Genetic analysis of longevity and performance traits of Sahiwal cattle in Kenya

Abstract

Genetic and phenotypic parameters for longevity, genetic relationship between longevity and growth, milk yield and fertility traits and rate of inbreeding were estimated for Sahiwal cattle in Kenya. The aim was to assess the genetic diversity and inbreeding depression for performance traits. Data utilized were for cows born between 1972 and 2004 and with milk production records between 1976 and 2008. Measures of longevity related to productive life were: time between birth (Long_1) or first calving (Long_2) and last milking record in days, number of lactations initiated (Long_3), total number of days in lactation over all lactations (Long_4) and total milk yield over all lactations (kg) (Long_5). Measures of longevity related to survival were defined as survival from birth to 44 months (Long6_44), 56 months (Long6_56), 80 months (Long6_80), 92 months (Long6_92), 104 months (Long6_104), and 128 months (Long6_128). Longevity was also defined as survival from first calving as survival from for 12 months (Long7_12), 36 (Long7_36), 60 (Long7_60), 84 (long7_84) and 96 months (Long7_96) from first calving. Longevity measures related to productive life were analysed using linear models while those related to survival were analysed using threshold models. Effects of inbreeding on the traits were determined by fitting four regression models (linear, quadratic, exponential and Michaelis-Menten) to the errors generated by the animal model. Estimates of heritability for longevity measures related to survival were higher (0.084±0.053 to 0.119±0.035) compared to those of measures related to productive life (0.038±0.032 to 0.097±0.04). Comparatively, survival from first calving to predefined ages had higher heritability estimated (0.090 to 0.119) compared to survival from birth (0.084 to 0.104). Long7_96 had the highest additive genetic variance and heritability estimate, and therefore should be used for genetic evaluation of longevity in Sahiwal cattle in Kenya. Genetic correlations between measures of longevity and first lactation milk yield and fertility were positive (0.41 to 0.99) and negative (-0.02 to -0.85), respectively. First lactation milk yield had the highest genetic correlation and should be used as a selection criterion for longevity. Michaelis-Menten model had the highest significant (P < 0.001) for all the traits studied. Inbreeding shortened calving interval and age at first calving and increasing lactation length. Inbreeding depression was greater after 15% inbreeding. Genetic evaluation of the Kenyan Sahiwal should account for inbreeding. This study has also provided genetic and phenotypic parameters to enable inclusion of longevity in the breeding objective for the Sahiwal cattle improvement programme