I didn’t realize you were keeping them with the horses. Brilliant. (Well.... for the DPs 

 )
I kept my yearlings around the chickens sharing pasture and it was the BEST desensitization ever. The chickens scratched through the manure immediately which was great for parasites but in my humid area there were still areas flies hatched. I have yet to find a chicken that efficiently goes after flies once they’re buzzing around.
I had more horses in less space though so that was my problem, so I moved them to a bigger field.
To the OP... for me it was my laying hens. I appreciate and respect my chickens and as I did more research on the meat chicken industry, I realized I would personally feel better eating chickens I raised and was in control of the process.
Whether or not it’s healthier for me I cannot tell, but I know mentally I feel enlightened and the mind can do incredible things for the body. I know for sure the chickens are happier and for sure it’s cheaper than purchasing any kind of chicken raised to any sort of standards close to what I can do, while maybe not cheaper than the rotisserie birds at the grocery store that have been dipped in bleach.