The province says the one-time amendment will help farmers move grain to strategic locations from where it can be shipped to market.
Weight restrictions are usually imposed in the spring when rain and thawing can weaken the road.
Transportation Minister Steve Ashton says helping get surplus grain to market is the government's top priority.
A bumper crop combined with a transportation bottleneck has left grain sitting in bins across the Prairies.
Ottawa has ordered Canada's rail companies to double the amount of grain they ship in a week or they will face fines.