| dc.description.abstract | Data from a purebred Jersey cow herd at Kenya
Agricultural Research Institute's Regional Research Centre,
Mtwapa were utilized to characterize its production and
reproductive performance. A total of 1489 performance
records collected from 421 daughters of 80 sires covering a
29 year period (1960-1988) were utilized in the study. The
data were analyzed by the least squares maximum likelihood
computer programme of Harvey (1987). Heritability was
estimated by paternal half-sib analyses, repeatability was
estimated from variance components, with cow as a random
effect, while phenotypic, genetic and environmental trends
of 305-d~y milk yield and calving interval were estimated by
best linear unbiased prediction (BLUP) procedures.
The least squares means (± S.E) for 305-day milk yield,
lactation length, butterfat percent, annual milk yield, age
at first calving, calving interval, number of services per
concept 10n, and length of productive 1ife expressed in years
and as number of lactations were: J983±15 kg, 293±1.5 d,
4.9±0.05%, 1692±20.4 kg, 918±7.3 d, 414±3.9 d, 2.1±0.07 and
3.2±0.17 years and 3.4±0.15 lactations, respectively. In
general, production level was lower while reproductive
performance was comparable to the performance of other
exotics raised in the tropics.
The repeatability and heritability estimates for 305-
day milk yield, lactation length, butterfat percent, annual
milk yield, calving interval and number of services per
conception were: 0.31±0.031 and 0.28±0.079, 0.21±0.031 and
0.12+0.056 0.25+0.047 and 0.30+0.113, 0.35+0.035 and
0.18±0.076, 0.22±0.035 and 0.10±0.064 and 0.13±0.054 and
0.10±0.089, respectively. These indicate that useful
genetic variability exists for production traits but not for
reproductive traits. Genetic correlations of 305-day milk
yield with lactation length, calving interval and annual
milk yield were positive (0.83+0.152, 0.39+0.289
0.91+0.049, respectively) while genetic and phenotypic
correlations between 305-day milk yield and butterfat
percent were virtually zero (-0.28+0.360 and -0.004,
respectively) . The phenotypic correlation between 305-day
milk yield and calving interval was low (-0.001) while that
of unadjusted milk yield with lactation length (0.49) and
annual milk yield (0.82) were , respectively, moderate and
high.
Among the systematic environmental factors, season had
significant (P<0.05) effect on calving interval and annual
milk yield, while parity had significant (P<O.05) effect on
production traits. There was significant (P<0.05) variation
due to year of birth on age at first calving and of year of
calving on all other traits. During the period of study,
there was no deterioration in age at first calving but for
the major production traits and calving interval, there was
a decline in performance which was particularly marked in
the latter period of the study. The estimates of genetic
and environmental trends for milk yield and calving interval
showed that there had been no changes in the genetic merit of
cows and that the major decline in performance was
attributable wholly to environmental factors. In the
absence of any systematic climatic change, the deterioration
in performance must have resulted from other environmental
factors such as nutritional level, disease control and
general management factors.
The high average replacement rate (103%) will have had
major repercussions on herd profitability and will have
limited replacement selection possibilities. Therefore, to
increase profitability and the scope for selection at herd
level, improvements in husbandry are required particularly
in the raising of calves (and specifically female calves).
The cause or causes of the short reproductive life (average
of 3.3 calvings)should also be investigated. | |
