Journal of Dairy Science, Vol 77, Issue 3 883-890, Copyright ©
1994 by American Dairy Science Association
Effect of sampling status and adjustment for heterogeneous variance on bias
in bull evaluations
R. L. Powell, G. R. Wiggans and H. D. Norman
Animal Improvement Programs Laboratory, Agricultural Research Service,
USDA, Beltsville, MD 20705-2350.
Holstein bulls were assigned to sampling categories (AI stud, AI
nonstud, or non-AI) based on bull code, controller number, and age at
semen distribution. The AI stud bulls were sampled through
traditional progeny-testing programs of 13 AI organizations; AI
nonstud bulls had AI semen collection reported by another
organization or by multiple organizations. The non-AI bulls had no
reported AI semen collection. Actual daughter yield deviations for
these three groups of bulls were compared with expected performance
(parent average) to provide an indication of whether evaluations were
free from bias for daughter yield deviations. Mean difference of
daughter yield deviation from parent average was close to 0 kg for
animal model evaluations of all 22,930 bulls but was positively
biased by 46 kg of milk for AI nonstud bulls. Mean PTA and
reliabilities for parents were highest for AI stud bulls and lowest
for non-AI bulls. The AI stud bulls varied least and were
intermediate for mean management, approximated as mean daughter yield
minus bull PTA. Management was highest for AI nonstud bulls, which
suggested that adjustment for heterogeneous variance might reduce
bias. However, the effect of this adjustment on mean difference of
daughter yield deviation from parent average was small.
