FlowchartRequired · Today 13:32
Yes, you are right about hay and straw. Back in those days (pre-car), people would be used to horses being used for transport and cavalry (linking back to the army), and they would know that horses - generally, as some will eat clean straw - eat hay and shit/piss on straw.
In the past, straw was starvation rations for military horses. Hay is much finer and greenish brown, whereas straw is thick, tough and yellow.
Interestingly, modern cows often have shredded straw mixed into their winter diet of silage, as it can make them constipated, the straw provides roughage.