Enteric methane (CH4) production from beef cattle accounts for more than 71% of the total enteric CH4 fermentation from ruminants. Many nutritional strategies have been investigated in vitro to mitigate CH4 production from ruminants. Saponin is a plant extract that has been demonstrated to be effective in vitro. In this thesis, a series of studies were conducted to investigate the effects of dietary inclusion of steroid (Yucca schidigera) and triterpenoid (Quillaja saponaria and Camellia sinensis) saponins on animal and manure-derived CH4 and other gaseous emissions. In addition, the effects of adding saponin extracts to manure on manure-derived CH4and other gaseous emissions were also investigated. Dietary inclusion of up to 1.5% of quillaja and yucca saponins or 0.25% of tea saponin did not change animal-derived CH4 emissions, while CH4 emissions were significantly reduced when steers were fed 0.5% tea saponin. The reductions of CH4 production can be possibly attributed to reduced DMI in 0.5% tea saponin treatment. Manure-derived CH4 emissions were reduced in steers fed 0.64% yucca saponins, increased in steers fed 1.5% quillaja saponin treatment and not affected in steers fed 0.25% tea saponin treatment compared to the control treatment. However, direct saponin addition to manure showed no effects on CH4 emissions. Feeding steers up to 1.5% yucca saponin or 0.5% of tea saponin did not affect animal-derived NH3 emissions. Manure-derived NH3 emissions were reduced in 0.64% yucca saponin treatment. Increased animal-derived NH3 daily emissions were observed in 1.5% quillaja treatment in one of the studies, whereas in another study, 1.5% dietary quillaja saponin supplementation did not change NH3 emissions. The differences may be explained by variation among animals. Animal-derived H2S, NMTHC and N2O emissions were not influenced by dietary saponin inclusion or direct addition. Dietary inclusion of 1.5% of quillaja saponin reduced manure-derived H2S emissions, increased NMTHC emissions but did not affect N2O emissions. Both NMTHC and H2S emissions from manure were reduced as a result of dietary inclusion of 0.64% yucca. Dietary inclusion of 0.25% tea saponin treatments reduced NMTHC, H2S and N2O emissions. Overall, dietary inclusion of all saponin sourced failed to change animal-derived CH4 emissions without affecting growth the performance. Effects of dietary saponin supplementation on manure-derived air emissions were varied by saponin type. Direct application of saponins to manure had no effects on manure-derived air emissions.
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