I did some calculations regarding the feeding of artillery horses to give an example of what they had to deal with logistically. I have posted it in threads in the past, here is a copy if it can help you.
As far as feeding horses, I did some calculations and the logistics would be even difficult today.
From the field artillery manual regarding horses:
"The Daily allowance of oats, barley and corn is 12 pound;that of hay, 14 pounds."
One Battery would have 120 horses (for mounted. For horse artillery, add about 75 more)
Note: the following numbers are based on an assumption that all batteries had a per-regulation amount of horses. In reality, I am quite sure that many did not have a full per-regulation complement of horses, but the only realistic numbers to use are per-regulation.
Per Day for a battery:
Oats, barley and corn: 1,440 Pounds
Hay: 1,680
a modern square bale of hay weighs about 40 pounds.
a modern bag of grain 50 pounds
Per Battery
42 bales of hay
29 bags of feed
At Gettysburg there were approximately 65 union artillery batteries.
65 * 120 horses = 7,800 artillery horses
65 * 1440 Pounds of oats,barley and corn= 93,600 pounds/day
65 * 1680 pounds of hay= 109,200 pounds/day
in modern square bails: 109,200/40=2,730 bales of hay/day
modern bags of feed; 93,600/50= 1,872 bags of feed/day
plus add in water
plus, what goes in eventually comes out - don't know how many pounds come back out of the horse, but for that many horses, that's still a major problem to clean up.
The reality is that there is no way that could be provided. Living off the land was notpossible (that many horses would have the ground stripped to dirt within days.)
also, here is a link to a document regarding Union Artillery at Chickamauga - it may have some information you can use:
Union Artillery at Chickamauga