>  kill that grass cover crop early so as to not have to pay the carbon penalty.

Here, I worry less about the carbon penalty and more about the can't-get-it-planted penalty if I terminate early.

We had a triticale/clover cover on some ground last year that was supposed to go to corn, and killed it when the trit was 12-14" tall. Didn't think there was much risk because the stand of covers was a bit thin and the fields were well drained...

...but not thin enough. After a couple rainy weeks there was enough dead material to prevent the soil from ever drying out. If we had been planting green there would have been plenty of opportunity to plant corn, but this ended up being the last ground that got planted--to soybeans, not corn--around the 23rd of June.

Soils are likely different where you are, but here early termination is always a risk if going to corn.