Genetic response for milk production traits, somatic cell score, acidity and coagulation properties in Italian Holstein–Friesian population under current and alternative selection indices and breeding objectives

The aim of this work was to estimate the annual genetic response for milk yield (MY), milk components, somatic cell score (SCS), milk acidity and milk coagulation properties (MCP: rennet coagulation time, RCT; curd firmness 30 min after rennet addition, a30) in Italian Holstein-Friesian (IHF) population under different selection indices and breeding objectives. Current selection index (PFTSI) of IHF breed includes fat (FY) and protein (PY) yields, fat (F%) and protein (P%) percentages, and SCS. Annual genetic response (GRy) for the aforementioned traits and correlated response for MCP, MY and milk acidity were estimated using selection index theory. Genetic parameters for all traits were assumed from different sources and validated for positive definite matrix. Alternative selection indices were simulated by adding MCP as direct (MCPd) or predicted (MCPp) traits to current selection index. Direct measures were assumed to be recorded in laboratory by computerized renneting meter, whereas predicted measures by mid-infrared spectroscopy (MIRS). Current breeding objective (PFTBO) includes FY, PY, F%, P% and SCS with relative emphasis of 13.6, 61.0, 3.4, 5.1 and -16.9%, respectively. Six alternative breeding objectives were formulated according to the relative weight given to MCP: 5% (MCP5%), 12.5% (MCP12.5%), 25% (MCP25%), 50% (MCP50%), 75% (MCP75%) and 100% (MCP100%). Current selection index does not affect significantly MCP traits, whereas MCPp to improve MCP12.5% resulted in a gain of about 0.10 genetic standard deviations per year for both RCT and a30, without losses in GRy for MY, FY and PY. The inclusion of MCP traits in the selection index of IHF is expected to increase casein to protein ratio and to decrease SCS.