If you’re an apple grower, then pandemic aside, did you have an ‘on’ year or an ‘off’ year in 2020? Mike Williams, CEO of California-based OMEX® Agrifluids, takes a closer look at the problem of alternate yield bearing, explaining why it’s an agricultural issue to resolve, rather than a natural occurrence to overcome.
This phenomenon, where small yields of large fruit (off years) are followed by large yields of small fruit (on years), is a recurring problem for apple growers. But why does it happen, and how can we reduce the problem?
It’s a common question that customers put to our team of agronomists; with orchards dormant for the winter, now’s a good time to discuss it and think about taking appropriate steps to banish it for 2021.
We usually concern ourselves only with the current crop as it swells and ripens on the branch, but in fact the apple tree grows three crops at once. Of course, there’s the crop we see on the branch. But in development are not just the flowers for the next crop, but the budwood for the crop after that too. Thus our 2021 agronomy year will also influence our 2022 and 2023 harvests.
Herein lies the problem. With these three crops developing simultaneously, albeit at different stages, branches in different parts of the growth cycle compete against each other for resources. To ensure even production across seasons — eliminating the alternate yield bearing — the plant needs to allocate equal emphasis to vegetative growth (shoots) and reproductive growth (flowers for next harvest, as well as fruit development for current harvest).
Under non-agricultural conditions, the tree would manage this process itself; we rarely see alternate yield bearing occur in nature. In fact, it’s largely an agricultural phenomenon, created by the way we farm crops and influenced by prevailing weather conditions. And what’s the single most important influence on where the plant allocates its growth? The level and form of nitrogen available.
Not all nitrogen is created equal
You see, not all forms of nitrogen have the same effect. Take nitrates, which are transported to the leaves for processing. As nitrate levels accumulate, the plant responds by increasing production of the plant hormone auxin — a stimulant of vegetative growth and apical dominance. When nitrates are the dominant form of nitrogen, we see increased internode length (the space between leaves on the stem), faster shoot growth and increased height. Together, these create the classic ‘leggy’ appearance, characterized as a ‘nitrate effect’.
Amine nitrogen, conversely, behaves quite differently. It’s processed in roots and favors production of the cytokinin hormone. In cases of amine dominance, the plant emphasizes fruit and flower development. Internodes are shorter and there’s slower, more even shoot growth: the ‘bushy’ effect.
That’s the laboratory view. But how does this pan out in the field?
Well, when we apply conventional fertilizers to a crop, no matter what kind of nitrogen we apply — nitrate, amine, ammonium — that nitrogen is quickly transformed into the nitrate form, which the plant takes up. The more nitrogen we feed, the more unbalanced the crop becomes: it allocates a disproportionate resource to vegetation at the expense of flowers and fruit.
Played through successive seasons, this creates the cyclic ‘boom and bust’ that leads to alternate yield bearing. Then we’re compounding the effect: every year when we prune, we prune back the yield potential, because much of our applied nitrogen has gone into the very stems and branches we’re meticulously removing.
How do we break the cycle?
The secret lies with a stabilized form of amine. After application, its nitrogen remains available in the amine form, without degrading to nitrate. OMEX® Cell Power® SizeN®, developed by scientists at the British firm Levity Crop Science, does precisely this: it holds nitrogen in the amine form.
Taken up by the tree, this stabilized amine can influence growth partitioning — the way in which the tree allocates growth. It’s a nitrogen supplement rather than a solo source; growers should use it in conjunction with conventional inputs, rather than as a substitute. Used on perennial tree crops, it helps the tree distribute growth more evenly between vegetative and reproductive development — delivering the more even fruit and flower development we want.
Shortening the internodes allows more fruit per unit length of branch. It also reduces the need for pruning (keeping that valuable nitrogen in the plant, rather than on the clippings pile) and makes for a better canopy shape. This ‘growth in the right places’ strategy will bring yield benefits to all three parts of the growing cycle — and make alternate yield bearing a thing of the past.
Cell Power® SizeN® should be applied regularly through the season, using drip or foliar application at 2qts/acre per month.
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© OMEX® Agrifluids, Inc. 2020.