Hiya & welcome to TAS

Your questions.......
The difference between outdoor and indoor is the most obvious, it's not gamey so much, more that it's just full of flavour. If you're looking for 'gamieness' in the pork look to Iron age or Wild boar - but see my notes* below

Tamworths around 85Kg liveweight would be the 'optimal' time to slaughter (it's
usually around 6 months, but many things can affect it - so go by weight rather than age) after 85Kg they don't convert the food as efficiently.
You can't reduce feeding by much, even if they are outside - hunger would be a contributing factor to breakouts and possibly result in a bit of fighting over food

You're far better to stick to a pound per month of age daily (or what they can eat in 15 minutes) up to a maximum of 6lbs and just keep an eye on their condition - you
may be able to reduce it a bit, but you'd have to see how they got on.
I feed pig nuts or rolls - it's easier and less hassle than getting licences and having storage for mixing your own, but there are one or two folk on here who do mix their own I think, so they'll be able to advise on that

You have to be very careful with regard to not feeding anything which has been in a kitchen - completely illegal.
Litter size will vary with age of pig, breed of pig, boar used, condition & health status of both and can be helped by you with good provision of post farrowing care - probably around 8 would be average. And yes, most folk will be aiming to get 2 litters a year to keep conception rates and litter size high (they'll drop a wee bit if you're only having one litter a year)
I'm aware I'm bombarding the forum with questions but these are the last details that I hope to check off before diving in at the deep end.
I would advise against diving straight in

Not to put you off, but there's lots to learn before you start breeding. I'd advise raising a few batches of weaners first to make sure you enjoy keeping pigs and allow you to get to grips with the husbandry side and decide if you really want to go down the breeding route.
* Wild boar - NOT a starter's breed unfortuately, and you'll need a dangerous animal licence and all that goes with that to keep them. Iron age are a cross with WB and Tamworth, and don't require the dangerous animal licence but again, not really a breed for your first venture into pig keeping. Start off with something lop eared and a bit easier to manage

Have a read back through the pig section on the forum - loads of good advice there too

HTH
Karen
