Characterisation of pure bred Jerseys in the Coastal sub-humid zone of Kenya
Njubi, D M
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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.