Breeding value estimation for milk coagulation properties in the Italian Holstein Friesian bull population

Abstract

Cheese manufacturing is the primary destination of milk produced in Italy. The transformation of milk into cheese is a process consisting of several stages. Coagulation is one of the most important stages, in which milk changes from liquid to gel. Commonly, the main milk coagulation traits studied are milk rennet coagulation time (RCT, min) and curd firmness (a30, mm). Recently, mid-infrared spectroscopy (MIRS) has been applied as a cheap technology to predict milk coagulation properties (MCP) at population level. In the Veneto region, 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 value for milk coagulation properties (IAC) combining RCT and a₃₀ in equal weight has been developed using a repeatability single-trait animal model, which included herd-test-day, days-in-milk, age at parity and season of parity as fixed effects, and cow permanent environment and animal additive genetic as random effects. However this direct IAC is not yet available at whole population level, which is mainly due to the absence, up to now, of a collection of individual cow data on a large scale. For this reason, a stepwise approach was chosen in order to combine existing official EBVs for traits published by the Italian Holstein Friesian Cattle Breeders Association (ANAFI) as indirect predictors 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 predictors retained after the regression analyses were the EBVs of protein and fat content, and somatic cell score and the genetic variants for k-casein. The predicted IAC is an official index for ANAFI since December 2013, and is used by the farmers in order to choose those bulls with better cheese capabilities. In the near future it is expected to pass to the direct estimation of IAC at national level.