I would get a soil test to know more. In addition to iron you want to see selenium. Without these plan on feeding minerals. Kelp is a good source of minerals. A cast iron frying pan or other iron in their water trough, or better yet whey trough, is a good thing. With good soil the iron shots aren't necessary when the pigs are on the soil. In the winter we have the issue that we're up high on snow pack for four to six months. In addition to the kelp I make up pails of dirt in the fall that I give to the pigs as well as simply taking up a scoop of dirt with the backhoe when I move hay around to feed in the winter. Big pigs don't seem to have a problem in the winter, just piglets, weaners, shoats and growers.
Cast iron in the water trough! Woah. Thats awesome! Yeah, it seems soil is answer to everything isn't it?
Where we are we have wonderfully mild winters and ferociously hot and dry summers. In the winter the soil turns luscious with worms and water and goodies, and the summer it's a clay brick. Making up for deficiencies with seasonal drought resistant fukoaka seed ball cereals combined with salatin rotational style seems to be the key to good pastures for pigs here.
Photo Blog of our Off The Grid Permaculture Farmstead:http://mulchutopia.tumblr.com