Effect of feeding practices and manure quality on CH₄ and N₂O emissions from uncovered cattle manure heaps in Kenya

Countries in sub-Saharan Africa (SSA) rely on IPCC emission factors (EF) for GHG emission reporting. However, these were derived for industrialized livestock farms and do not represent conditions of smallholder farms (small, low-producing livestock breeds, poor feed quality, feed scarcity). Here, we present the first measurements of CH₄ and N₂O emissions from cattle-manure heaps representing feeding practices typical for smallholder farms in the highlands of East Africa: 1) cattle fed below maintenance energy requirements to represent feed scarcity, and 2) cattle fed tropical forage grasses (Napier, Rhodes, Brachiaria). Sub-maintenance feeding reduced cumulative manure N₂O emissions compared to cattle receiving sufficient feed but did not change EF_N2O. Sub-maintenance feeding did not affect cumulative manure CH₄ emissions or EF_CH4. When cattle were fed tropical forage grasses, cumulative manure N₂O emissions did not differ between diets, but manure EF_N2O from Brachiaria and Rhodes diets were lower than the IPCC EF_N2O for solid storage (1%, 2019 Refinement of IPCC Guidelines). Manure CH₄ emissions were lower in the Rhodes grass diet than when feeding Napier or Brachiaria, and manure EF_CH4 from all three grasses were lower than the IPCC default (4.4 gCH₄kg^-1 VS, 2019 Refinement of IPCC Guidelines). Regression analysis revealed that manure N concentration and C:N were important drivers of N₂O emissions, with low N concentrations and high C:N reducing N₂O emissions. Our results show that IPCC EFs overestimate excreta GHG emissions, which calls for additional measurements to develop localized EFs for smallholder livestock systems in SSA.