Plenty of factors can affect the rate at which a pig gains weight, among them breed and gender, age, feed type and quality, feeding frequency and quantity, ambient temperature and exercise amount. While you can't control all factors, you can control enough of them to reasonably expect to help a pig to gain weight as rapidly as possible.
What to Expect
The Washington State University Extension website says you can expect a pig to gain between 1.5 pounds and 1.7 pounds per day until he weighs about 110 pounds. After that, a pig should gain between 1.8 and 2.2 pounds per day. To keep your pig on the high side of these numbers -- growing as fast as possible -- feed starter ration until the pig weighs about 100 pounds, then a good growing ration until he weighs about 170 pounds, followed by a finishing ration to get him to the desired weight. Expect to feed a single pig at least 4 pounds of feed per day to encourage growth.
A pig's dietary requirements change with his size. Younger, smaller pigs should be allowed to eat as much pig starter as they want. Pig starter feeds are 75 percent to 80 percent carbohydrate sources such as corn and sorghum. When a pig reaches about 170 pounds, switch to a feed that is lower in fat; otherwise his weight gain will be mostly fat.
Encourage feed intake by adding flavor-enhancing supplements to your pig's food. Doing so works best when you are not employing free-feeding; a pig whose feedings are scheduled will eat till all the food's gone, by habit -- so you can add a little more at a time. Boost consumption by adding up to a half-pound of whey, milk replacer or liquid protein supplements on top of his food at each feeding. Or, crack two raw eggs over his food each time you feed him. A pig must always have fresh, clean water.
Other Ways to Spur Growth
Make sure your pig is healthy. Worm him regularly so he gets the full benefit of his feed. Keep him at a comfortable temperature, because if your pig gets too hot, his growth will slow. He'll do best at about 60 to 70 degrees Fahrenheit. One way to keep him cool is to hose him down several times a day. Keeping him cool will encourage him to eat more feed.
A bit of exercise, such as a short walk three times a week, will reduce fat stores while allowing muscle growth and development, but too much exercise may slow his growth. Add garden or table scraps to your pig's feed to add flavor and calories, and to encourage your pig to eat more.
Foods to Avoid
A pig grows best when he's fed a balanced diet of food made specifically for pigs. While it's okay to add some extras for flavor -- which will increase consumption -- it's not a good idea to replace any significant amount of your pig's diet with anything else for any length of time. Some feeds to avoid include dog food, sweet feed and cracked corn. While your pig will eat these foods, they are more likely to slow growth than to boost it.