Functional groups
Fibrolytic (cellulolytic) bacteria
Attach to plant fibre and degrade it slowly. Sensitive to low pH: fibre digestion tends to decline when pH stays low. e.g. Fibrobacter, Ruminococcus
Amylolytic bacteria
Grow quickly when starch is abundant. A sudden surge of rapidly fermentable starch can favour lactate producers. e.g. Streptococcus bovis, Ruminobacter
Lactate-utilising bacteria
Convert lactate into weaker acids. They adapt more slowly than lactate producers, which is one reason gradual diet changes matter. e.g. Megasphaera, Selenomonas
Proteolytic bacteria
Degrade rumen-degradable protein. Ammonia is used for microbial growth when enough fermentable energy is available. e.g. Prevotella
Methanogenic archaea
Use hydrogen to form methane, which keeps fermentation running but is an energy loss for the cow. Propionate-rich fermentation tends to leave less H₂ for methane. e.g. Methanobrevibacter
Protozoa
Engulf starch and bacteria, which can moderate fast starch fermentation. Often decline when pH stays low. e.g. entodiniomorphid ciliates
Anaerobic fungi
Penetrate tough plant tissue with rhizoids and open it to bacteria. Favoured by long, fibrous diets. Neocallimastigomycota
Diet scenarios
- Balanced diet: Fibre and starch fermentation coexist; acids are produced and removed at similar rates.
- High starch: Amylolytic activity rises and propionate increases. If pH stays low, fibrolytic activity tends to decline.
- High forage: Fibre degraders and fungi dominate; acetate-rich fermentation and more methane per unit of feed tend to occur.
- Low effective fibre: Less chewing and saliva: buffering falls, the rumen mat weakens and fibre degraders are disadvantaged.
- Sudden grain increase: Starch fermenters respond within hours while lactate users lag behind; lactate can accumulate. This is why grain is increased step by step.
- Gradual adaptation: Given time, lactate users and rumen papillae adapt, so the same starch level is handled with less disturbance.