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Effects of some methodological factors on in vitro measures of gas production (GP, mL/g DM), CH 4  production (mL/g DM) and proportion (% CH 4  on total GP) were investigated by meta-analysis. These factors were considered: pressure in the GP equipment (0 = constant; 1 = increasing), incubation time (0 = 24; 1 = ≥ 48 h), time of rumen fluid collection (0 = before feeding; 1 = after feeding of donor animals), donor species of rumen fluid (0 = sheep; 1 = bovine), presence of N in the buffer solution (0 = presence; 1 = absence), and ratio between amount of buffered rumen fluid and feed sample (BRF/FS; 0 = ≤ 130 mL/g DM; 1 = 130–140 mL/g DM; 2 = ≥ 140 mL/g DM). The NDF content of feed sample incubated (NDF) was considered as a continuous variable. From an initial database of 105 papers, 58 were discarded because one of the above-mentioned factors was not stated. After discarding 17 papers, the final dataset comprised 30 papers (339 observations). A preliminary mixed model analysis was carried out on experimental data considering the study as random factor. Variables adjusted for study effect were analyzed using a backward stepwise analysis including the above-mentioned variables. The analysis showed that the extension of incubation time and reduction of NDF increased GP and CH 4  values. Values of GP and CH 4  also increased when rumen fluid was collected after feeding compared to before feeding (+26.4 and +9.0 mL/g DM, for GP and CH 4 ), from bovine compared to sheep (+32.8 and +5.2 mL/g DM, for GP and CH 4 ), and when the buffer solution did not contain N (+24.7 and +6.7 mL/g DM for GP and CH 4 ). The increase of BRF/FS ratio enhanced GP and CH 4  production (+7.7 and +3.3 mL/g DM per each class of increase, respectively). In vitro techniques for measuring GP and CH 4  production are mostly used as screening methods, thus a full standardization of such techniques is not feasible. However, a greater harmonization of analytical procedures (i.e., a reduction in the number of available protocols) would be useful to facilitate comparison between results of different experiments.   Electronic supplementary material  The online version of this article (doi:10.1186/s40104-016-0094-8) contains supplementary material, which is available to authorized users.

Methodological factors affecting gas and methane production during in vitro rumen fermentation evaluated by meta-analysis approach