My point was that methane is a much worse green-house gas than carbon is and that keeping cows alive longer increases the methane production.
We do care about green house gasses still right? It isn't just mindless hate of plant food, right?
The bolded statement isn't meaningful in terms of net contribution to atmospheric CO2. Yes, if a cow lives twice as long it emits (say) twice as much CO2 in the form of methane. But it consumes twice as much grass, and that grass captured twice as much carbon from the atmosphere. This is an equilibrium state. This would be true even if a grass-fed cow were NEVER slaughtered and lived to a ripe old age.
Injecting "new" carbon into the atmosphere can occur only if fossil fuels stored in the earth are burned (or to a very small extent, if volcanoes erupt) and/or if the CO2 held in living things decreases (for example, if rain forests are turned into deserts).
Using corn for cattle feed depends on burning fossil fuels, which breaks the equilibrium, so corn-fed cattle are not as carbon friendly as grass-fed cattle. (Edit: This assumes "natural" grass grazing in situ. That is, this assumes that the grass used as cattle feed isn't grown as a commodity and shipped to cow-producing areas.)