The effect of mechanical forage treatments on the growth of Clostridium tyrobutyricum and Listeria monocytogenes in grass silage

After cutting of the grass with a mower conditioner and a 24 h wilting period, the grass was subjected to three different mechanical treatments, namely untreated (L), cut in a stationary cutter head (C) or in a precision chop harvester (P). These three differently treated forages were inoculated with 100 (log 2) g−1 Clostridium tyrobutyricum (Experiment 1) or with 2 000 000 (log 6.3) g−1 Listeria monocytogenes (Experiment 2). Each kind of forage was ensiled in three mini-silos (volume 25 l) and sampled with a corer on Days 0, 35 and 95. Each core was sectioned into nine consecutive samples in order to assess the homogeneity within silo. The objective was to investigate if the different mechanical treatments influences silage homogeneity and if silage homogeneity affected the number of reisolated clostridium and listeria bacteria in the silages. Homogeneity was expressed as coefficient of variation (CV) of the analysed constituents. In both experiments L-forages had a larger in-silo variation of dry matter (DM) than P-silages. The level of DM variation in this study, however, was small if compared to practical farm silages. The P-treatment increased the lactic acid concentration and reduced silage pH significantly in both experiments compared with L- and C-treatments. However, the P-treatment could not inhibit a fast and intensive proliferation of clostridial spores in the clostridium experiment (>log 7 g−1). In the listeria experiment, P-treated silages produced substantially more lactic acid and counts of L. monocytogenes dropped to log 0.1 g−1 on day 35 while counts in C- and L-silages were still on a high level (log 3.5 and 5.8 g−1, respectively). However, by Day 95 high listeria counts were detected in one of the three P-silages (log 3.6, log 1.0 and 0 g−1) but no L. monocytogenes were recovered from C-silages and L-silages contained very low numbers (log 0.6 g−1). No significant correlations were found between the DM or pH variation in the forages and the number of inoculation bacteria in the silages. Probably due to the low degree of variation in the forages, the study could not prove that forage homogeneity affected the number of inoculation organisms in the silages.