Effects of a Moderate Increase in Dietary Starch Content and Partial Replacement of Dietary Protein Source with Canola Meal on Methane Emissions and Lactation Performance in Dairy Cows

This study aimed to evaluate the impact of moderately elevated dietary starch levels and the partial replacement of dietary protein source with canola meal (CM) on lactation performance, methane (CH4) emissions, ruminal fermentation parameters, and serum biochemical parameters in dairy cows. Initially, two in vitro gas production trials were conducted: Trial 1 involved diets formulated with starch levels of 24.0% (Low starch), 27.4% (Medium starch), and 30.8% (High starch) on a dry matter (DM) basis. In Trial 2, compared with a control diet without CM (CONin_vitro; 25.7% starch, the mean of 24.0% and 27.4%), a diet with moderately increased starch content and the inclusion of CM at 10.73% of DM (HSCMin_vitro; 29.1% starch, the mean of 27.4% and 30.8%) on a DM basis was formulated. In the subsequent in vivo experiment, thirty mid-lactation Holstein cows were randomly allocated to two groups: a control diet (CON, comprising 26.1% starch without CM, on a DM basis) or an experimental diet (HSCM, containing 28.9% starch with 7.68% CM, on a DM basis). In vitro results in Trial 1 showed that increasing dietary starch linearly decreased CO2 production (p < 0.01) and CH4 production (p = 0.03), but no significant differences between groups were detected. In Trial 2, a 3.4% increase in dietary starch combined with partial inclusion of CM showed a tendency to reduce CH4 (p = 0.06) and CO2 production (p = 0.08) compared to the CON diet. For the in vivo experiment, cows on the HSCM diet exhibited a tendency for greater dry matter intake (DMI; p = 0.06) and feed efficiency (milk yield/DMI; p = 0.09), and significantly improved milk yield (p < 0.01), energy-corrected milk (ECM; p < 0.01), and fat- and protein-corrected milk (FPCM; p < 0.01). The yields of milk fat (p < 0.01), protein (p < 0.01), lactose (p = 0.03), and total solids (p < 0.01) were also significantly improved, while the milk urea nitrogen (MUN) concentration was decreased (p < 0.01). The HSCM diet significantly decreased CH4 production by 14.1% (p = 0.02), as well as CH4 yield (p < 0.01) and CH4 emission intensity (p < 0.01) in terms of g/kg milk yield, ECM, and FPCM. Rumen fermentation parameters indicated lower molar proportions of isobutyrate and isovalerate in the HSCM group (p < 0.01), with no differences in other rumen parameters between groups. Additionally, the HSCM group exhibited a higher serum total protein (TP) concentration (p = 0.02) and a decreasing trend in blood urea nitrogen (BUN; p = 0.09). These findings demonstrate that a moderate increase in dietary starch content and partial replacement of dietary protein source with CM is an effective nutritional strategy for simultaneously improving lactation performance and mitigating CH4 emissions. However, as starch and CM were altered simultaneously in the in vivo experiment, the independent contribution of each factor cannot be determined and warrants further investigation.