Hi Mr Paul,
1. The herbicide has to be applied two days after planting else it will kill the seeds.
2. Yes you will have to use man power for weeding right before the 2nd application. Because using herbicide at this point will be harmful on the maize plant.
3. As for the harmful effect on the soil research is still ongoing. But by and large youre a business man and u need to reduce expenses. Weeding in itself is expensive doing it more than once will be expensive.

I also forgot timing of planting is critical. As early planing always yields better. Same as harvest. Time your harvest for when the rains have gone or are about to go.

Truth be told for anyone to be successful in maize farming you need the following.
1.high yield seeds. Preferablely one that gives yields above 3tonnes per hectare.
2. Know the appropriate seed quantity per space. I, e for open pollinated seeds 20kg per hectare and for hybrids 15kg per hectare.
3.spacing. your loss will be compounded if you dont space properly. I. e 25 cm in between stands and 75 cm between rows. For efficiency avoid manual planting. Hire a planter.
4. Fertilizer. Fertilizer can be applied in 3 split applications as follows at planting, four weeks after planting and a third at six weeks to eight weeks. Use Npk and and urea. For efficiency budget a minimum of 4 bags per hectare. 3npk 1 urea.
5.ensure your farm is free of weeds from the 3-10 leaves stage. Generally make sure ur farm is free of weeds.
6 avoid land prep manually. Use a tractor as follows. Ploughing and harrowing if youre using a planter.
7. Use a powerful pre and post emergence herbicide two days after planting then follow up with weeding as necessary.
8. Harvest only when maize has dried fully, for better accuracy shell at 150days. This is to avoid losses due to Mycotoxin growth during storage.
9. Selling dried shelled maize is by far more profitable than selling fresh. In Niger state where I farm by November a bag was sold for 11,000. I will be happy to entertain more questions and look forward to your success this farming season.