I switched from a Purina brand feed to a semi-locally sourced feed that is soy-free and non-gmo. These hens have pretty much been on Dumor or Purina all their life, and I don't think they ever needed the oyster shell with those feeds. All of my hens eggs had begun to weaken (they always had shells harder than supermarket eggs), and one was laying eggs that were just the inner membrane. With adding the oyster shell to the feed the worst one started to lay eggs that had a super thin shell, so thin that it was already broken when I would go to collect them, but did not continue to improve. Since I realized the hens were having issues with the calcium and I was feeding the same brand feed to my chicks I started blending the chicks' feed with another brand of soy-free and non-gmo feed. I have not noticed any signs of calcium deficiency in the chicks. I plan to start blending the feeds for the hens as well, and will stop adding the calcium when I do to see if the soft shell problem reoccurs. So long as it does not happen on a regular basis I will only add the calcium when it is an active problem.
With the calcium spray I was doing 4-5 sprays on their feed once a day (I have 5 hens). Kept that up for about a week and a half. Then started spraying about every other day. Going to take it down to once a week. The spray has a minimum calcium value of 1700 ppm, that is from three different types of calcium. I started to see a vast improvement in the eggs after 3 days, still a weak shell but it made it out of the coop.
With the powder you could probably offer it free choice like you would the oyster shell. Mine won't touch the oyster shell, or I would have gone that route to solve this issue. I even tried mixing it in with the feed, the only stuff that they ending up eating was the really finely ground stuff, the larger bits were all left in the bottom of the feed dish. Tried that for a couple of weeks before resorting to the spray.
Birds are not all that different physiologically than reptiles, so just follow the dosing instructions on the powder or spray. Typically you would lightly coat the insects or other live food to be fed to the reptile. You do not want to do that to the feed, smaller bits mean more surface area that can be coated by the powder. If you are worried about overdosing them get some insect treats to coat with the calcium to start off with. There is probably a youtube video that will show how to dust crickets. Go for a coating about like fresh cinnamon and sugar donuts, not packaged powdered sugar donuts. You could also gut-load some mealworms or crickets with the calcium, or offer soldier fly larvae which are naturally higher in calcium. Gut-loading means feeding the calcium to the insect then feeding the insect to, in this case, chicken. You can always start off with once a week for a couple of weeks to see if there is improvement. If there is improvement stick to the once a week regimen until the shells are back to normal, then discontinue and dose again when the shells begin to weaken.
Another option you could try is grinding eggshells to offer free choice. I'm pretty sure that I have seen a post on here about doing that. With an existing soft shell problem I would mix it with the feed for awhile.
Since I plan to combine the hens in with the chicks in a new coop, in the future I will be offering either ground eggshells or calcium powder free choice. I won't be able to use a layer feed, and will be using a flock feed since I have roosters now. Will have to wait to see how the current hens' eggs do after this switch. If they go soft again I think the easiest way to get them calcium with be the gut-loaded insects for treats, that way I can more easily control which ones are being "force-fed" the calcium.
Hopefully all that helped to explain. If you still need clarification please ask.
