It doesn't pay to feed Pumpkins to Pigs. Most of you may know this but many Newbs and I did not. I know that you get one Pumpkin per seed. I was unable to play earlier today and am a little late in growing my pumpkins so I decided to feed my piggies instead of taking time to go to the Vendor. I removed 30 pumpkins from my SDB and went to the piggies in my back yard (only 3). I fed each one 10 pumpkins. The options showed that I could do that. Well, even though I was told I was feeding 10 pumpkins to each pig, I, in fact, only fed one at a time! Each plop gave me only one seed (not unexpected). I had to feed 30 times to get those pumpkins into the 3 pigs. This gave me only 18 seeds! The result is, it is tedious and unprofitable to feed pumpkins to pigs (unless they are starving).
ETA: I did this again. I fed one Pumpkin to each pig and got one seed back. I repeated this 3 times and each time I got one seed per pumpkin which is what I expected to start. I think the difference may be that the first time I did it I fed each pig 10 pumpkins according to the options when, in fact, each pig only ate 1. I did that feeding of 10 several times. this may have something to do with why I got only 18 seeds from 30 Pumpkins. Tedious though it is, you can eventually get one seed per pumpkin if you take your time.