GENETIC ANALYSIS OF TEST DAY RECORDS OF MILK PRODUCTION IN DAIRY CATTLE

Test day records of British Holstein-Friesian cows were considered as either multiple traits or repeated records in genetic analyses of performance of dairy cows in the second lactation. Estimates of genetic parameters of monthly test day and completed lactation records for milk, fat and protein yields were obtained using multivariate restricted maximum likelihood methods fitting a sire model. Generally, heritabilities for monthly test day yields were lower for early and later tests than for mid-lactation tests. The average estimates of correlation between adjacent test day records of all traits were 0.91 to 0.96 (genetic) and 0.56 to 0.71 (phenotypic). In general, the mean estimates decreased the further apart the test days. Estimates of genetic correlation of test day yields with corresponding lactation yields were highest (0.94 to 0.98) for mid-lactation records. The phenotypic correlations were lower than the corresponding genetic correlations, but both followed the same pattern. Test day yields in the first three lactations were analysed using bivariate models to estimate genetic variance components for a trait between pairs of lactations. One year's records were considered in the analysis so that cows had records in only one lactation. The bivariate heritability estimates were generally lower than within lactation multivariate estimates for all traits. The heritability estimates were lower for test day yields at the beginning and the end of lactation than for test day yields taken in mid-lactation for all lactations. The heritability estimates for the complete lactation yields in lactations 1, 2 and 3 were all around 0.20 (ranging from 0.17 to 0.24) for milk, fat and protein yields. Genetic correlations between lactations 1 and 2 for test day yields taken in mid-lactation were higher (0.92 0.95) than between lactations 1 and 3 (0.83 0.92) and between lactations 2 and 3 (0.81 0.89). Genetic correlations between lactations for test day records taken at the beginning and the end of lactation were, generally, low for all traits. Analysis of test day records was attempted assuming a repeatability model by considering all 10 individual test day records as repeated measurements of the same trait. The heritability estimates for milk, fat and protein yields were lower than the corresponding predicted estimates using average parameters from a multivariate model. Under a repeatability model, only a few test day records taken at mid-lactation may be considered most important with the intuitive assumption that heritabilities are equal for all the test day traits used and that the genetic correlation among all pairs of records is near unity. Partial yields derived from test day yields were used to express the relative proportions of yield in three successive stages of lactation and to provide ratio measures for persistency of milk production. The ratio measures of persistency had moderate heritabilities and low genetic and negative phenotypic correlations with peak yield. The ratios based on partial variations in test day production were found to provide a good measure of persistency. Acknowledgements I would like to thank my supervisors, Professor Bill Hill and Dr Robin Thompson (Roslin Institute). I am grateful to the National Milk Records and Animal Data Centre for making the data available. I would also like to thank Dr Susan Brotherstone for offering computational assistance when needed. My heartfelt thanks to Dr Raphael Mrode of the Animal Data Centre for making useful comments at various stages of the study and also for his continuous encouragement and motivation. Neither do I forget those friends and colleagues who helped me with my work, especially Mehmet Firat of the Department of Mathematics and Statistics and those who shared an office with me for their useful comments and discussions. Finally, my thanks go to the Ghana Government for funding my study and the University of Cape Coast, Ghana for granting me study leave; and to my wife, Felicia and children Bernard, Eric and Lorraine and my entire family, both nucleus and extended, for their sustained love and moral support. And to God be the glory!