I manually feed. Sometimes I can't tell if they are eating all that I throw out. Its a half acre pond and I usually put in about a 16 oz cup full a night.

We definitely have a pond scum problem.

More than likely that's not too much food - it depends on what you have in the pond.
Better answers will be coming forthwith.....

Mike it coudl be aiding in pond scum either uneaten falling to bottom of jst in more fish poop. So do you want to stop feedign no. I owudl combat algae with other means. ALso if you do not kow i like to have the food gone in 5-10 mins if taking longer then you may be indeed be feeding too much.

Slightly different issue, but still on feeding and time of year.

I am 85 miles South of St. Louis, and have noticed that since the nights have gotten cooler, the water temp is dropping, my feeding is received with less vigor than 2-3 weks ago. I don't necessarily want my feeding to become an issue on the pond bottom if not eaten.

What are the 'general' conditions/time of year when feeding become less effective an should be stopped?

IIRC you should see feeding slow down around 65* water temp and lower, stopping feeding @ 55* water temps for warm water fishes. I don't know what temp to stop feeding cool water species.

my one acer pond witch if only a yr old but was stock with 100 full size blue gill and 250 3in which are about 6 inch now are all feeding and i only can feed about half a 16oz cup full

Thanks guys. I'll cut back on my portions (and do the same for my fish, lol).