I read a couple of articles and a reference study, trying to find the answer. Those did say grinding was the preferred method of feeding, but the reason behind that was directed more at large scale feed operations. One reason, as Fred stated, is that feed intake is influenced by particle size. The concern is that the poultry won't eat whole peas. I've found that my own chickens will eat whole peas after all the other choices have been consumed.Thank you so much!! I could not think of a reason why but everything I read says to crack them.
There is also higher digestibility, for broilers, if peas are ground with enzymes added and pelleted. This wouldn't apply to someone wanting to feed a small flock.
It also stated a risk of buildup of material in the beak if fines are produced by grinding. I would think this would be more of a issue for someone grinding their own peas. So leaving the peas whole would work best for most poultry people.
You are really fortunate to be able to get all that feed! I'm going to try growing our own field peas this winter to reduce the cost of feed for my omnivores.