Traits for sire selection related to udder health and management.

Traits related to udder health were discussed: bacteriological measures, clinical frequency, and milk properties (somatic cell count, enzymes, etc.). Bacteriological testing of quarters maximizes information on infection but is impractical on a population scale. Clinical recording is an objective measure but fails to indicate subclinical cases and is not available in most countries. Cell counts are economical to obtain but are highly variable, difficult to interpret, and not sensitive indicators of subclinical infections. Several milk enzymes are possible indicators of tissue damage. Heritabilities are about .2 for cell counts and .1 for other measures, reflecting genetic variation in teat defenses, phagocytosis, or immune response. There appears to be genetic antagonism between cell count and milk yield (positive gentic correlation of .1 to .2) in contrast to the negative phenotypic relationship between milk yield and cell count. A program to select sires whose progeny have lowest cell counts should be formulated carefully; more needs to be known about interpretation of cell count as a defense mechanism. Heightened cell counts decrease the probability of infection upon subsequent challenge with pathogens. Is there heritable variation in cell count of cows before exposure? A combined criterion of cell count, lactose, and enzyme concentration should be studied.