Here's a few tips for successful breeding and subsequent offspring production. Once you tame another boar for breeding, it's a good idea (but by no means essential) to have a small pen inside your larger pen. Push the breeding couple into the smaller pen and throw down a stack of 2-4 foodstuffs. They should come together over the food, and while in close proximity to one another, you will see a burst of hearts explode from the boar which was successfully inseminated. Your character has to be in the general vicinity of the boars for them to both eat, mate and produce offspring. Feeding them, then running away and zoning out of their vicinity will not produce any results. It is a good idea to have your boar pen in either close proximity to where you sleep, or an area you are active in for extended periods, like your gardening plots. I keep gardens inside my large boar pen, which is also right beside my sleeping quarters. So we are in close proximity to one another when I am planting and when I am sleeping. I sometimes also turn auto-loot off while harvesting certain portions of my crops, and leave them laying on the ground for my boars to eat.

If you use a smaller pen inside a larger pen for breeding, make sure to empty it out regularly, as boar will not produce offspring when the area they occupy is cramped. I also have a star shaped boar pen, and throw food down in the points of the star, as it forces them into close proximity to one another as they contend for the food, which facilitates successful breeding. Only happy boars will breed. If I feed them before I sleep, I place 2 food at least for every boar in the pen, this ensures they will have a meal before I sleep, and they will have enough food to eat in the morning. Boar are always hungry again after a sleep cycle.