The effect of oxygen concentration on changes in the microbial population, temperature and dry-matter content in grass silage

The effects of transfusing cores of grass silage with mixtures of oxygen and nitrogen containing 0%, 0·2%, 2%, 5% and 20% oxygen were investigated. Changes in populations of yeasts, lactobacilli and streptococci were measured and the relationship between populations after 7 days' transfusion and oxygen concentration were tested against the predictions of two models, based on Monod and logistic growth functions. Yeasts were related to oxygen by the Monod model, while lactobacilli and streptococci were related by the logistic model. The differences were ascribed to yeasts initiating deterioration by growing aerobically, while lactobacilli and streptococci grew fermentatively once the environment had changed following yeast growth. Dry-matter losses were related to oxygen by the Monod model. Changes in temperature were related to dry matter loss (through measurements of CO₂ evolution) using a simple heat balance model.