Horse collars invented (int China) before 500AD, Europe didn't adopt until 1000AD.
!wiki horse collar
!wiki horse collar
Look at that article again. Some early form of the horse collar may have been around in China around AD200-250, but the finished form wasn't until 450-500. Rome fell in 476.
Besides, consider what kind of contact Rome had with China. Nobody traveled all the way from Rome to China; instead, goods were relayed: China to India, India to Arabia, Arabia to Constantinople, Constantinople to Rome. And the caravans through Arabia didn't use horses; goods were carried on camels. So there was no route for the horse collar to take, as a live technology. Somebody in China could have tried exporting horse collars, but it wouldn't have worked--only very valuable items were worth shipping all that way.
Europe didn't adopt until 1000AD.
We weren't talking about Europe, we were talking about the Roman Empire.
That caveat "as a live technology" is not strong: many technologies Did pass between cultures even though they weren't attached to the camel.
(Western) Rome wasn't around in 1000AD, Europe was.
True, but your original claim was about Rome ignoring a technology that wasn't available until 524 years after Rome fell. And I'd still like to see some evidence that gunpowder existed before 476 as well so that Rome could ignore it.
Western Roman empire fell in 476.
Wasn't available until after Rome fell? That was my point - it was there all the time (perhaps since 200AD, certainly at 500AD).
(All according to Wikipedia.)