Relevance and Potential Use of In vitro Gas Production Measurements to Evaluate Varying Ratios of Roughages and Protein Sources for Ruminants

This study explored the relevance and potential of measurements obtainable from in vitro gas production technique (IVGPT) in predicting nutritive value of ruminant rations comprised of poor quality roughages (RG) and different protein sources (PS). The RG were grass hay (GH) and maize stover (MS). The PS were Lucerne hay (LH), Sericea lespedeza hay (LPZ) and sunflower oil cake (SFC). RG:PS ratios used were 100:0, 80:20, 60:40, 40:60 20:80 and 0:100 on DM basis. Evaluation measurements included degradability (Deg), microbial yield (MIC) gas production (GP), time taken to half total GP (T_½), Deg/total GP ratio (PF) and Deg/(half total GP x T_½) ratio (DEF). The Deg increased linearly as LH (P<0.003) or SFC (P<0.03) increased in GH rations while a quadratic increase was obtained in MS rations. Increasing LPZ caused a linear decrease in Deg in both GH and MS rations. The PS differed (P<0.001) in GP in GH rations but protein source ratio (PSR) had no effect. In MS rations, the PS, PSR and PS x PSR interaction affected (P<0.001) GP. There was a linear decrease (P<0.02 to 0.002) in T_½ as LH or SFC ratios increased in both GH and MS rations while increasing LPZ ratio tended to cause a quadratic decrease (P=0.17). The MIC and PF showed no clear trend among all the PS and PSR. The DEF was consistent and increased linearly (P<0.002 to 0.001) as the ratios of LH or SFC increased, while increasing LPZ ratio caused a quadratic increase. It is concluded that the confounding results obtainable in Deg, GP, MIC and PF can be resolved by including DEF and/or T_½ among the evaluation measurements in order to improve the accuracy of predicting the nutritive value.