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 wit h 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, f at and lactose content of daughters were analysed weekly. In three gen erations, 412 heifers completed their first lactation. To estimate gen etic differences between lines, records were adjusted for age at calvi ng, calving year X season interaction, and percentage of Holstein Frie sian genes. Traits considered were milk, fat, protein, fat and protein corrected milk and lactose yield (kg) and fat, protein, and lactose p ercentage, and protein/fat-ratio. Genetic and phenotypic parameters we re estimated using an animal model. The difference in protein/fat-rati o 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/f at-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 dem onstrate the possibility to genetically change the protein/fat-ratio i n dairy milk. (C) 1998 Elsevier Science B.V.