The aircraft was being operated from a strip which had been cleared in a hay paddock. The pilot was aware that the strip was of marginal length and had therefore reduced the load to be carried. On take-off, the aircraft accelerated normally to about 40 knots but the performance then appeared to stagnate. The pilot attempted to dump the load, but only partial dumping was achieved before the right main gear struck a fence post as the aircraft became airborne. The impact displaced the gear, however the aircraft remained under control and the pilot diverted the aircraft to a more suitable aerodrome. The right main gear became completely dislodged during the landing. A subsequent inspection of the strip revealed that it had a soft sandy surface, covered with short and thick grass. Heavy rain had fallen in the area during the night and early morning, and the grass was very wet at the time of the take-off. When calculating the load he could safely carry from the strip, the pilot had not appreciated the degree to which the surface conditions would affect the take-off performance.