A truss is a tight bundle of hay or straw. It would usually be cuboid, for storage or shipping, and would either be harvested into such bundles or cut from a large rick.

Hay and straw were important commodities in the pre-industrial era. Hay was required as fodder for animals, especially horses, and straw was used for a variety of purposes including bedding. In London, there were established markets for hay at Smithfield, Whitechapel and by the village of Charing, which is still now called the Haymarket. The weight of trusses was regulated by law and statutes were passed in the reigns of William and Mary, George II and George III.

The standard weights of a truss were:

new hay, 60 pounds

old hay, 56 pounds

straw, 36 pounds

and 36 trusses made up a load.

More Info: en.wikipedia.org