In the literature persistency is mosty defined as lactation or milk yield persistency, and persistencies of fat and protein yield are seldom considered. Different persistency measures were described in the literature.
Three great tpes of measures can be defined: l) me^cures based on ratios between total, partial or other felds; 2) measures based on variation of testday lelds and 3) measures based on mathematical modets. Environmental
infuences on persistency are well described in the literaturg even if there are diferences between persistenry measures and population studied. The most important effects are milk yields and seasonal effects. The heritabilities of persistency measures reported in the literature range from under 0.05 to over 0.30.