This is an April fool joke transferred to hay-harvest. The joke is this: some greenhorn is sent a good long distance to borrow a rickmould, with strict injunction not to drop it. The lender places something very heavy in a sack or bag, which he hoists on the greenhorn's back. He carries it carefully in the hot sun to the hayfield, and gets well laughed at for his pains.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894