I LOVE rubbish jokes. Here's my contribution:
Did you hear about the scarecrow that won a nobel prize?
He was out standing in his field!
(hmm, in light of Obama's recent win just to emphasise that is not some kind of veiled racism :-) )
Did you hear about the magic tractor?
It went down to road and turned into a field...
(ok, I got both of those from Farmers Weekly)
Did you hear about the lorry full of tortoises that crashed into a van-load of terrapins?
It was a turtle disaster!
And finally, my best terrible joke:
Why did the koala fall out of the tree?
Because it was dead.
Why did the second koala fall out of the tree?
Because it was in the first koala's pouch.
Why did the third koala fall out of the tree?
Peer pressure!