I have successfully harvested yeast from the primary after racking to secondary but, when I was racking to bottling bucket I noticed a significant layer of yeast in the bottom of the secondary. My question is can this harvested yeast be used or will it be exhausted and insufficient to brew another batch?
I've successfully brewed with harvested yeast from secondary. It took a three step starter to get the proper pitching rate though. Now I just wash the yeast from primary. In short, the answer is yes you can. I have also read that the yeast isn't as viable coming from the secondary compared to the primary.
2 reasons why I normally harvest from primary rather than secondary. First, I was told the cells that have dropped in the secondary are less flocculant, and second, I normally throw a ton of dry hops into the secondary.
On Tap: House IPA, Mt Hood Kolsch, Fall Rye Red Ale
You can use the secondary trub but it will not be a viable to begin with and of course there isn't as much. I would suggest that you use the primary yeast unless you are in a bad scrape, and in that situation you could use it but I woud plan on a 2 or three step starter especially if it sits any length of time.