Richard E. Muck

Lucerne and two maize crops were ensiled with and without Lactobacillus plantarum and fermented for 4 or 60 d to assess the effect of inoculant on in vitro rumen fermentation of the resulting silages. Water and 80% ethanol extracts of the silages were also analysed for effects on in vitro rumen fermentation. The inoculant affected lucerne silage characteristics but had little effect on the maize silages. In vitro fermentation of the silages showed few effects except increased microbial biomass yield (MBY) at 24 h in the inoculant-treated lucerne silages. In vitro fermentation of the lucerne silage water extracts produced no differences due to treatment except for reduced MBY in the inoculant-treated extracts. The ethanol extracts produced results inconsistent with the in vitro results of the silages. Consequently it appears that the factor in in vitro fermentation of inoculated silages causing increased MBY was in neither the water nor ethanol extracts.

Recent advances in silage microbiology are reviewed. Most new techniques in silage microbiology use the polymerase chain reaction (PCR) to make copies of a portion of the DNA in microorganisms. These techniques allow us to identify and quantify species as well as do community analysis. The PCR-based techniques are uncovering new species, both bacteria and fungi, during storage and feeding. Silage inoculants are widely available, but of greater interest has been research investigating why inoculants are so successful. Various inoculant strains have been found to produce bacteriocins and other compounds that inhibit other bacteria and fungi, improving their chances for success. In vitro ruminal fermentation research is showing that some inoculated silages affect rumen microorganisms, reducing methane in some cases and increasing microbial biomass production in others. Better understanding of silage microbiology will allow us to better manage silos and develop better inoculants to improve silage quality.

Ensiling is a method of preserving a moist crop. A moist crop can support the growth of a wide range of microorganisms, most of which will degrade the nutrient value to livestock. However, ensiling generally controls microbial activity by a combination of an anaerobic environment and a natural fermentation of sugars by lactic acid bacteria on the crop. This fermentation and the resulting low pH primarily suppress the growth of other anaerobic microorganisms. The fermentation can also inhibit yeasts, molds and aerobic bacteria, but the anaerobic environment is essential to preventing most of the spoilage microorganisms from growing. Inoculants have become the dominant additives for making silage. Homofermentative strains help guarantee a rapid suppression of anaerobic stains early in storage, increase dry matter recovery and have improved animal performance by means that we do not fully understand. Inoculants containing Lactobacillus buchneri, a heterofermentative species capable of fermenting lactic acid to acetic, are recent additives. The added acetic acid inhibits yeast and mold growth, increasing aerobic stability of silages at feeding.

The effect of 14 microbial inoculants on the fermentation and nutritive value of alfalfa silages was studied under laboratory conditions. The first cut (477 g of dry matter/kg) and second cut (393 g of dry matter/kg) of a second-year alfalfa stand were ensiled in 2 trials. In both trials alfalfa was harvested with standard field equipment. All inoculants were applied at 1.0 x 10(6) cfu/g of crop. Uninoculated silages served as controls. After inoculants were added, the chopped forages were ensiled in 1.0- and 0.5-L anaerobic glass jars, respectively, at a density of 500 g/L. Each trial had 15 treatments (uninoculated control and 14 inoculants), with 4 silos per treatment. Silos were stored for a minimum of 30 d at room temperature (approximately 22 degrees C). In first-cut silage, all inoculants but one reduced pH relative to the uninoculated control, and all but 2 of the homofermentative strains shifted fermentation toward lactic acid. In second-cut silage, the epiphytic lactic acid bacterial population was 2.7 x 10(7) cfu/g, and only commercial inoculants produced significant shifts in fermentation. Overall, microbial inoculants generally had a positive effect on alfalfa silage characteristics in terms of lower pH and shifting fermentation toward lactic acid with homofermentative lactic acid bacteria or toward acetic acid with heterofermentative lactic acid bacteria, Lactobacillus buchneri. These effects were stronger in the commercial products tested. In spite of the positive effects on silage fermentation, 48-h in vitro true DM digestibility was not improved by inoculation with lactic acid bacteria.

Alfalfa silages from 2 similar trials were analyzed for in vitro ruminal gas production. In both trials, there were 15 treatments: alfalfa treated at ensiling with 1 of 14 lactic acid bacterial inoculants or untreated alfalfa. First-cut (477 g of dry matter/kg) and second-cut (393 g of dry matter/kg) alfalfa were ensiled in glass jars for a minimum of 35 d at room temperature (approximately 22 degrees C). At opening, a portion of each silage was wet-ground with a mixer. Each silage was then assessed for in vitro ruminal gas production in 3 replicate runs with the wet-ground silage, 1 on the fresh silage and 2 on frozen and thawed silage. In vitro gas production was measured in 160-mL sealed serum vials incubated at 39 degrees C. One gram of silage was incubated with 17.1 mL of nutrient solution, 0.9 mL of reducing solution, and 12 mL of ruminal inoculum (1:2 vol/vol mixture of rumen fluid and buffer). Gas production was measured manually by using a pressure gauge at 3, 6, 9, 24, 48, and 96 h. At 96 h, the rumen fluid was analyzed for pH and volatile fatty acids. In the 2 trials, the untreated control silage produced either numerically the highest or one of the highest levels of gas production per unit of dry matter incubated. In first-cut silage, 9 of the inoculant treatments at 9 h and 4 treatments at 96 h had reduced gas production compared with the control. In second-cut silage, 10 inoculant treatments at both 9 and 96 h had reduced gas production compared with the control. Furthermore, in first-cut silage, the fraction of total gas production at 3, 6, and 9 h was numerically the highest for the control, and only 4 treatments were not significantly lower than the control at 9 h. In second-cut silage, 2 of 14 inoculated treatments produced faster fractional rates of gas production than the control, but most inoculated treatments had numerically slower fractional rates (4 significant) in the first 9 h. The in vitro fermented wet-ground control silages had one of the highest acetate:propionate ratios in both trials, significantly higher than 12 and 8 of the inoculated treatments in first- and second-cut silage, respectively. The response in acetate:propionate ratio in both cuts was similar, even though the control silage was highest in lactic acid in one trial and lowest in the other. Overall, inoculation of crops at ensiling appears to affect in vitro ruminal fermentation of wet-ground silages, even in the absence of large effects during silage fermentation.