This is not correct. We are already in a steady state. Methane levels due to cattle are already at their maximum (given a fixed amount of cattle).
At any point in time, gras will absorb some CO2 from the atmosphere, cattle will convert some gras / carbon into methane and emit the latter into the atmosphere, and some methane in the atmosphere will decompose into CO2. This is a continuous process, that's happening every second, and if the amount of cattle is kept constant, there is a maximum amount of methane that you can reach in the atmosphere as well as a maximum amount of CO2. After all, the total amount of carbon involved in the process is constant – energy conservation dictates that mass doesn't appear out of nowhere.
In fact, since we've been doing agriculture for a time much longer than the time scale in which grass grows, a cow digests grass or methane decomposes into CO2, this whole closed loop has already settled into a steady state: At any point in time, the amounts of methane and CO2 in the atmosphere due to cattle are constant. The resulting radiative forcing will thus stay constant, too.
