Pigs needs to be fed a daily ration all year food with the main ingredients being a compound feed or a mixture of straights. Free range pigs obviously have a chance to root for added extras and pick up minerals from the soil but even then those mineral levels will depleted over a number of years of keeping pigs at pasture if the land becomes pig sick.
John and I both add minerals to our compound feed as we feel even scientifically formulated rations can be deficient and certainly mixed straights are likely to need extras.
Minerals are readily available and you just add to the food at feeding time. Tithebarn.co.uk will be happy to help.
We also add seaweed which is a good supplement for all livestock.