Two common genetic variants of β-lactoglobulin (β-lg), A and B, exist as co-
dominant alleles in dairy cattle (Aschaffenburg, 1968). Numerous studies have shown
that cows homozygous for β-lg A have more β-lg and less α-lactalbumin (α-la) and
casein in their milk than cows expressing only the B variant of β-lg (Ng-Kwai-Hang
et al. 1987; Graml et al. 1989; Hill, 1993; Hill et al. 1995, 1997). These differences have
a significant impact on the processing characteristics of the milk. For instance, the
moisture-adjusted yield of Cheddar cheese is up to 10% higher using milk from cows
of the β-lg BB phenotype compared with milk from cows expressing only the A
variant (Hill et al. 1997). All these studies, however, describe compositional
differences associated with β-lg phenotype in established lactation only. No
information is available on the first few weeks of lactation, when there are marked
changes in the concentrations of β-lg and α-la (Pérez et al. 1990).