Altering milk protein/fat ratio: results of a selection experiment in dairy cattle.

Starting in 1987, two divergent lines were established from a Holstein Black and White base population by selecting sires for either a high ratio of breeding values for kg protein to kg fat or a low ratio. Each year, per line, five sires were selected, having a breeding value with a repeatability of at least 85% based on a Dutch proof and a minimum level of breeding value for kg milk, fat and protein. Milk protein, fat and lactose content of daughters were analysed weekly. In three generations, 412 heifers completed their first lactation. To estimate genetic differences between lines, records were adjusted for age at calving, calving yearxseason interaction, and percentage of Holstein Friesian genes. Traits considered were milk, fat, protein, fat and protein corrected milk and lactose yield (kg) and fat, protein, and lactose percentage, and protein/fat-ratio. Genetic and phenotypic parameters were estimated using an animal model. The difference in protein/fat-ratio between lines was 0.054, 0.088 and 0.126, respectively in the first, second and third generation. Heritabilities ranged from 0.36 to 0.57 for yield traits, 0.72 to 0.83 for percentages, and 0.79 for protein/fat-ratio. The genetic correlation between protein/fat-ratio and fat % was highly negative (-0.77) and between protein/fat-ratio and protein % slightly positive (0.18). The results of this experiment clearly demonstrate the possibility to genetically change the protein/fat-ratio in dairy milk.