Estimating methane
A standard way to estimate enteric methane is to take the gross energy a cow eats, multiply by the share lost as methane (called Ym), and divide by the energy in a gram of methane. This model uses 18.45 MJ of gross energy per kg of dry matter and 55.65 MJ per kg of methane.
Well fed cattle on forage diets typically lose about 5.5 to 6.5 percent of their feed energy as methane, and high grain diets lose less. The straight line this model draws between about 3 percent and 6.5 percent is a teaching simplification.
Try it: hold intake at 22 kg and slide forage from 100 percent down to 30 percent. Daily methane falls by roughly 40 percent, because starch ferments toward propionate, which ties up hydrogen that methanogens would otherwise use.
Why it matters
Methane is a potent greenhouse gas and also lost feed energy. Feed additives, better forage quality and breeding are all being studied to shift rumen fermentation away from methane without hurting the cow.