FWIW, there is an interesting article in this week's Wisconsin Agri-View newspaper Livestock section that quoted David Lalman from Oklahoma State. Lalman says data shows it takes 27 pounds of dry matter to the cow to increase weaning weight of a calf 1 lb through milk. The article then went on to say it takes around 9 lb of dry matter to put on 1 lb of gain on a similar weight calf when fed directly to the calf.

This article was about calf weaning weights and how they have plateaued. It suggested the way for a cow/calf producer to make better use of his feed resources is for the cow/calf person to background and feed his own weaned calves (to maybe 800 lb?) and take advantage of a 9:1 DM conversion rate rather than a 27:1 DM conversion rate when the calf is on the cow.

So if the OP's calves gained just 1 lb/day does that mean they were only getting 9 lb/day of dry matter?

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In my own situation, my current calves near weaning (around 600 lb) are eating hay as well as nursing the cow so they are getting some 9:1 gain as well as some 27:1 gain through the cow. It would be interesting to know what that proportion might be.