Erin, Personal opinion only - no scientific background, but I think your cows know a treat when they see it. There are lots of trace elements in leaves of trees, and I think I am right in saying that maple leaves generally, and sugar maple in particular will also taste sweet. Cows under range conditions will browse many trees in the same way that goats will, and they just seem to have that knack of knowing what is and what is not good for them. In drought conditions in Australia we used to cut all sorts of trees (some reckoned to be better than others) for the cattle to eat the leaves and small twigs. Old McDonald.

Very toxic: Wild Cherry or just cherry trees. If memory serves, they are more dangerous in the wilt stage, such as broken from tree or frost. I believe it is a cyanide poison and a handful will kill a cow.....so I have been told.

Thanks Old McDonald. I never in a million years would have thought they would like the leaves, but b/c they are sugar maples, the leaves probably are a bit sweeter than most.
I bedded them w/ straw on top of the leaves just in case!

I kenw that cherry leaves were problematic. but I know that there are zero cherry leaves in the bunch. Pretty much all sugar maples.

shouldn't be a problem for the cows.
Red maple(A.rubrum) leaves - fresh, wilted, or dried can be deadly to horses - and I'd be inclined not to purposely feed sugar or silver maple to horses, just in case - so, don't use them to be a horse stall!
The cherry leaves are largely only problematic in the wilted state - as in an uprooted tree or large branch blow-down - fresh, crisp & green off the tree, yellow dehiscent or dry & brown on the ground should not be a problem.

Here is a link that might be useful: Red maple toxicosis

Lucky, thanks for the info.

Both of my calves have been eating all the dead leaves that have blown into the pastures for the past 2 months now. The cow isn't interested. Must be something in there that they need.

Prunasin, the cyanide-containing molecule of the black cherry leaf, is neither destroyed nor altered when the leaf wilts. The completely healthy leaf and the completely dried leaf contain prunasin. Wilting occurs when a leaf losses water and, If unchecked, dries to a brittle state. The cyanogenic potential of the leaf does not change during this process.

To liberate cyanide gas, the dried leaf must be crushed and mixed with water; the intact leaf, just crushed. Crushing results in prunasin mixing with an enzyme also found in the leaf. This mixing initiates a process that leads to the liberation of cyanide gas. A second compound, benzaldehyde, is also produced at the same time. Benzaldehyde has the distinctive odor of bitter almonds and at room temperature becomes noticeable 15-30 seconds after a fresh leaf is crushed. If you smell benzaldehyde you are inhaling cyanide.

To see a demonstration of the killing power of the dried leaf of the back cherry: https://www.youtube.com/watch?v=Fj7q5ORjxkg

For more information: https://facultyweb.cortland.edu/fitzgerald/three%20caterpillars.html