You didn't cause a problem with the acv, but it's probably that with her light weight and the panting/heat she needed more both in the way of electrolytes, and the nutrients. I suspect more that she might have been weak - but vision changes aren't yet ruled out so let's hope it's not that.
I understand that Buckeyes need more protein than other birds. Also she's a combo-purpose bird (meat and laying, not a broiler) and she's coming into laying age but isn't there yet. Correct? Would you say that her comb is still pale, tight, small, and dry? Not reddening, getting waxier and larger? If so, I would actually switch them to a feed more designed for developing birds for laying, not for slaughter (which is what broiler feed does). However I understand that Buckeyes are a breed that requires a little more protein so for her if you want to go that route, I'd use a gamebird grower formula. I know the last recommendation sounds odd but it's a higher protein and still aimed not for birds for short-lives, but longer lives as it's also for breeders. But I'm not sure I'd use broiler developer. Just my opinion.
When their combs start to enlarge, redden, and get waxier, that's when you'll start integrating laying crumbles into their mix as well as offering oyster shell, free choice, for those hens whose needs exceed the usual 6:1 cal/phos ratio of laying feeds. I like to give yogurt daily when I see the comb changes to help with a little extra calcium, protein (slight amount), and D3 as well as boost their gut for a period of stress. I find that doing this makes the first laying days problem-free mostly. Certainly not egg shell issues.
On her, I like the recommendations of all the others, too. Yogurt for gut bacteria in case there's something brewing and, if there's not, to improve her overall vigor and food efficiency. All the gals can have this. Personally because you describe them as being on dirt, and now free ranging, I would worm with Wazine. You can actually save the ivermectin if you still have that and follow up the wazine worming in 2-4 weeks with a BB-sized portion of ivermectin in the beak. We've had a few readers use this dosage, and the paste serves to slow the ivermectin down as it moves through the system making it a little more effective than the injectable used orally.
Then worm at least twice a year with the ivermectin. If you want, you can try more natural methods inbetween the twice-annual worming such as Cayenne, DE (food grade only), VermX, etc. I haven't before, but I'm about to try the cayenne personally. Then you just worm twice a year, or any time your gals get lighter than you like.
On the wazine, It really is easier to use the 17% piperazine for swine/cattle (liquid) if you can get it and return the dog/cat ... but in a pinch. I'm really unsure of the dosage conversion using dog/cat.. But we can try if you wish us to.
Keep her on the vitamins/electrolytes in the water for at least four days, and honestly I'd do it through til four days after worming to let her get caught up on nutrients.
I like your mash idea - try throwing a boiled egg in there was well as yogurt. That should be a nice power-snack for her to keep her energy up.
If you're concerned about her vision, try this trick: use a light (flashlight pen light etc) and shine it into her eye. Her pupil should contract, dilating again when you remove the light source. The eye shouldn't be greyed over, and the pupil should be normal in shape and size as the other birds - not contracted, not misshapen.
Incidentally, cedar shavings are bad. Old cedar aged wood for coops are not unless the coop is very closed in. I'd also make sure they all have a lot of free-flowing air and shade. There are some awesome posts on this board for helping during the heat if you're interested in checking them out.
Personally I think she's a Buckeye that isn't utilizing her nutrition well for some reason - either breed, feed, or worms - and that tweaking the nutrition as above as well as a worming as she's free ranging now should help. I hope so. In the mean time, look for ANY small signs of changes or things you didn't think about. Sometimes the tiniest bits of information really solve the mystery. let us know if you think about anything new, etc. Also look back to when this started happening and any changes that happened within 2 weeks of that period.
And personally, I would switch from grass clippings to horse stall shavings.
TSC apparently has great small ones that are super dry, not tending to mildew. It's cheaper than petstore or
walmart prices. Grass clippings tend to hold mildew and they can eat them. They won't want to eat the shavings (hopefully).
Be sure to check her crop (smell it as well from her throat) and make sure she's not slow or sour smelling. Check her vent for any clinging droppings and let us know what you find.
A summary of my advice:
Yes, worm with wazine. Reworm in 2-4 weeks with the ivermectin. Repeat ivermectin 2x's yearly at least, or more often as needed.
Yes, continue your 'glop'. Add yogurt, possibly boiled eggs. All can have the latter as well.
Possibly change the coop bedding to horse stall pine shavings.
Continue use of the vitamins/electrolytes in the water.
Take extra precautions against heat.
Change the food possibly to ramp her up for laying - add oyster shell at comb changes.
Let us know anything else you find.
Please do update us either way.
