Your birds are likely suffering from malnutrition. If they have worms, then it could be even more exaggerated.
Layer usually has about 16% protein. Scratch around 8%. Chickens need a MINIMUM of 16%. Scratch should be given as a treat at no more than 10% of their total daily intake. Grapes and pasta, same thing. Start thinking high protein treats like meal worms, left over beef, chicken, or fish, peas, any extra eggs... that type of stuff. I would cut the scratch completely
until you see improvement and then after that use it as a special treat that they come running and associate with you.
The issue with scratch is that it has calories but not much nutrients. It's on par with a bag of chips or candy bar IMO. Although chicken feed is often mostly corn, it has added vitamins, minerals, and amino acids. It's actually the amino acids in the protein which are key. Plant protein is where they get a lot in their feed, but they NEED animal protein. Feeds that don't have animals products in them, have the amino acids added in separately.
Now your hens that stopped laying, are they old enough and actually just had a molt? Feathers are made of 90% protein and hens stop laying in order to put the energy to growing new feathers. It can be a very stressful time for them. And lot's of times illness will rear its' ugly head during this time when the birds are in a little weakened state. Adding protein to their diet will help them to recover faster and maybe even return to lay a little sooner.
I feed a flock raiser with 20% protein and offer my oyster shell free choice on the side. Anything 16% or higher and 30% or lower is acceptable... Reason I do this is chicks, broody's, molting hens, roosters and juveniles don't need the extra calcium in layer and it *could* (doesn't mean will) cause kidney failure if fed long term. And I have those all in my flock most of the time. In fact Marans and Silkies are the two I breed.
Oh wait, I just realized we are talking about Marans which is a dual purpose bird and not a light bodied layer! They should be raised more like meat birds than layers. I wouldn't feed anything less than 18%, personally. A link to feed guidelines... (one misprint says 5-17% and should say 15-17%)...
http://ucanr.edu/sites/poultry/files/186894.pdf
I will also note as a breeder, I have read studies showing better hatches and chick vitality when the parent stock were fed a higher protein diet. To me that says better nutrition in the eggs that are being fed to my family.
This is only one part of the equation and why I always ask what people are feeding.
I would likely also stop the Corid since I haven't heard anything that sounds like cocci. And I would get some vitamin supplements, maybe some electrolytes, and probiotics on board right now. And start making plans to send a bird out for necropsy if you do have another pass. It's the only way to know for sure what you are dealing with. Some states do for free as a means of protecting our nations food supply chain. It cost $40 here in CA.
I will be back in a moment with a homemade electrolyte recipe.
One more question... how many birds in what size space, and you current or recent weather conditions?
I'm only sharing my experience and am NO expert. If something I say doesn't make sense to you, please do your own research and then do what you think is right for you and your birds.
