Anybody who knows what the 'meat percentages' are for the different rare breeds?
Eve - great question, as it gets to the heart of the difference between forumulas and guidleines.
In the really commercial indoor world, a pig producer needs consistency is his product - every finished pig should be the same as far as possible. To achieve this you need to control 3 factors, source, input and output. Source means the breeding of sows and boars that are as perfectly close to each other as possible - ie every sow they buy from specialist supplier of breeding stock (and there are several in the UK) should be the same. Specialist breeders work very hard to achieve this. Their customers don't want sows where one produces long pigs (lots of chops) and next door produces chunky pigs (lots of ham), they want every pig to be the same so if they want long pigs, every pig they buy should produce long pigs. Second input - feed in commercial units is very tightly controlled, both in quantity and formulation, and changed at every stage of growth to be the optimum for the pigs. Finally output - what energy the pig expends. This covers movement and energy needed to keep warm. Hence indoor units are temperature controlled and the amount space enough that the pigs move (meat = muscle so if you don't work muscles they don't grow), but not too much that they burn off and waste energy.
Now in this indoor world, you can talk of formulas as the final product is so tightly controlled, and indeed adjusted to get to the perfect answer.
Contrast that with us rarebreed lot – On source - there is lots of different bloodlines, and lots more variation as these are intermixed, hence why most litters only have a few that conform to standard. Within any typical litter you will see long piglets and chunky piglets, these will produce different meat ratios – if you want bacon you choose a long pig, as that’s where the bacon comes from.
On input – most people keep theirs outside where they pick up nutrition from the environment as well as what we feed, and most of us don’t weigh the feed and ensure every pig has the same. Many have varied and wonderful diets which will affect bone growth (size), muscle and fat.
On output, our outdoor pigs will need to expend energy to keep warm, and energy to forage.
Now since all three of these factors contribute to the amount of meat in which areas of the pig, it is not possible to produce formulas for different breeds, at best you can get an average – a guideline. So just as you can say that the average white british male is somewhere around say 5foot10, it doesn’t mean my child won’t be a Tom cruise or dragon Peter Jones (5’7 and 6’7” respectively).
Now having said all that some races do have traits – eg Chinese will tend to be smaller, African Americans are more likely to play basketball as height is needed. But just look around at your family to see how much variation there is within the broad trends.
Switch that to pigs, and yes some breeds tend to be longer, some tend to have larger hams, but not consistently enough to give % formulas on eating, as which piglet you choose from the litter, what you feed it, and environment play far larger parts in the finished product.
This is why the “which breed is best” question asked on all forums every month is really irrelevant, and why I say go for what you like the look of!