATIC CELL CONCENTRATION WITH MILK YIELD IN DAIRY COWS

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Abstract
The relationship between somatic cell count (SCC) and milk yield was studied using the data of 2181 composite milk samples collected once every 2 mo during 1982–1983 from 665 cows of the Ottawa herd of the National Cooperative Dairy Cattle Breeding Project. SCC was treated either as a continuous (log SCC) or a classification variable. The data, when SCC was treated as a continuous variable, were analyzed separately for the first and later lactations. The model contained the effects of genetic group, stage of lactation, season of calving, and log SCC as a covariable. As a classification variable SCC was partitioned into 10 classes and the data for the first and later lactation cows were combined. The model included the effects of genetic group, parity, stage of lactation, season of calving, and SCC classification. Lactation SCC was calculated as the average of actual individual SCC tests during the lactation. The model for the lactation milk yield included the effects of genetic group and parity, with log SCC and days in milk as covaribles within parity groups. The effects of genetic group, parity, stage of lactation, season of calving and SCC were significant (P < 0.01) for daily milk yield. Average daily milk yield loss was 0.5 kg and 0.7 kg for the first and later lactation cows, respectively, when the SCC increased from 200 × 103 to 400 × 103 cell mL−1. Lactation milk yield loss per unit increase in average log SCC was 74 kg in the first lactation and 88 kg for later lactations. These relationships were linear, suggesting that loss of milk yield per unit increase in SCC was greatest when SCC was low. Key words: Somatic cell count, milk yield