Irrigation
In conventional overhead systems a crop is watered at 100% pan evaporation replacement throughout its life or by using pan evaporation multiplied by a crop factor. In the reduced pass harvest system trialled in Manjimup 100% pan evaporation replacement was used early in the crop life.
As the crop matures plants are increasingly water stressed until curd or head formation occurs. To achieve this stressing without damaging the crop, soil moisture needs to be monitored using tensiometers, TDRs or other soil moisture sensors. Irrigation is only applied when the sensors reach a certain reading, which varies depending on the age of the crop and is specific to soil type.
From 34 to 47 days after transplanting crops are irrigated only when the tensiometer reads 10 centibars (cb). Each watering should return the soil to field capacity.
From 48 to 60 days after transplanting the threshold reading is increased to 20cb, and increases again to 30cb from 61 days after transplanting until head formation (Table 2). These readings are based on a shallow tensiometer placed 20cm below ground.
From head or curd formation to harvest, irrigation is returned to 100% evaporation replacement to ensure that plants are not stressed at harvest. This maintains head or curd quality and shelf life is not reduced.
| Transplant to 33 DAT | 34 DAT to 47 DAT | 48 DAT to 60 DAT | 61 DAT to head/curd formation | Head/curd formation to harvest | |
|---|---|---|---|---|---|
| Conventional irrigation scheduling | 100% evaporation replacement | 100% evaporation replacement | 100% evaporation replacement | 100% evaporation replacement | 100% evaporation replacement |
| Reduced pass harvest irrigation scheduling | 100% evaporation replacement | 10cBars tensionmeter threshold | 20cBars tensionmeter threshold | 30cBars tensionmeter threshold | 100% evaporation replacement |
DAT = days after transplanting
When air temperature reaches 30ºC additional water should be applied when using either irrigation regime. This cools the plants and provides only minimal additional water to the crop.
Additional strategies
Beyond fertiliser and irrigation management other strategies which help to reduce the spread of crop maturity are:
- Source seedlings which are uniform in age and size.
- Try to maintain single plantings on a similar soil type. Avoid planting across different soil types.
- Take care when handling seedlings during transplanting to reduce transplant shock.
Judging harvest time
It is important the correct day of harvest is chosen to ensure the optimum number of plants are harvested. It is recommended this be done by checking a sample of 20 plants. When 80% of those plants reach maturity the crop is ready to be harvested. Both broccoli and cauliflower can mature very quickly leading up to harvest, so daily checking is essential. Misjudging the harvest date can lead to a reduced yield due to a high number of over or under-mature curds or heads.
If you are planning to harvest a crop in a single pass, it is recommended that you gain experience in judging the best day for harvest on test crops beforehand. This will give a better idea of how quickly plants mature and how often you need to check your sample plants before applying the technique to a whole crop. Existing broccoli and cauliflower plantings can be observed and single harvest dates estimated even if the crop is intended to be selectively harvested.
Benefits
The major benefits from using the reduced pass harvest system should be savings in labour costs from shorter harvest time and savings in water and pumping costs through fewer irrigation applications and less total water being applied to the crop.
In trials, one-pass harvest cauliflower crops took about one third less time to harvest than selectively harvested crops. Time is saved st as there is no decision making or curd inspection required by the harvesting crew. While this takes only a small amount of time per curd, it adds up in a selectively harvested crop and it is faster when the harvesting crew simply cut every curd. Fewer skilled people may also be used when there is a shortage of suitable labour.
Under reduced-pass harvest system trials, water use efficiency was increased by 30.4% for cauliflower and 17% for broccoli. This saves on pumping costs as well as the amount of water used.
Challenges
Planting blocks and irrigation systems need to be arranged so different aged crops can receive different irrigation schedules. A higher percentage of the crop is wasted compared to conventional multiple harvest crops because generally only 80% of the crop is ready to harvest at one time. Some of the loss through wastage is offset by the savings in labour, water and pumping costs.
If there is a market available for out-of-specification curds or heads, such as those that are over or under-size on the single harvest day, then wastage is reduced. Growers should carefully work out the costs involved and likely return before using a one-pass harvesting system.
Acknowledgements
Horticulture Innovation Australia, utilising the National Vegetable Levy, funded this work through project VG02051.