From Diammonium Phosphate MSDS sheet for fertilizer:
"Ingestion (Swallowing): Low degree of toxicity by ingestion
TOXICOLOGICAL INFORMATION:
LD50 (oral, rat) = 6500 mg/kg, >2 g/kg; (Which is g/(kg of body weight) taken orally by rats to kill 50% of the population.)
Inadequate mutagenicity, target organ or developmental toxicity data located for diammonium phosphate.
No carcinogenicity data located for diammonium phosphate."
if we use 2g/kg, then 3g/gal -> .375g/pint, so a population of 180lb (82kg) rats would have to each drink 437 pints all at once to kill half the rats at the pub?
That's just back-of-the-napkin calculation by a lay person, but someone needs to double check that.
However the yeast are going to consume some of that in some way, right?
edit: Turns out that the weight is about 3g in a tsp. I screwed up the calc using molecular weight. I corrected my numbers above. This looks a lot better, but someone please check.