Most people forget, grain IS grass. Yes, there are problems with a heavy grain diet, but remember, some of these steers are going to slaughter around 14-18 month. That is nearly twice as fast as grass fed/finished, depending on the quality of your grass.
In the winter grass doesn’t have much nutrition to it. Kind of like trying to survive on iceberg lettuce. You have to feed hay then that generally still has the grain attached to it.