Evaluation of the nutritive value of maize silages using a gas production technique

The potential of a gas production technique (GPT) to predict fractions of rumen fermentable organic matter (OMf), crude protein (CP) and starch escaping rumen degradation (EP and ESTA), as well as in vitro organic matter digestibility (OMd) were studied with 30 samples of maize silage. Rumen degradation parameters were determined with four cannulated lactating cows by incubating finely cut fresh silage in nylon bags for 0, 12, 24, 48, 72 and 288 h. In vitro OMd was measured according to Tilley and Terry [Tilley, J.M.A., Terry, R.A., 1963. A two-stage technique for the in vitro digestion of forage crops. J. Br. Grassl. Soc. 18, 104-111], using rumen fluid from two sheep. Gas production (GP) was continuously measured by incubating dried and ground samples in buffered rumen fluid from two sheep for 72 h, and GP curves were analysed with a three-phase model. Parameters studied were GP in the first 3 h of fermentation (GP3 h), between 3 and 20 h (GP3-20 h), the time to reach half of these values (b1 and b2), the maximum fermentation rate during the second phase (R2 max) and total GP, and OM fermented after 72 h of incubation (GP72 h and OMf72 h). Multiple regression was used to predict OMf, EP, ESTA and OMd based on GP parameters and chemical composition (i.e., DM, NDF, starch, CP). Fractions of OMf, EP, ESTA and OMd were 0.580 ± 0.040, 0.350 ± 0.062, 0.260 ± 0.106 and 0.732 ± 0.028, respectively. The GP3 h, GP3-20 h and GP72 h were 52 ± 5, 177 ± 13 and 323 ± 15 ml/g OM, respectively, b1 and b2 were 0.9 ± 0.2 and 8.7 ± 0.6 h, respectively, and R2 max was 15.6 ± 1.6 ml/h. All rumen degradability parameters could be predicted by a two-term regression with b2 as most important parameter. For OMf, the combination of b2 and GP3 h resulted in a determination coefficient (R2) of 50.4% and a residual standard deviation (R.S.D.) of 0.028. The EP could be predicted by b2 and CP (R2 = 68.9%; R.S.D. = 0.035) and ESTA by b2 and DM (R2 = 74.3%; R.S.D. = 0.054). In the case of OMd, OMf72 h was the single explaining variable (R2 = 77.9%; R.S.D. = 0.013).