Laminitis, could it have been the farmer's grass?

[MadJ]

My pony has been diagnosed with lami today. I'm absolutely mortified as we've always been so careful and closely monitored his waistline and when the vet was out last month, commented on how well we were doing with keeping his weight down. Even the vet today said he's around about right condition wise.
I've spent all day wracking my brain as what could've triggered this. We don't fertilize the paddocks and they're all on restricted turnout so they don't turn in to great fatties.
The paddock they've been in the last few weeks borders the farmers crop field next door with a post and rail fence and I've realized I've spotted the Welsh one of several occasions with his head through the fence having a munch on the crops the other side. The horses he's out with are too big to do this.
I'm kicking myself that I didn't consider the implications of this sooner as having been an inspected that fence line, I'm horrified that he seems to have been having a private feast. There's now a line of electric running between the fence rails to prevent him doing this once he's recovered.
Could it really be this that's caused the lami?

 

[Honey08]

I guess it could have been, or just the grass coming through in general.

Did they test him for EMS? I had a pony that had laminitis despite not being over weight. Once she recovered and was sound and working we sold her a few months later to make sure she would be used and worked (she had been outgrown at ours). A year down the line she got laminitis again, the new owner's vet tested her and she had EMS...

 

[MadJ]

I'm going to ask about EMS when I next speak to the vet. I've read about it but don't really understand.
He's only 5 and, despite not being ridden yet, looks the fittest he ever has due to the work he's been doing.
Fingers firmly crossed, she thinks we've caught it very early as although uncomfortable on the concrete, he was non reactive to hoof testers and have acted promptly to get him as comfortable as possible. He was bouncing round the stable tonight.