Patchy,
From my point of view the pigs which you will be breeding from should have the absolute best feeding there is, in order to grow effectively with the right balance of protiens, vitamins and minerals. This will ensure their breeding is successfull. By that I mean they'll not be too fat to concieve and their bodies will be perfectly balanced in order for the piglets to grow well inside the mother and once they're born.
I know one of the feed manufacturers was talking about introducing a food especially for breeding gilts, but I'm not sure if it is available yet.
HTH
Karen x