Fiber content should not be an issue unless other nutrients are particularly low. If your base diet is something like a layer ration where formulation is intended as the sole nutritional source, then dilution of the protein content by addition of a high fiber item that is also lower in protein can result in an overall reduction in bird’s protein intake. Most knowledge based on controlled experiments involves the base diets that are very close to the minimum needed to give good production (least-cost diets). If you are in the habit of using more nutrient dense formulations with higher protein contents then increasing fiber is much less likely to be problematic. In some cases upping fiber content can be beneficial with otherwise richer diets. In some instances fiber can interfere with availability of other nutrients, especially minerals like phosphates, when such are otherwise only minimally available in the formulation.
My free-range birds consume far more fiber than just about any confined birds, especially when later consume at least some feed or grains yet the nutritional status of the free-range birds is very difficult to match by birds fed even the richest formulations. The birds thus can compensate within limits for high dietary fiber levels. In my case the birds seem to target animal prey preferentially (also a source of fiber) and simply eat more when dietary fiber levels are otherwise high. Chickens are well suited for processing fiber and evidence for abundance of its intake can be found in feces as well as in the crop.
