| dc.description.abstract | Data consisting of 4475 lactation records of Kenya Ayrshire cattle made from 1980 to
2005 were used to study the genetic and environmental effects on production and
reproductive traits. The data were analysed by the least square techniques of Proc GLM
of SAS@program. The overall means were 3009.8 ± 1098 kg for 305-d milk yield (MY),
487.5 ± 151.6 days for calving interval (CI), 39.4 ± 7.2 mo for age at first calving (AFC)
and 357.8 ± 171.4 days for lactation length (LL). Respective heritability estimates
obtained were 0.12 ± 0.05 and 0.044 ± 0.032, 0.091 ± 0.05 and 0.12 ± 0.04, while
repeatability estimates for 305-d MY, LL and CI were 0.35 ± 0.01, 0.38 ± 0.01 and 0.10
± 0.001 respectively.
The genetic trend in CI and AFC were -0.6 d/year and -0.01 mo/year respectively and
significant (P<O.OOl). The respective phenotypic trends were -0.35 d/yr and -0.31 mo/yr
and non-significant (P>0.05). Positive, and non-significant (P>0.05) phenotypic (11.7
kg/yr) and environmental (13.8 kg/yr) trends for 305-d MY were obtained. A negative
and significant (P<0.05) genetic trend of -2.1 kg/yr over the study period for 305m-d MY
was estimated, indicating a decline in milk yield.
The phenotypic correlations between AFC and CI, MY and CI and MY and AFC were
0.088, -0.008 and 0.024 respectively. The genetic correlation between CI and AFC was
0.57 suggesting that these traits could be controlled by similar genes. A positive genetic
correlation between AFC and MY in the first lactation obtained implied that cows with
high potential for MY in first lactation were older at first calving. However, low and
negative genetic correlation (-0.48) between AFC and MY in lifetime lactations obtained
indicate that cows calving at early ages had on the overall high lifetime MY. The
moderate negative genetic correlation between MY and CI (-0.48) indicate that high
producing cows had shorter calving intervals. | en |
