GENETIC AND PHENOTYPIC PARAMETER ESTIMATION OF FIRST LACTATION, LIFE-TIME YIELD AND LONGEVITY TRAITS IN HOLSTEIN CATTLE

Abstract

Life-time yield and longevity are of major economic importance in dairy cattle (VanRaden and Wiggans, 1995). Longevity affects profitability by reducing replacement costs because of fewer num- ber of replacement heifers needed to be raised with higher selection intensity and increasing the proportion of cows produc- ing at mature level with more opportuni- ties to voluntary culling (White, 1973; Burnside et al., 1984; Van Arendonk, 1985; Jairath et al., 1994; Boettcher et al., 1998; Pryce and Brotherstone, 1999; Nakano and Terawaki, 2002; Rogers, 2002; Canavesi et al., 2003; Powell and VanRaden, 2003; Sewalem et al., 2005; Tsuruta et al., 2005).
Longevity reflects a cow’s ability not to be culled. Culling is a complex issue and many factors are involved. Cows could be culled for one or more reasons that include low production, health prob- lems, low fertility and other reasons. Culling a cow for low production irrespec- tive of her health is usually referred to as voluntary culling while culling for disease and/or low fertility regardless of her performance relative to her herdmates is called involuntary culling (Silva et al., 1986; Ducrocq et al., 1988; Dekkers, 1993; Grohn et al., 1998; Pryce and Brotherstone, 1999; Rogers, 2001).
Longevity can be defined and measured quantitatively in many different ways. It can be defined as the ability of the cow to both produce and reproduce for many years (Hoque and Hodges, 1980). Van Doormaal et al. (1985) and Rogers (2001) defined it as the length of time a cow remains productive in the herd. It can be expressed as age at last calving, number of lactations initiated or com- pleted, length of life from first calving to disposal and survival to various ages (Van Doormaal et al., 1985). Vollema and Groen (1996) reported that lifetime lon- gevity traits can be expressed as number of lactations, total milk production, num- ber of days in lactation, herd life (culling age), and length of productive life.
Two approaches have been sug- gested to include longevity in a breeding program. The first approach is to include a direct measure of longevity in the selec- tion index (VanRaden and Wiggans, 1995). The other approach is to select for other traits, which are genetically corre- lated with longevity but are expressed earlier (Vollema and Groen, 1996).
Genetic improvement of life-time production and longevity by direct or indirect selection requires estimates of genetic parameters of these traits and their relationships with early lactation traits. Jairath et al. (1994) stated that knowledge of relationships between lifetime perform- ance traits and early lactation traits is important for prediction of expected correlated response to selection.
The objective of this study was to estimate genetic and phenotypic parame- ters of first lactation milk yield, life-time yield and longevity traits expressed as: the number of lactation completed, length of productive life, culling age and total lifetime months in lactation.