ChileanJAR

Data collected from five dairy farms belonging to the Instituto de Investigaciones Agropecuarias included in a research project to investigate the effect of subclinical mastitis on milk yield and milk composition were considered. Milk yield and somatic cell counts of 1,000 cows were monthly recorded. This work represents the first evaluation ofthe relationship between daily milk production and somatic cell count; to achieve this, the information obtained during three years of the project was considered. The data base was 18,247 observations ofwich 5,979 were for first lactation animal s and 12,268 for cows with two and more lactations. The statistical model used considered the somatic cells as a continue variable expressed as the natural logarithm of the cell counts (Ln SCC). R2 values indicate that this model would explain 34 and 45% ofthe yield variation in firts lactation and in older animals, respectively (P < 0.0001). In both animal groups, the correlation between daily production and LnSCC was -0.14 (P < 0.0001), showing a Iineartype relation between both variables. Accordingly the daily yield would be reduced in 0.61 kg (first lactation) and 0.73 kg (two and more lactations) for each unit in crease in the Ln SCC, wich represents a decrease of 3.4 and 3.7% respect to the corresponding real average of daily production. Considering a level of < 100,000 cell/ml as a reference cell count, the decrease in milk production would fluctuate aproximately between 4-18% (first lactation) and 4-19% (two and more lactations) for counts of 100,000-200,000 and >5,000,000 cellsl mi, bearing statistical significance from 200,000 cell/ml both for first lactation and older animals.