The expert witness, called by the defence, described the condition as "complex" and said researchers did not yet fully understand it.
He added: "There were an awful lot of animals that were in good condition. I could not see, in my mind, why some animals would die while others were in extremely good condition.
"To me, if animals were being starved, then the whole lot would show it."
Mr Parker also said witness accounts of Mr Gray pulling a horse onto a trailer by its tail and mane showed a misunderstanding of the correct treatment of horses.
He said: "I have got horses out of water jumps in that manner myself. Actually it's the way to do it. You can't hold them by their legs or their reaction is to give you a wallop.
"I quite accept that to a lay person this does not seem very good."