Effect of feeding practices and manure quality on CH4_{4} and N2_{2}O 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 CH$_{4}$ and N$_{2}$O 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 N$_{2}$O emissions compared to cattle receiving sufficient feed but did not change EF$_{N2O}$. Sub-maintenance feeding did not affect cumulative manure CH$_{4}$ emissions or EF$_{CH4}$. When cattle were fed tropical forage grasses, cumulative manure N$_{2}$O 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 CH$_{4}$ 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 g CH$_{4}$ kg$^{-1}$ VS, 2019 Refinement of IPCC Guidelines). Regression analysis revealed that manure N concentration and C:N were important drivers of N$_{2}$O emissions, with low N concentrations and high C:N reducing N$_{2}$O 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