Taking yields to the limit

Dr. Stuart Tustin, a plant physiologist and science group leader at the Institute of Plant and Food Research in New Zealand, spoke during several sessions at the 2014 Washington State Horticultural Association meeting. (TJ Mullinax/Good Fruit Grower)

Scientists in New Zealand have planted a prototype of a “super orchard” that they hope will produce up to twice as many apples as a typical productive orchard.

Dr. Stuart Tustin, a plant physiologist and science group leader at the Institute of Plant and Food Research in New Zealand, said he and his colleagues are working to find out what the biological limit of an orchard is, in terms of production, and to identify factors that prevent growers from achieving that.

Tustin, who spoke during the Washington State Horticultural Association’s annual meeting, said productivity is influenced by two factors: the amount of light intercepted by the tree, which fuels photosynthesis, and how the tree allocates resources between its various parts and functions.

Research has shown that in current orchards, yields increase as light interception increases. But beyond 60 to 75 percent light interception, the relationship falls apart.

“In my research experience, and measuring light interception in highly productive orchards, the highest I’ve ever measured is in the 60 to 65 percent range,” Tustin said. “And getting above that is rather challenging.”

Dr. John Palmer, plant physiologist with Plant and Food Research, has extrapolated from the systems that he’s measured that at 90 percent light interception, yields could theoretically be in the range of 150 to 200 bins per acre.

In comparison, Washington’s current average yield is 48 bins per acre, according to agricultural economist Dr. Des O’Rourke, although some modern plantings are yielding well over 100 bins per acre. Average U.S. apple production is around 35 bins per acre.

Rethink

Designing an orchard to achieve the maximum biological yield potential of 150 to 200 bins per acre will require a total rethink, Tustin said. “How we manage our orchards and how we operate them will have to change.”
One of the challenges is that gains in productivity can’t come at the expense of fruit quality. Light distribution within the canopy is important, as well as light interception, to ensure that all the fruit receives adequate sunlight.

Super orchard

(Click to enlarge) Apple trees could theoretically yield more than 150 bins per acre, says New Zealand scientist Dr. Stuart Tustin, but it would require a complete orchard redesign. Source: New Zealand Institute for Plant and Food Research Limited. (Jared Johnson/Good Fruit Grower Illustration)

In designing their super orchard, the New Zealand scientists considered every aspect of the orchard and redesigned the whole system. Their attitude was that there was nothing that could not be changed in an attempt to attain 85 percent light interception and yields of 170 bins per acre of high-quality fruit.

The system they developed is similar to the Upright Fruiting Offshoots that Dr. Matt Whiting at Washington State University developed for cherries. It employs trees with multiple vertically oriented fruiting units (which Tustin refers to as stems), rather than horizontal branches. This reduces the number of trees needed per acre, and minimizes establishment costs.

It also capitalizes on the natural growth habit of the trees, whose “brain” is in the apical bud, Tustin said. While the apical bud of a shoot is intact, it inhibits branching down the stem. Though lack of branching may be a problem with three-dimensional trees, it’s something that can be taken advantage of in a planar system.

By training the shoots vertically and having leaves spaced out up the shoots, light interception is increased. The system leaves no room for side branches, but this lack of tree structure allows for optimum light distribution through the canopy.

The new system turns traditional tree spacing on its head. In the prototype planting, established in 2013, the trees are planted 3 meters (10 feet) apart with 1.5 to 2 meters (5 to 6 feet) between rows. The relatively wide in-row spacing is needed because each tree has 10 upright shoots, forming almost 22,000 stems per hectare or 9,000 stems per acre.

Bi-axis

The trees are developed in the nursery as bi-axis trees, starting from budded bench grafts on Malling 9 rootstocks. Tustin said a bi-axis nursery tree has 30 percent more mass than a single-leader tree, which is a benefit in terms of rapid development of the canopy. After the tree is planted in the orchard, the two cordons are supported until they resume growth and are then laid down in either direction. Upright shoots, or stems, are trained from the cordons every 30 cm (12 inches). The multiple stems per tree help to manage tree vigor by dispersing growth.

Fruit density on the shoots will be managed by spur extinction (bud removal). Early adjustment of the crop load allows the tree to direct more resources into growing the remaining fruit. To achieve a yield of 170 tons per hectare/bins per acre, each stem will need to produce 45 apples, which Tustin said looks like a fairly low crop load. Provided the leaves have enough radiation to drive the growth of that fruit, he is convinced that such yields are possible.

The prototype planting, with Envy as the scion, produced 10 to 15 bins per acre of fruit on the cordons in the second leaf. That was a planned crop because the trees produced plenty of high-quality spurs and were quite capable of cropping, Tustin said. The stems can be grown up to 3.5 meters (11.5 feet) high. “But if we achieve 85 to 90 percent light interception with a canopy height of 2.5 meters (8.5 feet), that’s all we need,” he said.

Harvest index

The trees will be minimally pruned so that they can channel most of their resources into producing fruit. Each year, a tree grows fruit, shoots, and leaves and adds growth to its branches, trunk, and roots. The proportion of the total seasonal growth that ends up in fruit is known as the harvest index.

The harvest index (and thus fruit production) is enhanced by the use of dwarfing rootstocks, reduced pruning, and increased tree manipulation, Tustin said. “The greatest way to get a high harvest index is to reduce cutting of the canopy because the plant will naturally try to regenerate that part of the canopy that’s been removed.

“We will establish the canopy and have very minor intervention with pruning and training, simply because there’s no necessity to do it,” he added. “Then, we’re getting very close to pushing toward the upper limit of what yields might be.”

Renewal

There will be no rule about renewing the vertical shoots. “If there’s nothing wrong with the fruit, nothing wrong with the buds, nothing wrong with the flowering, why would you replace them?” Tustin asked. “If you’re not replacing them you’re conserving the growth of the tree and the plant won’t have to regrow that and will be more likely to direct more of its seasonal energy into growing the fruit.”
How will the fruit be harvested with the rows so tightly spaced?

“In a whole-system design, anything or everything can or should be changed to optimize the system,” Tustin said.

He sees such challenges as opportunities to engineer new solutions. He envisions that much of the orchard work could be done using small machinery with no more than a 20 horsepower engine. Small, planar canopies are more amenable to mechanization, automation, and ultimately robotics.

“A platform won’t fit down there,” he admitted. “But what’s wrong with a platform with one person on it? It’s a matter of allowing your imagination to range a little bit. Agriculture is the only segment of our society and our economy that hasn’t miniaturized yet. That’s pretty weird when you think about it.” •