How much is in a scoop? I cannot see that. Looks like they say 1 scoop per pound of feed.
My initial thoughts, before knowing the scoop volume are below.
1,000 mg niacin per pound of rooster booster. So one tenth of a pound would give 100 mg niacin, that would go into a gallon of water. But the directions are to add it to feed.
hmmm.
My Best Guess on Brewer’s Yeast Equivalents:
Okay, two tablespoons of brewer's yeast contains 10 mg of niacin, based on this page http://nutritiondata.self.com/facts/custom/1323569/2
To convert the Storey's Guide recommended amount of niacin in water to how much brewer's yeast to put in food . . . The guesswork is in how much water a duckling actually ingests in a day. If a duckling actually drinks a half a cup of water a day, it would be getting (16 cups in a gallon, 32 half cups in a gallon . . . . 150 divided by 30, more or less) 5 mg of niacin from water with 150 mg niacin per gallon. Which would be about a tablespoon of brewer's yeast.
So if you think a ducking drinks more than that, then that would increase the amount of brewer's yeast added....
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Many folks add a tablespoon of brewer's yeast per cup of food.