Determination of live yeast

[Ken Hazen]

Hello,
I would like to determine how much of a yeast culture is active or
alive. I am going to conduct a trial to determine the benefits of
feeding active yeast on animal performance. Not only do I want to learn
if their is a benefit to feeding yeast to animals, but does it have to
be active to show the response.  Is there a simple test to determine
amount of active (live) yeast in a feed grade yeast product?
Thank you in advance for any help you can give me.
Ken Hazen