What's that then?

[rebbonk]

Farmer was driving his tractor pulling a trailer loaded with cow dung and as he passed the mental institution, one of the inmates was looking over the wall.

The inmate shouted to him 'Hey mister, what's that on your trailer?'

The farmer replied 'It's cow's shit mate.'

The inmate shouted back 'What do you use that for?'

Farmer says 'I put it on my rhubarb.'

Spoiler:

'You should come in here then, we get custard on ours.'