The pilot had been carrying out spraying operations for most of the day. The flight in question was to be the first for the day from this particular strip. The pilot had pumped 644 litres of water based spray into the hopper, having previously carried this sized load from the 604 metre strip. Normal take-off procedures were employed, however the aircraft failed to become airborne as expected. Almost immediately after lift-off, the left gear leg struck and severed the top wire of a fence. The left flap, horizontal stabiliser and elevator collided with a fence post. The pilot was able to retain control of the aircraft, dumped the load and returned for a successful landing on the strip. Although the pilot had previously operated with a water based spray load of 644 litres from this strip, he had overlooked the fact that the loads had different specific gravities. On this occasion, the specific gravity of the load was significantly higher and resulted in the aircraft being about 190 kilograms above the gross weight permitted for agricultural operations. Under the conditions there had been insufficient strip length available to permit a safe take-off.