Methane (CH4) in the Earth's atmosphere is a powerful greenhouse gas with a global warming potential (GWP) 84 times greater than CO2 over a 20-year time frame. Methane is not as persistent as CO2, and tails off to about 28 times greater than CO2 over a 100-year time frame.
https://en.wikipedia.org/wiki/Atmospheric_methane
Converting CO2 to grass then methane is significantly worse than having it go back as CO2 in a stable cycle. We need to rapidly stop greenhouse gas emissions in the next few decades, because we're already starting to see how bad climate change is going to be.
Also, the population of cows currently on earth is unnaturally high, and convincing more people to adopt a carnivore diet would only raise it more.