Before departing from his home base for his first operation from the particular agricultural strip, the pilot had been briefed not to land down the slope unless there was a strong headwind component favouring that direction. A normal landing was made, and the pilot was then given details of the task to be performed. He departed for an aerial inspection of the area, and on return noted that there appeared to be a headwind of about 25 knots for landing down the slope. However, on late final approach to land in this direction, the pilot realised that the wind strength had decreased to about 10 knots. He continued with the landing, but was unable to avoid over-running the end of the strip. A groundloop was initiated, during which the left gear assembly collapsed. The strip was only 340 metres in length, and had a downslope of about 6 degrees. The over-run area was about 100 metres long, with a downslope of 9 degrees. While the pilot was conducting the inspection of the area to be treated, a shower of rain made the strip surface wet and slippery. The strip appearance was deceptive from the air, and probably influenced the pilot to continue with the approach after the headwind component reduced.