This is fully dependent upon how it's fed, you can simply state that most of the pepper powder will be consumed as there is never a shortage of 'powder' waste in my feeder that goes uneaten every week, as I'm sure others notice as well, if you are using a small amount of pepper powder that waste could be a huge percentage of what was fed... The amount consumed in either case will depend greatly on how it's offered and fed...
There is also nothing saying the alfafa can't be milled into a powder or mixed into a fermented feed, thus making the potential 'waste' likely very similar to powdered pepper fed the same way...