Degradation of fructans by epiphytic and inoculant lactic acid bacteria during ensilage of grass

Abstract

Fructan utilizing strains of lactic acid bacteria were assessed for their potential as silage inoculants during ensilage with herbage harvested from a mixed sward of ryegrass. The experiment included five treatments: uninoculated herbage as a control, herbage inoculated with one of three fructan utilizing strains (Lactobacillus paracasei ssp. paracasei P4134 and Lact. plantarum V54/6 and V57/5) or inoculated with a non-fructan degrading strain (Lact. plantarum P3775). It was clearly demonstrated that inoculant strains dominated the lactic acid bacteria population during the first 14 days of ensilage. All inoculated silages underwent a rapid lactic acid fermentation with lactic acid bacteria numbers increasing to > 109 by day 2 and pH falling to ≤ 4.0 by day 3. Rates of fructan degradation were, however, considerably reduced in silages inoculated with Lact. plantarum P3775 compared with the fructan utilizing strains. Inoculation with Lact. plantarum P3775 also resulted in slightly lower levels of lactic acid production during the first 14 days of ensilage compared with other strains. While rates of fructan degradation in untreated herbage were comparable with those observed in herbage inoculated with fructan degrading strains, resulting silages were less well fermented. The majority of lactic acid bacteria isolated from untreated silages were unable to utilize fructan and the observed rate of breakdown is attributed to plant fructan hydrolases and possibly activity of other micro-organisms. The proportion of L-isomer in total lactic acid formed during ensilage was also analysed. The highest proportion of this isomer was detected in Lact. paracasei ssp. paracasei P4134 inoculated silage (≃ 70%), with intermediate values in untreated silages (≃ 50%) and lowest values being found in silages inoculated with strains of Lact. plantarum (≃30%). This study highlights the potential of fructan degrading strains of lactic acid bacteria as silage inoculants and also that of strains of Lact. paracasei ssp. paracasei to manipulate the relative proportions of lactic acid isomers formed during ensilage.