I think it was said pretty well above, in that you can get a great product without MH light during veg just like you can get a great product with CO2. However I think it goes even beyond that. To this day, after doing experiments, looking at other peoples experiments, and doing my own research, I have yet to find physical and easy to see evidence that shows that 6000k light during veg will really help that much more. Itve tried used T5 HO during veg, Ive used MH during veg, and still I have seen no difference in yield, no difference in quality, and no difference really even in growth. If you look at all the bigtime growers, guys who are pulling 2-3 pounds per light, almost NONE of them use metal halide during veg. Its HPS all the way through. The plants are vibrant, they produce huge buds, the growth of the nodes are tight and compact... Doing HPS all the way through means you dont need 2 different lights or bulbs, MH uses more electricity and produces more heat.

I think its a highly overrated part of growing cannabis. When I do controlled experiments with the same conditions, same strain, same everything but MH during veg...I get absolutely the same results. About the only advantage I ever see is that clones can have a difficult time adjusting to the HPS light if you get them too close in the start.

theres my opinion. with all that said, I use T5 HO lights at 6500k for my veg closet. The T5s have very little heat, can light a large area (4 feet long), and will keep your plants from growing too much. They also are great for establishing clones, where obviously 6000k is far more important that during normal vegetative growth.