After losing one of my cockerels, I discovered that my other 3 cockerels had similar, albeit not so severe issues. I did some online research, and determined that they were all suffering from "pendulous crop". Since chickens have no teeth, their food is ground in the crop, a specialized part of their esophagus that grinds the food, along with grit (dirt & tiny stones that they swallow) before it moves on down the digestive system. In some chickens, the crop can become impacted with food, or conversely, stretched and filled with liquid that doesn't move through. In some cases, this liquid can develop a yeast infection, called "sour crop", that can cause all sorts of digestive and malnourishment problems. This is what apparently happened to my 1st cockerel.
I realized that although the TSC chicks have been pastured on lovely, green grass, and moved every other day to a clean area, they do not have access to any grit to eat. In addition, I learned that although they get fresh water every other day, adding a smidge of apple cider vinegar to it can be very helpful in alleviating and preventing the yeast infections.
I immediately added ground oyster shell to the feed of all my young chickens, and vinegar to the water of the TSC chicks. Since then, 2 of the cockerels are greatly improved, and the 3rd, the one with the gimpy leg, is no better. It appears that we may need to butcher him sooner, rather than later.
I also researched meat chickens, and although I am still unable to identify the breed of chicks that I bought at TSC, I am convinced that they are meat birds. It is recommended that they be butchered between 5-13 weeks of age, usually at 6-8 lbs. I am working on a method to weigh them, but it may soon be time to have fried chicken, as they are 9 weeks old and quite large.
5 months ago