Meta-analysis of lipid- and starch-based dietary interventions to reduce enteric methane emissions, fecal organic matter, and fecal and urinary nitrogen excretion in dairy cows.

Dietary supplementation of dairy cow diets with lipid or starch is a known strategy to mitigate enteric methane (CH4) emission. However, the consequences of these nutritional strategies on N and OM excretion in feces and urine need to be jointly evaluated. This present meta-analysis assessed the impact of dietary lipid supplementation (LS) and increased dietary starch content (IS) on production or excretion (g/d), yield (g/kg DMI), and intensity (g/kg ECM) of CH4; fecal OM (FOM); and fecal, urinary, and total N (FN, UN, and TN) excretions. Records of treatment means were obtained from 172 experiments (678 treatment means) conducted with dairy cows. Univariate and multivariate statistical analyses evaluated the effect of LS or IS (analyzed separately) to quantify the effect of dietary ether extract (EE) or starch (STA) contents on CH4 emissions and FOM N excretions, with or without using additional explanatory parameters obtained from diet composition and animal characteristics. In simple univariate models for LS experiments, an increase in EE content of 1.0 g/kg DM reduced CH4 production, yield, and intensity by 2.03 g/d, 0.077 g/kg DMI, and 0.082 g/kg ECM, respectively; EE content alone did not influence FOM or N yield or intensity, except for a decrease in FN excretion (g/d). With IS experiments, simple univariate models revealed that increasing STA content by 1.0 g/kg DM reduced CH4 yield by 0.0085 g/kg DMI and CH4 intensity by 0.013 g/kg ECM. An increase in STA content increased FOM excretion g/d and decreased TN and FN yield and FN intensity. In complex univariate models, DMI and CP content (g/kg DM) exhibited positive relationships with N excretion (g/d) across both strategies. In the multivariate models, neither EE nor STA content showed trade-offs or synergies in intensity (g/kg ECM). However, an increase in EE decreased CH4 and TN production (g/d) in trivariate models, whereas an increase in either EE or STA content decreased CH4 yield but increased FOM yield (g/kg DMI) in bivariate models for LS diets. A decrease in DMI decreased both CH4 and TN production, as well as decreased both CH4 and FOM production in the bivariate model for both strategies. Similarly, decreased feeding level and NDF content reduced CH4 and FN production (g/d) in the trivariate model for LS diets. Decreased DMI decreased CH4, FN, and UN production, as well as CH4, TN, and FOM production (g/d) in starch-based diets. In trivariate models, for LS experiments, lower NDF content reduced CH4 and FOM yields (g/kg DM), whereas in IS experiments increased percentage of concentrate reduced CH4 and increased FN yields, and increased CP content led to increased TN and reduced FOM yields. In conclusion, reducing DMI and balancing key nutrients such as EE, starch, NDF, and CP can lower CH4, FOM, and N excretion, but trade-offs occur. In lipid diets, higher EE or STA contents reduce CH4 yield while increasing FOM yield, and in starch diets, higher CP content or concentrate proportion reduces FOM or CH4 yield while increasing N yield, highlighting the need for diet- and metric-specific optimization.