A systematic study of sheep faeces to be used as an alternative inoculum source in the Hohenheim gas test

In vitro studies of gas production (GP) using rumen fluid obtained from rumen-cannulated animals are common in feed evaluation for ruminants. The objective was to compare sheep faeces with rumen fluid from dairy cows as inocula in the Hohenheim gas test (HGT) using a large number of different-quality feeds. It then was evaluated whether GP obtained by using rumen inoculum (RI-GP) can be estimated from GP using faecal inoculum (FI-GP). Ninety feeds for ruminants including roughages (n = 27), total mixed rations (n = 6), commercial compound feeds for dairy cows (n = 18), cereal grains (n = 10), energy-rich by-products (n = 5), legume grains (n = 6), oilseed meals (n = 6), and other protein-rich feeds (n = 12) were incubated with either rumen inoculum (RI) or faecal inoculum (FI), and GP was recorded at 2, 4, 8, 12, 24, 48, and 72 h. Nonlinear equations were fitted to the GP data to determine GP kinetic parameters. Correlations between RI-GP and FI-GP were calculated for different time points. Linear regressions were calculated to estimate RI-GP from FI-GP using the entire data set or subsets of roughages and concentrates. GP kinetic parameters were lower for all feed categories when incubated with FI instead of RI. On average of all feed samples, the potential GP was 9 mL/200 mg dry matter (DM) lower and the rate of GP was 3.1%/h lower with FI than RI. The estimation of kinetic data for FI included a lag phase of 1.51 h on average, whereas no lag phase was estimated for RI. Estimated parameters indicated an overall lower fermentation activity of FI than RI. However, there was a very similar progression of GP curves of RI-GP and FI-GP. RI-GP after 24 h was significantly correlated with FI-GP in the time period of 24 to 60 h (r = 0.973 - 0.982, including all feed samples). Linear regression analysis showed that RI-GP after 24 h can be estimated from FI-GP after 24 h (Slope = 1.02, R² = 0.97) and 48 h (Slope = 1.1, R² = 0.97) including the complete data set. Splitting the data set into roughages and concentrate feeds did not lead to distinctly higher estimation accuracy. In conclusion, sheep faeces can be used as an alternative inoculum for studying a wide range of different-quality feedstuffs. The standard gas test may be modified without the need for rumen-cannulated animals.