Glucose is the major precursor of lactose synthesis in the mammary gland. Lactose the major carbohydrate and osmolyte of milk, controls milk volume and its concentration. Glycosylated haemoglobin (HbG) is a retrospective measure of mean blood glucose level and it is largely unaffected by recent physiological conditions and environmental events. The purposes of this study were to determine the correlations between lactose traits and other milk production traits in dairy cattle and to investigate whether HbG level can be correlated with milk and lactose production traits. Here, HbG percentage, milk and lactose production traits including milk yield, lactose, protein, SNF, total solid and fat percentages and yields were measured in 485 second calved Iranian Holstein cattle. Statistically significant negative correlations were established between HbG and milk yield (r=−0.88), lactose yield (r=−0.83), SNF yield (r=−0.81), protein yield (r=−0.79) and total solid yield (r=−0.74). Positive correlations were established between lactose yield and milk (r=0.96), protein (r=0.81), SNF (r=0.92) and total solid (r=0.79) yields. The negative correlation between HbG and milk and total lactose production is Vet Res Commun (2010) probably related to the higher glucose demands in the lactating mammary gland of more productive cows. The positive correlation between lactose yield and milk, protein, SNF and total solid yield indicates that the level of lactose synthesis influences milk production traits in ways other than merely via its osmolytic action.

The scope of this study was to identify sequence polymorphisms in the β-1,4-galactosyltransferase-I gene (B4GALT1), the gene which encodes the catalytic part of lactose synthase enzyme. Exons of the gene were scanned for the presence of allelic variation among a sample of 400 Iranian Holsteins, using the single strand conformation polymorphism technique followed by sequencing. Nine polymorphic nucleotide sites and 16 different genotypes of B4GALT1 were identified. We concluded therefore that the β-1,4-galactosyltransferase-I gene was polymorphic in Holsteins. Considering the biological roles of the B4GALT1 gene, we suggest that further studies are critical to investigate the biological effects of the SNPs reported in this study.