I forgot to mention that she's Morgan and apparently they are the poster child for IR. Some Morgan owners say that all Morgan's have IR but some are not symptomatic. So I figured better safe than sorry and added the supplement and reduced her feed.
It is true Morgan's are Poster Children for IR but, so are Arabs. Yet I have a 28 year old Arab who shows no signs in his blood work of high insulin or cortisol levels.
There are exceptions
That said, your Morgan looks great and it once upon a time, I'd say she is "bout near perfect" but these days I have two formally diagnosed metabolic horses, so if she lost a few pounds, it wouldn't hurt her
While I understand and sympathize with the barn owner's "one size fits all". It really is a stupid crock of you-know-what-concept. Its ONLY well-served purpose is the ease of feeding for the barn help.
In this day and age, the barn help AND the BO should know exactly how much that scoop of feed weighs on a scale, for starters.
Second behind that, I understand that some horses in the boarding barn are probably being schooled pretty hard and can easily tolerate the grain in that feed.
However, the barn owner should wise up and supply a low starch ration balancer for the horses who don't do much of anything. As hard as I used to trail ride, I was basically a weekend warrior because I worked 40 50 hours thru the week. More than one vet said my horses were being over-grained and would never burn all those calories up, so I cut everyone way back.
THAT conversation was in the late 90's, long before everyone was discovering they had horses with metabolic issues
Yes, this is a rant against your BO for that idiotic "standardized feeding program. She needs to offer at least two feed programs:
1. Feed for horses in serious, every day work.
2. Ration balancer for horses not doing much of anything and need low starch.
EDITED TO ADD: You posted those adorable pics while I was typing. I love love love the old Lippit Morgans. Ana's dam looks to have Lippit breeding. Years ago, I had a backyard bred Morab who was Eyptian Arab and Lippit Morgan cross. He was a solid blood bay and the most adorable, loving, loyal horse. I laid him to rest when he was 27 due to low ringbone from his previous owner's sons racing him against cars on the macadam road:(