I think it was around the 11th-12th centuries, around the time they also started breeding the bigger horses. The padded horse collar made it possible for the horse to put his formidable shoulders* into the job. I know the Limburg Bros show a horse being used for harrowing in one of the calendar pages of the Tres Riches Heures, which is very early 15c. Here's an image:

http://commons.wikimedia.org/wiki/File:Les_Tr%C3%A8s_Riches_Heures_du_duc_de_Berry_octobre_detail.jpg

Edit: Wikipedia is surely our friend for refreshing our memories on stuff like this, but I was pretty close:

http://en.wikipedia.org/wiki/Horses_in_the_Middle_Ages#Agriculture

*Edit II: Wikipedia says hindquarters rather than shoulders, but I thought the point of the collar was that it would engage the shoulders instead of the neck.