TRUCK BACKFIRED IN HAY FIELD WHEREAS A SPARK FROM MUFFLER IGNITED GRASS ON FIR UNDERNEATH TRUCK. CONSEQUENTLY TRUCK WAS LOADED DOWN WITH EIGHT LARGE ROUND ROLLS OF HAY WHICH IGNITED ON FIRE. C ONTRIBUTING CONDITIONS: 1. NO SUBSTANTIAL RAIN FOR 8 WEEKS CAUSING HAY TO BE VERY DRY AND UNCONTAINABLE. 2. WINDS GUSTING AT 20 MPH.