Relationships between milk yield, milk composition and electrical conductivity in dairy cattle

摘要

The purpose of this study was to investigate the relationships between electrical conductivity (EC) and milk yield and composition in Japanese dairy cattle. In total, 4380 quarters of 106 cows were used in this study. Individual cows were herd testedin the evening with a quarter-milker fortnightly on at least 6 occasions during the lactation. Herd test records were collected from 1990 to 1998. Each quarter was sampled for milk yield, concentrations of fat, protein, lactose and solids-not-fat (SNF) and EC. Each milk quarter sample was classified into one of 9 classes of EC value and one of 10 classes of quarter difference EC value (QdEC). The QdEC for each quarter was calculated as the difference between the EC value for that quarter and the EC value of the quarter with the lowest value within the same cow, on a given herd-test day. Statistical analyses were performed using a mixed linear model for repeated measures considering the fixed effects of quarter position, lactation stage and their interaction, parity, calving season and the EC or QdEC class, and random effects of cow and quarter within cow. Previous studies have recommended criteria for healthy quarters of < 6.3 mS/cm for EC and < 0.3 mS/cm for QdEC. In the present study, quarters withvalues above these criteria did have significantly lower milk yields (by 15 to 20%), but these decreases became apparent above values for EC and QdEC of 5.4 mS/cm and 0.2 mS/cm respectively, much lower than the suggested criteria. Milk yield was reducedby approximately 2% for every 0.1 mS/cm increase in the QdEC. The higher value classes for EC, and for QdEC both showed significantly lower concentrations of fat, lactose and SNF (P < 0.05). In the present study, decreases in milk yield and concentrationof fat and lactose occurred in quarters that would have been considered to be normal by the previously accepted criteria for EC and QdEC.