| dc.description.abstract | Production and reproduction records of Kenyan Friesian cows
obtained at the Kenya Milk Records (KMR) , covering the period 1968 -
1984 , were used to study heterogeneity of variance of 305 - day milk
yield and to develop parity and lactation length adjustment factors.
Two different methods of multiplicative correction factors and linear
regression were used to standardize milk yield to 305 day equivalent.
Coefficients of variation and standard deviations across herds were
used to test heterogeneity of variance. A fixed effect least squares
model was used to analyse the contribution of fixed effects to
heterogeneity of variance of milk yield, while lactation length and
parity adjustment factors were developed by ratio method.
The least squares means for 305M, 305L - day and annual milk
yields were 2740.3 ± 10.37 kg, 2813.2 ± 11.08 kg and
2722.8 ± 12.88 kg respectively. Significant heterogeneity of variance
of milk yield was found to exist between herds. Year of calving,
herd, herd class , parity , calving interval and lactation length
affected milk yields significantly and contributed to heterogeneity
of variance. Season of calving did not influence milk yield. Age
within parity affected 305 day milk yields significantly but not
annual milk yield. Average herd standard deviation and coefficient of
variation after correcting for the fixed effects were 672.2 Kg and
26.7% respectively, while scaled standard deviations between herds
were 180 kg and 3.4% . Use of parity adjustment factors reduced the
CV of 305M day milk yield from 24% to 20%, while the amount of
variation accounted for by parity reduced from 82% to 23% .
It was concluded that heterogeneity of variance of milk yield _
should be accounted for in sire and cow evaluation. The scaling of
observations within individual herds by sample standard deviation
minimises heterogeneity of variance and increases accuracy of
selection. Adjustment factors for lactation length and parity were
proposed as possible means of using all lactation records in sire and
cow evaluation in Kenya. | en |
