Efficiency of feed and energy use in primiparous and multiparous dairy cows fed contrasting dietary protein concentrations across lactation

Reducing dietary CP concentration can reduce feeding costs and N excretion in dairy production but may negatively impact productivity and efficiency. This study investigated the impact of reduced dietary CP concentration, across early, mid− and late−lactation stages in primiparous and multiparous lactating cows, on productivity, feed and energy use efficiency and CH₄ emission parameters. Twenty-four Holstein-Friesian (12 primiparous, 12 multiparous) cows were allocated to three experimental total mixed rations containing 12.2, 15.1 or 18.1% CP (LCP, MCP and HCP, respectively; DM basis), in a continuous study across lactation (days 1–305). Digestibility and gas exchanges were measured in metabolism units and indirect open-circuit respiration calorimeter chambers during early, mid− and late−lactation. Data were analysed using a linear mixed model, with repeated measures, with CP concentration, stage of lactation, parity and their interactions as fixed effects, and a random effect of cow fitted as the subject and stage of lactation as the repeated measure. When compared with LCP treatment, MCP and HCP had higher DM intake (DMI) (+1.9 and +3.0 kg/d), milk yield (+5.5 and +7.7 kg/d), energy corrected milk yield (ECMY) (+5.0 and +7.1 kg/d), and feed efficiency (ECMY/DMI, +0.13 and +0.15 kg/kg; milk solids/DMI, +16 and +20 g/kg; milk energy output (E_L)/DMI, +0.40 and 0.46 MJ/kg). Digestible energy intake (DEI)/gross energy intake (GEI), metabolisable energy intake (MEI)/GEI, MEI/DEI and milk energy output adjusted for zero energy balance (E_L(0))/MEI were higher in HCP (+0.02, +0.03, +0.02, and +0.06 MJ/MJ) than LCP while there were no differences between LCP and MCP for DEI/GEI, or between MCP and HCP for MEI/DEI and E_L(0)/MEI. Methane production per digestible DMI and ECMY were lower for MCP (−3.2 and −3.1 g/kg) and HCP (−3.8 and −3.4 g/kg), when compared with LCP. Methane energy per GEI, DEI and MEI were lower for the MCP (−0.007, −0.011 and −0.014 MJ/MJ) and HCP (−0.007, −0.014 and −0.017 MJ/MJ) than LCP. The significant interaction between CP concentration and stage of lactation on milk yield and ECMY showed that treatment differences (increased values in MCP or HCP vs LCP diets) reduced as lactation progressed. A diet containing 15.1% CP (DM basis) may be sufficient to maintain milk production and feed efficiency while reducing CH₄ yield and intensity. However, regression analysis suggests that productivity may increase further between 15 and 18% CP (DM basis) but the response to dietary CP may depend on the stage of lactation.