Relationships of Current Bacteriological Status of the Mammary Gland to Daily Milk Yield and Composition

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Abstract
Monthly data from 29 herds during 2 years were collected to determine relationships of bacteriological status with milk yield and composition on the same day. Test day results for a cow were classified as “positive” if bacteria were isolated from one or more quarters, otherwise “negative”. Staphylococcus epidermidis was most frequently isolated, followed by Staphylococcus aureus. Bacteriological status on test day had no significant effect on yield or composition of SLB cows. However, SRB cows positive on test day had 0.4 kg less milk, 0.033 kg less lactose, 0.06% lower fat composition, and 0.05% lower lactose composition than negative cows. Recovery of coliforms was associated with greatest decline in yield (2.5 kg less milk and 0.14 kg less lactose in SLB). In SLB, a stage of lactation by bacteriological status interaction existed, with positive test days having higher production than negative test days during early lactation.