I agree with gmil, over seeding brassicas just stunts there growth and if you end up with much growing they will be very short and not produce much forage at all. On the other hand if you dont put enough seed down you will have a very spoty plot with out much forage. Like mentioned before the best is to figure exactly how many pounds of seed you need for a given plot and stick with that. I tend to seed a little on the heavy side when using oats of something along those lines but use caution when using any kind of seed that canopies over when mature cause they do need room to grow.