I will kinda agree and disagree. If you are only going to supplement with only .05% of their body weight, you can skip a few days. Cattle can EASILY be started at 1% of their body weight with grain. If you are looking to get good growth, that should be increased to at least 3% of their body weight. If you are just wanting to keep them alive, with very minimum growth, hay/grass will be just fine.
Since you sound like you are extremely new at this, has anyone suggested DEWORMING your calves? This is probably more important than anything if they have not been done.
You said you will keep them thru winter - then what are your plans for them? Just sell or try to butcher?
I have never raised a dairy cross or fed such a low amount of feed.