If we were just talking about the CO2 exhaled by cows I would agree. Plants consume CO2 to grow, cows release a lot of that CO2 back into the atmosphere. It's a closed loop (if we ignore all the energy we spent on farming).
The problem is that cows also release a lot of methane. Methane is carbon based (CH4), but it acts very different from CO2. It decays faster, but it a much more potent greenhouse gas than CO2. It's around 80 times more potent in the first 20 years, or 25 times more potent in the first 100 years. So having plants absorb CO2 only for cows to turn it into methane is pretty bad on the timescales we care about.
It's not necessarily the biggest of the problems with cattle, but it's worth preventing on its own.
The problem is that cows also release a lot of methane. Methane is carbon based (CH4), but it acts very different from CO2. It decays faster, but it a much more potent greenhouse gas than CO2. It's around 80 times more potent in the first 20 years, or 25 times more potent in the first 100 years. So having plants absorb CO2 only for cows to turn it into methane is pretty bad on the timescales we care about.
It's not necessarily the biggest of the problems with cattle, but it's worth preventing on its own.