The effect of bacterial infection on the milk yield of the individual quarters of the cow's udder

A study has been made of the effect of udder infections on the proportionate yield and composition of the milk of individual quarters of over 100 cows, and the data from eleven of these cows have been presented. Typical examples have been given showing that subclinical Streptococcus and haemolytic Staphylococcus infections may persist for months and either cause a gradual reduction in the proportionate yield, and solids-not-fat percentage of a quarter or have no measurable effect, and that both types of reaction may occur simultaneously in the same udder. Examples also show that following penicillin treatment of clinical cases caused by streptococci and staphylococci there is little recovery in the proportionate yield of the treated quarter until the following lactation, when recovery may be complete.