Weed control in hemp farming

In general, weeds in hemp production are not a really big issue. This is because hemp grows rather fast and if put under the right conditions, hemp covers the ground shade with 90% in 3-4 weeks from planting hereby suppressing almost all the weeds. The key is to plant hemp early as soon as the soil is warm enough for the hemp to grow (Hemp University)
Moreover, hemp is even used as a weed killer on fields over a hundred years.

“Very few of the common weeds troublesome on the farm can survive the dense shade of a good crop of hemp… In one 4-acre field in Vernon County, Wis., where Canada thistles were very thick, fully 95 per cent of the thistles were killed…”
— Lyster Dewey, Hemp. USDA Yearbook of Agriculture, 1913.

“Hemp grows quickly, soon covers the ground and chokes out the weeds. So weed control is not necessary.”
— Eddy A. A. de Maeyer. 19941

Because of the competitiveness of hemp with weeds, the chemical input need for herbicides is also almost negligible which is good as in some states in Canada, very few herbicides are approved. This is also the case for the state Manitoba. here there are no registred crop herbicides for hemp. However, the do have the option of pre-seed burn-off with glyphosate. (www.gov.mb.ca) A research done in 2002 suggested that for conventional production of hemp the advised seeding rate would be 40-50 kg/ha in order to benefit from the density that shades out the weeds. And with a premium price paid for organic hemp, the advised seeding rate would be higher 50-80 kg/ha ( Plant Population and Weed Control in Hemp Production, 2002) 

Sources:

http://www.hempuniversity.com/hemp-university/growing-hemp/weed-control/
https://www.gov.mb.ca/agriculture/crops/production/hemp-production.html#weed
http://www.globalhemp.com/1994/01/hemp-as-weed-control.html