**1. Introduction**

Replacement heifers are critical to dairy herd productivity because they represent the future milking and breeding stock of all dairy operations. The goal should be to provide an environment for heifers to develop full lactation potential at the desired age with minimal expense. Animal health and well-being play vital roles in achieving this potential, and one disease that can influence future productivity is heifer mastitis caused by *Staphylococcus aureus,* the coagulase-negative staphylococci (CNS), and the environmental streptococci (*Streptococcus uberis, Streptococcus dysgalactiae*).

Unfortunately, most producers regard young dairy heifers as uninfected, and the presence of mastitis is not observed until freshening or until the first clinical flare-up in early lactation. Thus, animals may carry intramammary infections (IMI) for a year or more before they are diagnosed with mastitis (Boddie et al., 1987). The greatest development of milkproducing tissue in the udder occurs during the first pregnancy, so it is important to protect the heifer mammary gland from pathogenic microorganisms to ensure maximum milk production during the first and future lactations. In the USA, Louisiana researchers found that if bred heifers infected with *Staph. aureus* were left untreated, they produced 10% less milk in early lactation than those receiving therapy (Owens et al., 1991; Trinidad et al., 1990c). Likewise, research in New Zealand demonstrated that *Staph. aureus* mastitis in heifers resulted in significant production losses during the first lactation, which carried over into the subsequent lactation because of damage to milk-producing tissues of the udder (Woolford et al., 1983).
