Effects of lipid supplementation on the yield and composition of milk from cows with different β-lactoglobulin phenotypes

Responses to lipid supplementation differ between dairy breeds and genetic lines suggesting nutrition by genotype interactions. β-Lactoglobulin phenotype is associated with changes in yield and composition of milk. The response of cows with different β-lactoglobulin phenotypes to lipid supplementation has not been examined. Furthermore, we examined whether lipid supplementation alters milk protein composition. By using a randomized block design, we fed Holstein cows for 3 wk either a control diet containing 2.8% crude fat (n = 19) or an experimental diet that was supplemented with 4.2% tallow (n = 20). Before randomization, all cows were fed the supplemental tallow diet for at least 2 wk. Dry matter intake, body weight, milk yield, and milk composition were measured in the last week before and during the experimental period. Feeding supplemental tallow increased dry matter intake and yields of milk and milk components, including casein content, without decreasing milk component content or altering milk protein composition. On the low-fat control diet, cows with the β-lactoglobulin allele B had a greater milk and milk component yield than cows with the A allele, whereas no differences by β-lactoglobulin phenotype were observed in cows on the tallow supplement diet. Our results suggest that cows that differ in β-lactoglobulin phenotype respond differently to a low-fat diet and that feeding cows 4.2% of additional tallow increases milk yield without affecting milk component content and milk protein composition.