Modeling the effects of propionic acid bacteria in silage
A simulation model of the growth and fermentation of propionic acid bacteria (PAB) in silage was developed 
from literature data with pure cultures or mixed cultures with lactic acid bacteria (LAB). Effects of temperature, pH, 
water activity, and lactate, acetate, and propionate concentrations on growth rate and end product formation were 
included. The PAB were assumed to grow in succession to the LAB in silage and to ferment only lactate. A three-stage 
simulation was performed: LAB fermentation using a previous silage-fermentation model, PAB fermentation using the 
present model, and aerobic deterioration using a previous fungal-growth model. Inoculation with PAB was predicted to 
increase the aerobic stability of alfalfa silages of 20, 35, or 45% dry matter (DM), but to effect only minor improvements 
in 35% DM corn silage. Inoculation of PAB in combination with LAB at ensiling had a synergistic effect on aerobic 
stability. Comparison with two silage studies from the literature suggests uncertainty in the pH sensitivity of PAB in silage 
which may reduce the effect of LAB and PAB inoculation. In general, inoculation of PAB was predicted to have the 
smallest potential benefit for silages most prone to aerobic deterioration.