Regulated Deficit Irrigation: Is it appropriate for your operation?

Regulated deficit irrigation is the practice of reducing irrigation to obtain some type of desired stress level. This practice is often used during the initiation of the hull-split period to reduce hull-rot strikes. It also has been utilized during periods of water shortages to save 5-15% of the seasonal water use of almond. Due to the limited effect on yield, as well as the benefits, it has been advocated for application within orchards by the University and industry (including myself!). This practice, however, can have negative impacts when applied incorrectly and should only be used if deemed appropriate.

In theory, this practice is easy to apply. Water application rates should be cut to achieve -15 bars stem water potential for two weeks preceding the onset of hullsplit (e.g. Blank split). This stress should be maintained at this level for this period. At the end of the two weeks, regular irrigation resumes and the orchard prepares for harvest.

The difficulty in application has everything to do with accurately monitoring plant stress. Every orchard and orchard practice creates a different approach in application. For example, assuming similar irrigation levels, stress levels achieved in a mature orchard planted on sandy soil will occur in a shorter time frame than an orchard on a heavier clay loam. Every orchard site requires careful monitoring to determine when to cut and resume full irrigation.

I have observed numerous orchardists apply tree stress only to see a reduction in kernel yield. This is due to ongoing gains in nut weight that occur between the onset of hull-split and harvest. If the tree is significantly stressed during this period, the conversion of carbon to fats is reduced, impacting the final crack out percentage. One closely monitored orchard in which I worked, demonstrated a 10-15% reduction in kernel weights within blocks where they applied deficit irrigation. This was a loss of about $500/acre to the operation. I should add that the orchard was monitored daily with a pressure chamber and the orchard was maintained between -15 and -20 bars mid-day stem water potential. This is further supported by work within the Water Production Function trial, which showed yield decreases correlating with stress during the period of June and July.

These challenges could be viewed as reasons not to apply regulated deficit irrigation. They should not. Rather, they should be viewed as points to help farmers recognize that not every operation should be reducing water during hull-split. Here is a list of further considerations to help determine if this practice is correct for your operation:

1. Residual soil moisture levels remain high throughout the season. If high levels of irrigation are applied during the spring, it may be difficult to achieve a stress level that accelerates hull split. Well-watered and fertilized trees, especially on hybrid rootstocks, will experience delays in ripening, delaying harvest and increasing insect and hull-rot issues. Practices should be employed to reduce soil moisture levels going into hull-split without applying too much tree stress to help make the trees more responsive.
2. The irrigation system does not apply enough water to meet the daily demand. When I was a farm advisor, this was commonly observed. Either due to poor design, maintenance, or just under-calculation, many orchards were applying a maximum daily amount of 0.28-0.32” of water per day (6.3-8.0mm/ha/day). This is not enough water to meet daily demand during the peak summer use, and as such trees are already stressed. Cutting water further will only reduce yield.
3. Hull rot occurrence seems to be due more to nitrogen than water status of the tree. Multiple studies over 40 years have shown that a mid-summer leaf nitrogen % of 2.2-2.5 is adequate for maximum yields if the trees are properly sampled. Leaf N levels over 2.5% have been shown to increase hull rot, with personal observations of a five-to-ten-fold increase with every 0.1% leaf nitrogen over 2.5%.
4. Lastly, many orchards do not apply regulated deficit irrigation.These orchards have reduced hull rot, are able to start harvest at a “normal” time, and are able to shake the trees cleanly. In reviewing the operations, these farms tend to start irrigation a little later in the season after soil moisture levels are depleted to a desired level, irrigate to match almond demand, and have nitrogen fertilization practices that keep the trees between 2.2-2.5%. I have seen these orchards essentially irrigate up to the day before harvest, cut the water for 24 hours and begin shaking without bark damage. They are also achieving kernel turn-outs from gross nut weights that are consistently between 27-35% for all varieties.

Regulated deficit irrigation is a practice to achieve a desired outcome. That outcome is not applicable to all almond orchards. When determining if this is right for your operation, take into consideration the soil moisture status, the capacity to irrigate to meet daily demand, hull-rot incidence, kernel quality desired, and harvest operations