The following is an example of using a decision tree in a typical on-farm decision making process. The example chosen is for rainfall and weed control for a hypothetical location in south-eastern NSW.
A particular weed control is said to be 90% effective if no rain occurs within 12 hours of application and less than 1 mm of rain falls in the following three days, whereas it is only approximately 25% effective if rain falls within 12 hours. It is only 50% effective if 10 mm or more falls in the following three days. It has been a reasonably wet season and some rainfall has occurred during each of the past four weeks, and so weed control is needed reasonably soon.
The chances of 10mm rain over the next 3-4 days are very low (close to zero), but there is rain in the area (particularly to the east) and the Water and the Land rainfall forecast maps and Bureau forecasts indicate 1mm is possible for the next 3-4 days.
|Options||Apply weed control today – Day 1, take risk of no rain in a 12 hour period.||Apply weed control on Day 3-4.||Delay applying weed control for at least 5 days|
|Cost of chemicals, fuel and labour.
75% of costs lost if rain falls in the 12 hours
|Risks quite low, costs of delay might be low also||
Contract labour delays
Extra weed growth to control
Other farm operations requiring attention
|Other considerations||Risk of runoff of contaminated water if rain occurs||Looking for value for money.|
|Assessment||Assessment may be made by combining Costs information and the Other considerations in some way, e.g. using judgement|
The final decision is made by comparing the objective assessment of each option.
This page is produced with the support of Managing Climate Variability - a consortium of primary industry research and development corporations.