I'm afraid I can't help you about what you should be feeding your birds since the supply situation in Cameroon is obviously quite different from here in the UK, where we would just give layers pellets to pullets coming into lay. Most of us here feed layers pellets rather than layers mash since they're more economical, the birds can't pick and choose which bits of the food they eat but have to clear up the lot and so there's less waste and dust. Also I've no experience with industrial-sized egg production farms, though I'm impressed by what you're trying to do and I'm sure you'll get success quite soon. However, with my limited experience of raising hybrid pullets, I would say it's not usual for any birds to begin laying as soon as 18-20 weeks. I've had a variety of hybrid pullets over the years and mine have never laid before 22-23 weeks and some are later than this. The longest to mature took 28 weeks before her first egg. However, you can tell when they're ready to lay by 1) the colour of their combs, should be bright red and the combs need to be fully developed, with points along the comb. 2) when you run your hand along the back of a pullet about to come into lay, she crouches with her wings sticking out to the sides, as a sign she's expecting a cockerel's attentions, ie is nearly mature.
I expect you know that the first eggs they lay will be quite small compared with what they'll manage when they really get going? The first ones are called pullet eggs and the birds need a bit of practice as they mature fully and produce larger eggs.
The other thing which surprised me about your first post was that you seemed to be restricting their feed to what you considered to be the right amount. I think here most people keeping chickens would recommend feeding ad lib, ie as much as the birds need to eat, letting them regulate their own appetites. The feeder should never run empty especially whilst the birds are still growing. If they have plenty of clean water and food at all times they'll grow at their own uninterrupted pace. If my maths is right, you're feeding 1 kilo of food to 14 birds per day, ie about 70 grams each, which seems to me to be less than my pullets would eat. Mine also have access to quite a lot of good grass so don't eat as many pellets as they would if confined without green food supplement, and i don't expect yours have this advantage as there are so many of them. Could you let us know how you are keeping them ,ie are they in cages, or in barns, or free range outside?
One more point - have you wormed your birds? I would worm mine at 16-18 weeks, and every 6 months htereafter, especially if they're kept in commercial conditions where probably they're fairly restricted as to space. If they're not wormed regularly, egg production as well as other aspects of health will go down or maybe even be extra slow starting.
P.S. (Edit
I see in your first post you did worm them and I'm sure you're doing everything right - you just have to be patirent a bit longer, hard though it is!
Good luck with your enterprise and please keep in touch and let us know how you get on.
P.S.(Edit) Here in the U.K. we probably wouldn't feed mixed corn to growing birds or to layers, or would do so only in very small quantities as a 'treat' for those of us who have just a few tame domestic birds. This is because it's got lots of calories, ie is fattening, but not much protein, which hybrid layers need a lot of. Corn is warming, so in winter here, people sometimes give more to help the birds keep warm through long, dark nights when it's freezing cold, but I don't suppose this is a problem for you! Egglaying hybrids such as your Lohmanns are bred to consume a carefully-blended, commercially produced, high-protein diet from an early age, to get them into lay and to support them in producing an egg daily for the length of their lay. Sadly, this often results in health problems for the birds after 18-24 months, when egglaying falls off and many of the birds become prone to problems such as prolapse from their intensive rearing and egglaying. At this point, or even earlier, it's usual for commercial chicken farmers here to cull all their birds and start with a new batch. So maybe, (just maybe, I don't know for sure,) egglaying might have been delayed a bit for your birds because the diet they received as growers wasn't properly balanced for their specialised needs, and perhaps they weren't fed as much as they wanted to eat. If so this could have delayed their development, but as I say I don't really know. However, it may actually have helped them, if they developed a bit more slowly in a more 'natural' way, so maybe they'll go on longer even if egg production is lower than it might have been in Europe.
P.P.S; I found this on the web - http://www.hastavuk.com.tr/en/kitapciklar_en/0/brown.pdf
-ie, the breeders' management guide to the commercial production of Lohmann Brown layers, which may answer some of your questions about ideal feeding and management programmes for these hybrid birds. It certianly seems quite a specialised process, to rear them to peak production in the minimum time. Hope it helps.
