per percentage under- or
overestimation of REV (r = 0.38 and 0.29, respectively) as a result of the
variance–covariance structure of traits. The effects on TEM of under- or
overestimating trait REVs were non-symmetrical. TEM was most sensitive
to incorrect REVs for protein, fat, milk and lifespan and least sensitive
to incorrect calving interval, lameness, non-return and mastitis
REVs. A guide to deciding which dairy traits require the most rigorous
analysis in the calculation of their REVs is given. Varying the REVs
within a fairly wide range resulted in different bulls being selected by
index and their differing predicted transmitting abilities would result in
the herds moving in different directions in the long term (20 years). It is
suggested that customized indices, where the farmer creates rankings of
bulls tailored to their specific farm circumstances, can be worthwhile.