The 2012 word junior champion dominated race one, coming from a fourth-place start to gain a comfortable 15-second lead and take the win.
Starting the second and final race in 10th position, Grothues worked his way to third, but crashed while attempting to overtake for the lead and he became stuck under his bike, falling back to 12th.
He was able to make up some ground, finishing sixth, giving him 40 points to finish behind Finland’s Kim Savaste, who won on 42.
Fellow national team member Jett Lawrence, who turned 11 just three days before, won the 65cc 10-12 years class to be Australia’s youngest-ever world motocross champion.
He came first in race one and second in race two, giving him 47 points to win the title.
Australia finished fifth overall with the US, Germany and France taking the top three spots.
Grothues is now looking to the Australian Junior Motocross Championship in Appin, NSW from September 27 to October 4, competing in the 85cc big wheel and 125cc classes.