I for one cull at hatch, I know from hatching plenty and growing some out, what I'm going to get out of them if I don't cull. So besides wasting my time and money on birds that I know will be culled later on, I just do a hard cull at hatch. Since I am now hatching my 3rd generation off of the one line, I am not having to cull as hard at hatch due to getting better chicks now, and hopefully will not have to cull as hard either once they are around 5 to 6 months. It is a lot of work, but it is also worth it once you see where it takes you and your birds.