Nutritional influences on the composition of milk from cows of different protein phenotypes in New Zealand

The objective of this study was to investigate the effects of contrasting n utritional regimens on milk composition from cows of different protein phen otypes. Twenty sets of seasonally calving identical twin cows that constitu ted five different protein phenotypes (four sets of twins per phenotype) we re subjected to two nutritional treatments in crossover experiments during spring (early lactation) and summer (mid to late lactation). The phenotypes studied allowed a comparison of the AA, AB, and BE variants of both beta-l actoglobulin (beta-LG) and kappa-casein. Nutritional treatments were 1) ad libitum grazing (i.e., cows were allocated a pasture allowance of approxima tely 40 kg of dry matter/d per cow) plus 5 kg of a concentrate based on bar ley and 2) restricted grazing (pasture allowance of 20 kg of dry matter/d p er cow). Milk samples were collected from each cow near the end of each 14- d treatment period and were analyzed for a detailed range of individual pro tein and fat constituents. Diet had significant effects on the concentratio ns of all milk components measured. Protein phenotype affected some protein components but not fat components. Interactions between the effects of PLG phenotype and diet were noted for the concentrations of some milk componen ts. Diet and protein phenotype have important effects on the manufacturing potential of milk produced under the dairying systems of New Zealand, which rely heavily on grazing. The effects of nutrition on milk composition may depend on the beta-LG phenotype.