In the literature persistency is mostty
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.