Direct and indirect genetic indices for milk coagulation properties in Italian Holstein Friesian sires
Aim of this study was to define direct and indirect indices for milk coagulation properties (MCP) in Italian Holstein Friesian sires. A total of 315,700 individual milk samples from 49,183 cows were collected in 479 dairy farms from September 2011 to February 2014. Rennet coagulation time (RCT) and curd firmness (a30) were predicted using mid-infrared spectroscopy. Sire breeding values (EBV) for RCT and a30 were estimated using a repeatability single-trait animal model, which included herd-test-day, days in milk, age at parities and season of parity as fixed effects, and cow permanent environment and animal as random effects. The direct genetic index for milk coagulation ability (IAC) was defined as the combination of EBV for RCT and a30 with equal weight, and then expressed on a scale with mean 100 and standard deviation of 5. A stepwise method was chosen to combine official EBVs for traits published by the Italian Holstein Friesian Cattle Breeders Association in a genetic index able to predict the direct IAC. Only sires with at least 10 daughters (scored for MCP) in 5 different herds were considered. The official EBVs retained by the regression analysis were those of protein and fat content, somatic cell score and the genetic variants for k-casein. Results of direct and indirect selection for MCP are presented.