They will continue to produce milk even after babies are weaned IF you continue to milk them daily. Goats can stay in milk production over 10 months out of the year.
It all depends on how much milk you need. Nigerians have a higher percentage of butterfat...makes wonderful soap. Nubians also make wonderful soap....with either breed, do you research on bloodlines and make sure they come from good milking stock.
On the other hand, I have saanan x nubians with does reaching 155lbs. They make 2 gallons of milk a day. The herd queen milked through 2 years and 2 kiddings until I forced a dry off. Her milk is extremely fat heavy. When her milk is allowed to settle in the frig the cream rises to the tops of the jars and is 3 or more inches thick.
In my experience with goats and goat's milk and soap making - milk is milk it all works about the same, cow milk, goat milk, buttermilk, yogurt, sweet cream, etc. The % of fat in the product is what adds to the super fatting of the soaps.
Once your goats have kidded and the babies are doing well, around 6 - 8 weeks you can start milking for yourself. Once the babies are weaned completely milking them twice a day (every 12 hours) will keep them in milk for about 10 months at a time.
I do not like the lamancha's. My 2 oldest does make the lamanchas around here look small. LOL
I am trying to find someone with a big saanan buck to breed my herd queen to when she comes back in heat this fall. She needs a big boy this year. No more medium sized bucks for her. I want to breed some larger kids like I had the first year.
As for the milk, the higher the butterfat %, the better. Nubians are noted for having high butterfat, but not EVERY Nubian has read that review
If it's important to you, find a herd on test that can verify butterfat, it can vary greatly line to line. Saanans are like the "holsteins" of the goat world, Nubians are more like "jerseys" but some La Mancha lines have a huge amount of butterfat as well.
when i made soap.. i was milking 3 goats ...
a nubian... 1 3/4 gallons a day
a nubianX alpine? ... 2 gallons a day
and a LAmancha... 1.5 gallons a day
i only milked all 3 for one year ...to much work, way WAY to much milk.. i did not know what to do with it all.. i made soap and cheese and fed it to the pig, chickens, dogs, cats.... gave some to my friend and she made cheese....
i just mixed all the milk together, put it in the right portions for my recipes and froze it,, so when i wanted to make soap i just grabbed a premeasured bag from the freezer..
i also used the whole milk NOT just the cream..
i milked the lamancha for several years and when i sold her they milked her for many more years, she was a great goat.. and i bought her for $25 from someone trying to make dwarf lamanchas and she kept throwing earred kids, so they just wanted to sell her quick.. one of the best goats i have had..