Farmers who produce animals on pasture aren't really a homogenous group. Each farmer makes their own choice as to what sort of farm they'd like to have, and how much work they can devote to it.
Raising pigs on pasture means that the animals have a much wider area to roam, and are usually in a larger social group. the pictures above show a cohort of 110lb grower pigs at my farm. The animals in these groups will tend to hang around each other, and seem to move around the pasture by consensus. These pigs are after the water, and a little bit of the green grass that hasn't been killed by frosts.
While they do eat the green grass, and when that's not available, small amounts of hay -- a pound or two a day for a larger animal -- that forage doesn't provide anywhere near the amount of calories to promote pig growth. I'm not even sure that a pig could maintain weight on a diet based entirely on grass or hay.
But experience has shown that given a choice, they'd prefer a little grass or hay, and seek it out. That's part of the reason I use hay for bedding the pigs instead of straw. The pigs seem to prefer it, and every pound of hay they eat is probably a little less feed they'll eat. I'm buying hay for $120/ton, and pig feed for $300/ton, so pound-for-pound the hay is cheaper, and the volume of hay is much bigger as well.
There's a lot of discussion about raising pigs on "forage". The term describes both what an animal might find on its own, out in the pasture, or so-called "forage feeds", which are gathered and then transported to the animal -- like baled hay, or silage. I don't know of any farmers that are successfully raising pigs on a diet that is primarily forage. Every farmer I know is feeding their pigs something additional.
Free food
If you're thinking about buying a pig from a farmer to eat, it's perfectly OK to ask that farmer what they're feeding the animal, and in fact, that's the first question that I'd ask. The second would probably be breed of pig, and the third is price.
My biggest single cost in raising pigs is the feed. I happen to be located next to a highway, and get offers of food from time to time. Food banks that get a donation they can't use, people who ship food for a living who have a spill, accident or fire that prevents them from using the food for humans, or cold-storage facilities that have to dispose of food for various reasons. The cold storage facilities are actually a good place to look for food because they deal with food in industrial quantities -- nothing sucks more than to have to pull 1,000 loaves of bread out of individual wrappers, for instance. A
tote or supersack of bread dough is a lot easier to deal with. I've got no connection to that packaging company, they just have nice pictures of various bulk packages.
BEWARE FOOD RECALLS
You need to be very careful that the food you are offered is not the product of some sort of recall, or if it is, that the recall is for reasons unrelated to the wholesomeness of the food -- improper packaging, for instance. We live in a time when millions of pounds of food is recalled and has to be disposed of on a regular basis, and you don't want to get stuck with food that will make your animals sick.
The key element of all of those is that none of the feed can be gathered after it's been exposed to people. The simple reason for this is that pigs can catch human diseases, and if you're feeding your pigs post-consumer food, you're going to have to heat it to kill any pathogens that might be contained in it.
This typically doesn't apply if you're feeding your pig scraps from your own table -- whatever your family has probably won't hurt your pig, and the risk is fairly low. But if you're dealing with plate scrapings from the general public, the odds increase.
I'm not going to go into how to heat food because I don't do that, and I'm not an expert in it, but I am going to encourage you if you're looking for free food to be aware of the pre-consumer and post-consumer food issue.
So here's some stuff that you might find; these have all been offered to me in the last month or two, do any of these have a disease issue?
1) Expired cottage cheese from the dairy case
2) Wilted produce from a produce stand
3) 600lb barrels of condensed milk
4) Food scraps from a restaurant
5) Donated food that couldn't be used at a soup kitchen
6) one-ton totes of pureed pumpkin
7) Pressed soybeans from a tofu factory
8) buffet line food
9) spent grain from a brewery (byproduct of brewing beer)
When you're offered food, it's a good practice to ask why they can't use it. The pureed pumpkin in the above examples was caused by a forklift driver who speared one of them, smearing the outside of all of the other totes with pumpkin, and the recipient of the shipment refused to accept it. The contents were good, it was just messy.
Rejected food:
Examples 4 and 8 are both sources of food that I would likely not accept. I can't trust the restaurant to have successfuly managed to keep plate scrapings out of the food containers, and I can't even say that the containers they put it into aren't contaminated with food.
Suspect food:
Examples 3 and 6 you're going to need to ask why they have to get rid of it. #3 I've already explained; that was fine. #6 is suspect though. Dairy is a super food for pigs, but why are they getting rid of it? Sometimes it's as innocent as a poorly mixed batch; in other cases there's some sort of contaminant that makes it unfit for human (and in my opinion, pig) consumption. So ask, and use your best judgement as to whether you can trust the answer.
Spent grain, also known as distillers grain, is an interesting feed. It's essientially a low-calorie porridge of barley and wheat and cows, pigs, sheep and poultry all eat it. At times breweries will even pay you to take it away. So if you can find a microbrewery in your area, this is often a good source.
None of these feeds I've described are complete feeds. You'll probably have to either add or allow the animal to select, various trace minerals and vitamins that might be lacking from a particular source, and I really can't tell you how to do that efficiently. I look at my animals every day, and watch how they take their feed, how much, and what their poo looks like to evaluate their condition.