Livestock Research for Rural Development 19 (7) 2007 | Guide for preparation of papers | LRRD News | Citation of this paper |
Milk yield
data (n=120307) from116 Holstein-Friesian herds were
used to group herds into clusters and carry out genetic
characterization of the production environments in Kenya. Herds
were clustered based on herd mean 305-day milk yield, and herd
standard deviation of 305-day milk yield. Variance
components for the clusters were estimated by univariate animal
models using derivative free REML algorithm, and significance tests
were done using the F_{max} procedure.
Based on the descriptive variables, three production environments or clusters were identified. Phenotypic, additive genetic and residual variances varied across production levels: 1134608.1, 1513952, and 827057; 4144955, 503934, and 122837; and 1134608, 918189, and 661768, respectively for herd production environments 1, 2 and 3. The heritability estimates were 0.23 ± 0.04, 0.33 ± 0.04 and 0.14 ± 0.03, respectively. Differences in production environments are important and cause heterogeneous variances which should be accounted for in genetic evaluation for Holstein-Friesian in Kenya.
Key words: clusters, heterogeneity, Holstein-Friesian, milk yield, production environments, variance components
The dairy industry in Kenya is based on exotic breeds and their crosses with indigenous breeds. Bos taurus dairy cattle breeds such as Holstein-Friesian, Ayrshire, Guernsey, Jersey and crosses among themselves, and with Sahiwal or the East African Zebu are found in various agro-ecological zones where they are raised in different production systems. Over 76% of dairy cattle are raised under the smallholder production system while the rest are raised in production systems found on medium and large-scale farms (Peeler and Omore 1997). Smallholder production systems predominate where land sizes are small, while medium and large scale farms are common where land is not limiting.
The orientation of the breeding programme is towards increasing milk yield and both locally bred sires and semen from foreign bulls are used (Ojango 2000; Bebe et al 2002). In the genetic evaluation of locally bred sires, herds are fitted as fixed effects (Ojango 2000; Olukoye and Mosi 2002; Magothe et al 2006). Given the production systems, and the large number of herds, use of multiple trait models becomes increasingly computationally unfeasible. Inadequate genetic ties between herds can lead to erroneous covariance estimates. Variances of milk yield vary with the level of management and environment (Costa et al 2000) due to genotype by environment correlation and methods of feeding concentrates (Brotherstone and Hill 1986). Bias arises in genetic evaluations from differences in variation within herds, and may become more severe as intensity of selection increases (Vinson 1987). Where different production environments or herds exist (Olukoye and Mosi 2002), herds can be grouped according to management, climatic and genetic information (Naya et al 2002; Weigel and Rekaya 2000) to increase the precision of genetic parameter estimates in structural covariance models. Such clustering of herds can lead to borderless evaluation and even specific to each production system/environment or herd (Weigel and Rekaya 2000; Lohuis and Dekkers 1998).
Dairy cattle evaluation using Best Linear Unbiased Predictions (BLUP) requires appropriate variance components to provide solutions. Use of BLUP assumes independence of genetic and environmental variances from the mean and that they are homogenous across herds or environments, and that the genetic correlation between genetic values in different environmental variance groups is unity (Meyer 1998). Heteroscedasticity across production environments (Olukoye and Mosi 2002; Costa et al 2000; See 1998; Visscher et al 1991) reduces the accuracy of predicted breeding values relative to the population mean (De Mattos et al 2000; Verrier et al 1993) and can lead to favouring high performers from more variable herds over high performers from low-variance herds, causing a reduction in response to selection (Hill 1984). These biases in evaluations accumulate over time as dams and daughters tend to express records in the same herds or environments (Vinson 1987).
Non-genetic factors are important causes of heterogeneity of
variance at phenotypic level (Olukoye and Mosi 2002) in the
Holstein-Friesian population in Kenya. Response to selection is a
function of selection intensity, heritability and phenotypic
standard deviation (Falconer 1989), and therefore genetic variances
should be investigated for heteroscedasticity. Dairy cattle
evaluation using BLUP is just being implemented in Kenya (Magothe
et al 2006), and the usefulness of the evaluations will depend on
how well the assumptions of homogeneity of variance components
match the data. The objective of the study was to determine if
evidence exists for heterogeneity of variance of milk yield in
Holstein-Friesian population in Kenya using cluster analyses
305-day milk yield data was obtained from Dairy Recording Services of Kenya on herds participating in performance recording in Kenya. The data consisted of cows that calved between 1985 and 2005 and had completed the current lactation by the time of the analysis. Information in the data included pedigree of each cow, season and year of calving parity and herd.
Two variables were used to identify production environments. Mean 305-day milk yield (LMYD) and average standard deviation (SDLMYD) for each herd provided information about intensity of management on each farm.
The variables were defined as follows:
Standard deviation of milk
It provides a measure of production intensity in each herd. This parameter assumes that a more effective management elicits greater performance variability within a herd (Dong and Mao 1990; Naya et al 2002; Raffrenato et al 2003)
Average herd 305-day milk yield
This variable provides a measure of the intensity of feeding and general management. Lactations were extended to 305-day yield equivalents using Woods gamma function (Muasya 2005), where cows had dried off earlier or had not finished lactating.
Identification of the herd clusters was performed with cluster analysis techniques, using the variables defined above. The original data on the definitive variables were standardized to a mean of zero and a standard deviation of one using PROC STANDARD (SAS 2002). The resultant data was then subjected to hierarchical clustering under the PROC CLUSTER with the method of minimum variances within group of ward. Derivation of the appropriate number of clusters was based on the pseudo F statistic. After cleaning and editing, 12307 records were available for analyses.
Least square means and tests for statistical differences were carried out using PROC GLM of SAS computer programme (2002) using the following fixed effects model.
_{ } = 305-day milk yield
_{ } = Underlying mean
_{ } = Fixed herd cluster with i=1, 2, 3
_{ } =Fixed effect of parity with j=1 to 6
_{ } =Fixed effect of season of calving with k=1, 2, 3, 4
_{ } =Fixed effect of year of calving with l=1985=1 to 2005=22
_{ } =Random residual error NID (0, Iσ^{2}_{e})
Genetic characterization of the clusters was done by estimating phenotypic variance, additive genetic variance, residual variances and heritability using a univariate repeatability animal model.
The mixed model in matrix notation was:
where:
Y is a vector of observations,
X and
Z are known incidence matrix of fixed effects, and random
effects, respectively;
b and u are unknown vector of
fixed and random effects respectively, while
e is a vector of
residuals.
Variance components were estimated using the DFREML package
(Meyer 1998) and the F_{max} procedure was used to test for
homogenous variances.
Three clusters were formed and their basic statistics: Mean 305-day milk yield (LMYD) average standard deviation (SDLMYD), number of herds and the resultant number of records are shown in table 1. All clusters shared the same sires. The herd clusters differed significantly (P<0.05) for the two descriptive variables.
Table 1. Characteristics of the production clusters |
||||
Production environment |
No. of herds |
No. of records |
LMYD, kg |
SDLMYD |
Cluster 1 |
24 |
3689 |
5347^{a} |
1612 |
Cluster 2 |
18 |
2474 |
5875^{b} |
1361 |
Cluster 3 |
27 |
6144 |
3434^{c} |
1181 |
Different superscripts denote different means (P<0.05) A cluster is group of herds with homogeneous variance components |
Cluster 2 had the highest mean 305-day milk yield, while cluster 1 had the largest standard deviation for milk yield. Cluster 3 had the highest number of herds and total number of records but the lowest standard deviation for milk yield and mean 305-day milk yield.
Phenotypic, additive genetic, residual variances and heritability estimates, for each production level, are presented in table 2. Estimates of phenotypic, additive genetic, residual variances increased with herd production level, as did heritability estimates. Table 2 shows higher variance for cluster 1.
Table 2. Estimates of variance components and heritability estimates for milk yield by cluster |
||||
Production environment |
Phenotypic variance, (σ^{2}_{p}) |
Additive genetic variance (σ^{2}_{a}) |
Residual variance (σ^{2}_{e}) |
Heritability (h^{2}) |
Cluster 1 |
1134608^{a} |
4144955^{a} |
1134608^{a} |
0.23 ± 0.04 |
Cluster 2 |
1513952^{b} |
503934^{b} |
918189^{b} |
0.33 ± 0.04 |
Cluster 3 |
827057^{c} |
122837^{c} |
661768^{c} |
0.14 ± 0.03 |
Different superscripts denote different means (P<0.001) A cluster is group of herds with homogeneous variance components |
The F_{max} statistical test revealed that all the
variances (Phenotypic, additive genetic, residual variances) were
significantly different (P<0.05) across clusters. The results in
table 2 clearly indicate that there is heteroscedasticity of
variance components for milk yield.
Three distinct herd clusters were identified with the cluster analysis, which were all significantly different from one another in terms of the descriptive variables used (Table 1). The difference in clusters in their original variables (Table 1) show that milk yield varies with the level of management and environment (Costa et al 2000) and this could be due to general feeding and genotype by environment correlation and methods of feeding concentrates (Brotherstone and Hill 1986).
Phenotypic, additive genetic and residuals variances and the respective heritability estimates were different for all herd clusters (table 2). The results of this study agree with those of Neser (2002), Weigel and Rekaya (2000), Naya et al (2002) who demonstrated that variance components varied with change in production environment the differences in heritability estimates for the clusters affect response to selection(Hill 1984) and would result in reduction of the accuracy of predicted breeding values due to favouring of high performers in more variable clusters at the expense of their counterparts in less variable clusters (Hill 1984).
The biases that arise with heterogeneous variances can therefore
influence the selection of Bull-dams and superior sires in a
breeding programme such as Kenya's, and would cause a reduction in
response to selection (Hill 1984). Heterogeneous variances may give
rise to genotype by environment interaction implying that different
production environments may require a different set of sires.
There is evidence of heterogeneity of variance components in the Kenyan Holstein-Friesian population.
The herd clustering model grouped herds according to likeness rather than location or herd.
Breeding values can be estimated for animals in the different clusters, thereby simplifying selection as the best adapted animals get selected for each environment.
The effect of correction for
heterogeneous variances on estimated progeny differences for sires
need to be evaluated before they are recommended for
use.
The authors wish to acknowledge the LRC and DRSK for provision
of data and the Kenya Agricultural Research Institute (KARI) and
Egerton University (EU) for provision of computing facilities.
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Received 26 February 2007; Accepted 22 April 2007; Published 6 July 2007