Blummel and co-workers (1997) introduced the concept of apparent Fermentation Direction, defined as the ratio of degraded substrate to gas volume produced. They demonstrated an inverse relationship between gas production and microbial biomass yield when the variables were related to a given unit of truly degraded substrate.
aPF is calculated as: (short chained VFA yield + microbial protein production)/total gas production. Higher aPF values indicate fermentation patterns yielding relatively greater proportions of propionate and butyrate and lesser proportions of acetate, carbon dioxide, and methane. This would yield a greater supply of ATP, which could support greater microbial growth. However, acetate is a key substrate in the formation of milk fat, requiring a balanced approach to dietary manipulation of fermentation end products.
Interestingly, forages with a high PF (e.g. low gas production per unit of truly degraded substrate) exhibited higher intakes. Their dry matter intake prediction model included rate and extent of 24-hour gas production along with aPF; it accounted for 84% of the variation in the intake of fifty-four forages in their research.
