Mangels are very common here in Bulgaria for cattle feed. And the seed is very cheap, about 5 euros for 20kg.
We have grown mangels for two years for our pigs and poultry. Last year we sowed an area of about 15m x 24m (so 360sqm), and having learned from our first year, we sowed in February (before the snow had all gone), late March (at our last frost date) and end of April. For each sowing we harvested half at about 12 weeks after sowing, and the other half 16-20 weeks after sowing (very large).
We had learned that the pigs
generally prefer them when smallisheat them/take them whole - never chopped them even for piglets (sow showed them what to do)will eat the leaf tops any time, very good for hand feeding/tempting youngstersour boar could eat 10-15kg a day (no other feed) and would pace himself... there was never any left next morningour lactating sows would get half their "normal" feed plus about 10kg of beets which the piglets started eating within a week of farrowing
We sowed in rows into a pig-ploughed and human raked plot. I think the mangel is particularly good because the bulk of the beet grows above ground making it very easy to hand harvest - don't know what your agricultural setup is but we are hand-powered. We have very long hot summers from May to October and we don't irrigate our fodder growing areas.
We also mixed in a good quantity of the mangel seed with our pig paddock mix (rape, kale, parsley, stubble turnip, radishes [large cooking radish and french breakfast type], beetroot, plus any old/out of date cereal/maize seeds we scrounge from
local farmers). We use this mix to re-green the pig enclosures after we move them - well often we will broadcast the seed just before they are moved as we found that they push the seed in and make it harder for marauding birds to steal the seed. We then pull out all the bedding from the pig
shelter in the empty paddock, spread it around and also spread the soiled bedding from our poultry accommodation over it as well. 12-16 weeks later there has been lush growth in the paddocks ready for pigs to move back into. Last year, with the weird weather, two of our paddocks had three sowings very successfully. Pigs definitely graze hard before digging up a paddock - the trick is to get the timing of a paddock-shift right before they switch into crater-digging mode.
We have tried storing mangels for use during winter - and I don't know if the critters just went off them or they didn't like the stored ones, but they were not keen unless we either starved them for 24-36 hours or boiled them up with potatoes and other veg to use as a hot slop.
Other things we grow for the pigs:
Sugar beet - pigs really love themTurnipsDaikon RadishBeetrootComfreyTobacco (great natural wormer)Jerusalem artichokes - we plant these in fallow paddocks, as well in cultivated rows for harvesting - a good winter stored root for pigs and chickens and geese seem to like them cut small. The pigs and birds love the leaf too.
We tried growing our own maize but we had two crops completely destroyed by wind storms, so won't bother with that this year.