A diet two weeks before slaughter will knock off some excess fat.
After 6 months of age, most pigs will tend to start to put on fat, so at 6 months you need to start to regularly feel your pigs, and feed to condition rather than formula. Easily feel the backbone, too thin, cannot find backbone too fat. You should be able to feel that backbone when you press on the spine.
Feed wise, a proprietary pig nut will contain the correct balance of the various constituents needed - Energy, Protein, Minerals, and vitamins. When you deviate from this , you need to consider the impact.
Commercially gown pigs wil go through up to 8 different feed formulations in their brief 5 month life to get just the right amount and balance of nutrients to grow them fast. Not what I would want as the result is cardboard, but does show how key the right balance is to get a supermarket spec. pig.
Barley for instance does not contain enough lysine which is needed for muscle growth, so much is used for fat instead - as part of a sows diet this may be ok, and if just growing for food for yourself, again might be ok, but if selling the result to people who look for supermarlet style meat you need to watch the body condition.
Some people do make the mistake of thinking that any food source will grow pigs, so look for a "cheap" alternative to proprietary pig nuts. This can (can, not will!) be a false economy, as the feed just goes to producing fat rather than meat, so any "saving" is wasted. The correct balance of energy (fats/oils), protein (and esp. the right sort of protein), vitamins and minerals are needed to grow a piglet at a good speed and in the right way. If the cheaper alternatives worked well, the propriety feeds would just contain that and cost less !
Nwo don't mis-understand me, I'm not knocking those that do supplemental or alternate feeding of fuit/veg etc. or use other ingredients in home mixes - we regularly feed fruit/veg and those people who can do home mixing and save some money are great - but it is much harder to produce supermarket spec that way, so you should do this with your eyes open - and feel for fat.