Marker-assisted selection may provide the opportunity to make significant genetic gains in the improvement of economically important traits in livestock (Soller & Beckmann, 1983; Smith & Simpson, 1986). Implementation of this approach will first require identification of candidate genes or anonymous gene markers associated with the traits of interest. Candidate genes are those with a known relationship between physiological or biochemical processes and an economically important trait. In dairy cattle, genes associated with mammary gland growth, development and function are excellent candidate genes for milk production traits.

The polyamines are low molecular mass polycations that influence cell proliferation and growth (Tabor & Tabor, 1984; Pegg, 1986). Ornithine decarboxylase (ODC, EC 4.1.1.17) catalyses the conversion of ornithine to putrescine, the rate-limiting step in polyamine biosynthesis (Pegg, 1986). The level of ODC is induced in quiescent cells exposed to a wide variety of stimuli such as growth hormone, corticosteroids, testosterone and growth factors (Tabor & Tabor, 1984). There is ample evidence that growth factors influence morphogenesis and differentiation of the mammary gland (Imagawa et al. 1994). Trophic hormones that are associated with lactation, such as prolactin and growth hormone, are also required to induce differentiation of bovine mammary epithelial cells (Huynh et al. 1991; Flint & Gardner, 1994), and both mitogenic and lactogenic effects of prolactin, insulin and hydrocortisone appear to be mediated through the polyamine pathway (Rillema et al. 1977; Bedford & Zadworny, 1990; Golden & Rillema, 1993). Strange et al. (1992) have shown that ODC is involved in synthesis of a milk component; ODC has elevated expression in the lactating mammary gland, which declines sharply after weaning. It is therefore possible that particular variants of ODC could be associated with increased mammary gland function and thus influence milk related traits.

The objectives of the present study were to estimate allelic frequencies of ODC polymorphisms in Holstein bulls and to evaluate further the genotypic effects of ODC variants on milk production traits.