Being able to maximise total kernel recovery is the ‘flagship number’ of growing macadamias, as it is one of the determinants of the yield and profitability of the orchard.

What tools or indicators do growers have to help them ensure their trees are performing to return a maximum potential total kernel recovery rate (TKR) and to help predict what TKR might be before the crop is harvested?

These are two questions that Dr Rohan Orford* set out to answer in his presentation on factors affecting total kernel recovery at AusMac 2022.

The background

Dr Orford had a good basis for answering these questions as he had access to ten years’ of data for 170 farms in South Africa and Australia. As he delved into the numbers, he discovered that as with many things in life, the answers are not clear cut, rather involve a range of factors.

Two of the figures he had from each farm were TKR and altitude. This allowed him to come to an initial conclusion that geographic location and altitude are key determinants of TKR so that the higher farms are or the further from the coast they are – in both Australia and South Africa, the lower kernel recovery is likely to be.

While this did seem to indicate a pattern that associated altitude with kernel recovery, Dr Orford said that this only supported between 60 and 70 per cent of cases. As he explained it, this pattern was a “pseudo correlation”, which saw him on a quest to identify factors other than altitude that were also important.

Period 6–12 weeks after flowering

By looking closely at and comparing figures for different farms, Dr Orford identified that the period six to 12 weeks after flowering was crucial in determining TKR. During this time, water availability, nutrition, solar radiation, relative humidity, diurnal (day) temperature and leaf temperature all combine to produce conditions favourable or otherwise to high kernel recovery.

Figure 1. TKR is higher for all cultivars in areas with diurnal temperatures below 11oC six to 12 weeks after flowering.

Some of these elements – water availability (through irrigation and a well-mulched canopy floor), nutrition (through attention to the orchard nutrition program) and leaf temperature (though canopy management) – are all to some degree within the control of the grower. He says the others to do with climate are obviously “in the lap of the gods”.

Of the climatic factors, Dr Orford identified diurnal temperature as being crucial, in particular the temperature range from 5:00am to about 1:00 or 2:00pm, with a difference of between 5° and 10°C optimal for high TKR.

The key to diurnal temperature is that it is an easily obtainable and universal data parameter, and it is not as abstract as VPD, thus making it accessible to growers and other stakeholders as a handy decision-making tool.

“This difference grows the more you move away from sea level so that at a thousand metres, it can be between 12 and 14 degrees,” Dr Orford said.

In turn, as shown in Figure 1, this bigger temperature difference will tend to reduce TKR. The figure shows that this pattern for all cultivars is similar in all regions i.e., TKR is higher in areas with diurnal temperatures below 11°C six to twelve weeks after flowering. Where diurnal temperatures are higher than 11°C, TKR is lower.

Vapour pressure deficit the key

The story does not end with diurnal temperature, rather its importance is its relationship to vapour pressure deficit (VPD), the ultimate determinant of TKR.

Altitude affects diurnal temperature, which is related to relative humidity, which in turn drives vapour pressure deficit – something that changes “hourly, daily and monthly”.

Dr Orford makes the analogy to explain the significance of VPD in a macadamia orchard by describing how it dictates the opening and closing of stomata, which are the “tellers that accept the money (carbon dioxide) coming into the bank and direct the costs (water) going out”.

The carbon dioxide that enters the tree through stomata drives photosynthesis and determines the production of carbohydrates, which Dr Orford describes as “the currency for kernel recovery and yield”.

In simple terms, the ability of the orchard to build its carbohydrate capacity is what determines TKR and income, so it is crucial for growers to do what they can to develop this ability through good management – accepting that climate and geography are things that cannot be controlled.

Figure 2. A summary of optimal conditions for maximising TKR. 

Optimum conditions for maximising TKR

By analysing diurnal temperature, relative humidity and vapour pressure deficit, Dr Orford identified the optimal conditions for maximising TKR (see Figure 2).

In essence, the following conditions are ideal:

- VPD between 1 and 2kPa

- optimum temperature of 28°C

- relative humidity of 55%.

A climate map shows that these conditions apply along the east coast of Australia roughly between the Gold Coast and north of Bundaberg, at low altitudes (see Figure 3).

Figure 3. Where optimal conditions occur for maximising TKR in Australia.  

Application in the orchard

While growers cannot change orchard location, particularly if they are high altitudes, these findings are useful for a number of reasons, as follows:

- they indicate the importance of managing factors in the orchard that are within the control of the grower as a way of building its carbohydrate capacity and contributing to high TKR, particularly in that crucial period 6–12 weeks after flowering

- they show the importance of measuring vapour pressure deficit and managing the orchard to deliver moisture to the tree as much as is possible through irrigation and/or mulch

- they provide an indicator ahead of harvest of what TKR could be thus allowing for adapting harvest and delivery strategies.

*This article is from a presentation delivered at AusMac 2022 by Dr Rohan Orford from Macadamias Australia. Note. Dr Orford cautions that this work has not been scientifically reviewed, but rather is based on his own analysis of data.