In today’s bear market it has become more prudent that ever to aim for maximum kernel recovery to generate the kind of income that will cover costs and ensure future sustainability. Research suggests that one key number will determine whether your farm has the potential to produce kernel recovery able to keep farmers in business.

A myriad of factors influences the total kernel recovery (TKR) in a batch of nuts, ultimately determining income and feasibility. But what if a farmer knew before he or she bought a farm and established an orchard what the potential TKR would be?

After years of studying orchards in both South Africa and Australia, Dr Rohan Orford, grower technical manager of Macadamias Australia, believes he has found the magic number: 10°C. This is the diurnal temperature range (DTR) – the difference between the maximum and the minimum temperatures within one day. In the case of macadamias, the crucial month of November requires a very specific climate to allow for optimal nut formation, and consequently TKR.

Orford explained his findings at the recent AusMac conference held on the Gold Coast on the Eastern coast of Australia.

“Input costs have risen drastically over the last year, and at the same time nut prices have dropped. For growers to stay in business, they would need to obtain a yield of at least three tons per hectare, provided the TKR was high. If a high TKR can however not be achieved, the yield would then need to increase to make up for the shortfall,” Orford said.

Rohan Orford

His calculations show that 1,5 tons of kernel per hectare is the minimum growers need for success. This can be achieved with a three ton per hectare crop provided the orchard is in an area with a diurnal temperature not exceeding 13°C. For farms falling out of the range, the yield would have to reach at least five tons per hectare to achieve the 1,5 tons of kernel.

Considering the impact of climate on TKR, it makes it more prudent than ever to choose the right property and the right cultivar in which to invest.

However, if on-farm orchards are already established with trees in production, knowing the TKR potential means growers can at least estimate their income which allows for more solid future investment decision-making.

Open stomata

Orford’s research shows areas with a higher altitude that are further inland will generally bring a lower TKR. “Altitude can for the most part determine TKR, although there are outliers where pockets of micro-climates are ideal. Generally, areas with a maximum altitude of 600m above sea level will deliver good TKRs, which is why we see such high yields and kernel recovery in areas along South Africa’s Eastern seaboard. Looking at Nelspruit, the diurnal temperature throughout the year is not ideal, but during the crucial month of November, the 12°C diurnal temperature will result in a good nut set,” Orford said.

Some six to twelve weeks after flowering, Orford added was the most important time on a macadamia nut farm to determine potential income.

“Since carbohydrates are the backbone of maximum nut set, this will ultimately determine the price growers receive for their crop. TKR is closely correlated to altitude, because altitude is related to diurnal temperature. Diurnal temperature is driven by relative humidity, which is driven by vapour pressure deficit (VPD). The VPD determines whether the stomata are open or closed. If the VPD is too high, the stomata close to prevent too much water loss and prevent dehydration. When the stomata close, they can’t take up the gasses they need to produce carbohydrates and you are effectively losing production because the tree is not doing anything at this point – it has shut down. Carbohydrates determine TKR, and therefore income, so the bigger the stomata open, the more carbon dioxide they absorb and the more carbohydrates they can produce,” Orford said.

This means November leaf temperatures should not go beyond 30°C, or below 25°C for optimal stomata functioning and carbohydrate production. Relative humidity is needed, with the atmosphere being neither too dry nor too wet. Just as a high diurnal temperature is detrimental, so too would a temperature that is too low, dipping below 5°C. For best results, Orford advised farmers to focus on the temperatures between 5am and 1pm.

For optimal stomata functioning, an ideal VPD is required. VPD is the difference between the amount of moisture in the air and how much moisture the air could potentially hold when it is saturated. It is measured in kilopascal (kPa), with a 1kPa to 2kPa the ideal for nut production.

Managing risk

Orford’s research points to fundamental problems linked to where orchards have been established and the cultivars farmers choose. The bad news is that with climate change, the VPD is set to increase, reducing TKR further.

“For the most part, farms have been chosen based on temperatures. But temperature does not correlate to kernel recovery as much as diurnal temperature. So, we see many orchards planted in the wrong areas, and many farmers choosing cultivars for reasons other than kernel recovery,” Orford said.

Considering the impact of climate change, he said orchards planted on the fringe of ideal areas might still be producing good TKRs today, but this situation could change in years to come. “They could become even more favourable as the climate changes, but they could become completely redundant too. Forecasting how the climate will change is difficult and we don’t know which way the chips will fall. What we do know is that in South Africa the climate is becoming more hostile.

“On the positive side, this research shows there are areas suited to macadamia nut production that have not even been considered yet. There are opportunities to expand production. Since diurnal temperatures start rising as you move away from the coast, if you are in areas further inland, late flowering cultivars are a better choice as the temperatures during nut set are more favourable,’ said Orford.

Ultimately, farmers should aim for orchards as close to the coast as possible and concentrate on cultivars known for high kernel recovery, like A4s for example, and not Beaumonts, Orford said.


South Africa’s diurnal temperatures in November per macadamia producing region:

Nelspruit: 12°C

White River: 10°C

Barberton: 11°C

Malelane: 13°C

Pongola: 12°

Ballito: 9°

Port Shepstone: 7°

George: 10°

Levubu: 13°

Louis Trichardt: 14°

Piet Retief: 11°