|Abstract or Summary
- The investigations reported are based on two studies; the first
is concerned with the selection practiced for performance in one inbred
Angus and three inbred Hereford lines of cattle, the realized
responses, levels of inbreeding and associated performance, and
heritabilities of the performance traits.
Selection differentials for all performance traits were positive
when the averages for both parents were considered. Selection
differentials computed from the sire side of the matings were all
positive and were higher than those on the dam side as a consequence
of greater selection intensity through the sires.
Automatic selection against inbreeding occurred on the sire
side in conjunction with selection for increased performance. Due
to the low selection intensity for females, selection was for increased
inbreeding on the dam side of the matings.
Inbreeding increased in all lines over a 12 year period. Performance increased early in the inbreeding program, then
levelled off and subsequently declined. Score improved in all lines.
Generally favorable response in the Angus line resulted from a lower
initial performance, a broader genetic base and more animals from
which to select.
Data from repeat matings showed that more variability in performance
existed during the preweaning than in the postweaning period.
Zero and low levels of inbreeding were associated with high
preweaning performance and low postweaning performance. At
higher stages of inbreeding, the reverse was true.
Heritability estimates differentiated between the highly heritable
traits, postweaning rate and economy of gain, and the lowly and
moderately heritable traits respectively, preweaning gain and score.
In the second study the performance data on calves resulting
from a complete diallel cross between the three Hereford lines involved
in the first study were analyzed. In addition, blood samples
were analyzed at 450 and 750 pounds for amino acid nitrogen, urea
nitrogen and creatinine.
Differences in general combining ability between lines were
found for postweaning rate and economy of gain. The Lionheart line
was the best general combiner, followed by the David and Prince
lines. Specific combining ability differences in rate of gain favored crosses between the more rapidly gaining Lionheart and David lines.
The Prince and David cross was the lowest performing cross among
the specific crosses. The analyses of differences in economy of gain
were similar to those for postweaning rate of gain, but there was
also a significant difference in favor of linecross over inbred calves.
This was probably due to the economy of gain contributed by the
Prince line to the linecross calves. A highly significant association
existed between postweaning rate and economy of gain, with rate of
gain accounting for 77 percent of the variation in economy of gain.
Higher scores were given to Lionheart x Prince calves than
calves resulting from crosses involving the lower scoring David line.
Linecross calves were scored higher than inbred calves, with the
Prince and David inbred calves contributing to the low scores of all
inbreds. Line of dam differences in score at 800 pounds were due to
the less rapidly gaining David calves during the preweaning period
scoring lower at 800 pounds.
When all inbred and linecross calves were considered, blood
creatinine was higher at 450 pounds in calves from David dams which
were inferior milkers, and lower in the calves at 750 pounds sired
by the David bulls, due to lower gaining inbred David calves. A
significant difference in blood amino acid nitrogen at 750 pounds was
due to the low content in calves sired by the Prince bulls, which is
possibly a function of economy of gain contributed by this line. Urea
nitrogen at 750 pounds and feed required per 100 pounds of gain were
negatively correlated. At 450 pounds, blood creatinine and amino
acid nitrogen were highly associated.