Dissertation – Thinning
Crop Load Management
“Thinning” is a crop load management technique that has been practised in modern-day fruit production for many years for high yielding varieties, which involves removing excessive fruit from the trees. Effective crop load management is important for consistency in yield and ensuring a high fruit quality. For this experiment, we manually removed coffee cherries on the Caturra coffee trees with a high crop load at its seed ripening stage so as to leave a larger pool of sugars and nutrients for the remaining fruits to raise quality. The thinning process is also crucial because it can prevent a phenomenon known as "biennial bearing", where coffee trees alternate between high and low yields, or even suffer from branch dieback due to overbearing, by emphasising growth of new fruiting wood for next year. So, this not only improves the quality of the harvest for this year but also helps maintain consistent yields in the long term. While thinning isn't widely practised in coffee cultivation, we believe it holds promise for improving crop quality and yield stability.
Scientific Background
The relationship between sources and sinks
The relationship between sources and sinks are known to play a crucial role in regulation of vegetative growth and flower- and fruit development in fruiting crops. Sources are defined as being producing and net exporting organs or tissue of assimilates (photosynthates/carbohydrates/sugars), whereas sinks are net importing organs or tissue depending on the import of assimilates for its development. Leaves are the most important organ for the production and export of assimilates and their capability to do so is dependent on the leaf area and the photosynthetic activity. Sinks comprises any importing organ or tissue including fruits, flowers, roots, shoots, and young leaves.
Growth and development of plant organs is ‘source-limited’, meaning it is limited by the photosynthetic resources produced in the leaves. Though, leaf photosynthesis is regulated by the activity and presence of sinks. An increasing sink strength will influence the source activity prompting the source to increase its photosynthetic production and its transport to go faster, and conversely, down-regulate production when sink presence and demand decreases.
The sinks compete for these assimilates produced in the leaves, and the relative strength between sinks and their individual ability to attract assimilates will to a large degree determine the carbon allocation and hence the plant- and fruit development. Therefore, a good allocation of resources between sinks is crucial for a balanced development and to optimise the reproductive fitness of the plant.
Due to the competition among sinks and the limitation of sources it is essential to clarify the relationship and dynamics between sources and sinks. Way too few studies have been performed to try and elucidate these relations in the coffee plant. The following trials and their results are some of the first of its kind, which we wish to now scale and try on a larger scale with interested growers.
Dynamics in Coffee
The relationships between vegetative and reproductive growth in coffee are rather complex and poorly understood. In most regions, rapid vegetative growth and fruit development appear to take place at different times, suggesting a seemingly opposition or competition between the two processes. This is of considerable relevance because flower buds in coffee are initiated on the same wood only once, thus the amount of growth produced in the current season will largely determine the crop yield the following growing season.
Under plantation conditions, both assimilates and mineral resources might therefore be diverted to the fruit at the expense of the shoot apices and root system. Following consequences often include biannual bearing or overbearing dieback. However, if both the reserves and the supply of current carbohydrates are high, or if trees fruit lightly, then vegetative growth would not be expected to be greatly restrained.
In addition to diverting large amounts of carbohydrates/assimilates, coffee fruits, particularly during their rapid expansion phase, may draw over 95% of the current total uptake of N, bringing about N-deficiency symptoms in foliage and restricting vegetative growth at the expense of the shoot apices and root system. However, if trees fruit lightly, and reserves and production of assimilates are high, then vegetative growth would not be expected to be greatly restrained under well-fertilised conditions.
‘’Considering fruits as a sink organ, the relative strength will depend on the number of fruits and their ability to attract assimilates. Coffee fruits are strong sinks outcompeting vegetative organs for assimilates. However, since the system is source limited, the apparent increase in photosynthetic activity with increasing fruit number is usually not able to compensate for the greater number of sinks, likely resulting in lesser dry matter partitioning per sink organ. Conversely, this also suggests that the removal of fruit at high fruit/leaf ratios may promote dry matter concentration and fruit size in the remaining fruit’’ – Poma Coffee
Impact of Thinning & When to Thin
To highlight the impact of thinning, we initiated a trial to understand the effects of fruit-loads on the tree at different times during fruit development, and how this influenced fruit quality as well as vegetative growth for the cultivar Caturra. The goal was to find the framework to work with thinning within coffee crops to optimise the balance between crop yield, fruit quality, and initiation of flower buds for following years.
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First objective was to understand how fruit-loads at different stages of fruit development affected the developing fruits and vegetative growth to determine when to perform the thinning.
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Second objective was to understand the impact of different levels of fruit removal on fruit development and vegetative growth by removing fixed percentages of fruit.
Initially, it should be said that on a low cropping year, removing 50% of the fruit would be a catastrophe — therefore, using a ratio between leaves and fruit is a good work methodology to determine the need for thinning. We call this ratio LAI (Leaf Area Index) and is calculated by dividing the leaf area with the fruit mass. Aiming for a leaf area index between 12-17 is beneficial for coffee cultivation as seen proven in the following. In the following, you will see some development identification keys called ‘BBCH’. They refer to a certain stage of development, which can be seen on the following figure.
Table 1 - Timing of thinning
From table 1, we clearly see an opposing reaction between vegetative and reproductive growth — branch growth is greatly suppressed with a heavier fruit load and is relieved with thinning practices.
Early-Stage Thinning (BBCH 70/71)
Development Phase: At this stage, fruit development is primarily driven by cell division.
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Quality: Thinning reduces competition during cell-division, which result in larger fruits and seeds. Despite larger fruit size, sugar and nutrient concentration in seeds and fruits is diluted, resulting in minimal impact on overall quality
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Yield: Yield recovery is highest when thinning occurs early.
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Vegetative Growth: The vegetative response is strong, promoting better flowering in the next growitng season.
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Key Takeaway: Early thinning is effective for getting larger fruit and seeds and improving vegetative growth for flowering in the following season.
Mid-Stage Thinning (BBCH 75)
Development Phase: This stage focuses on seed maturation, with fruits reaching about 50% of their final size and the seed finalizing its development.
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Quality: Both fruit and seed sugar concentrations increase significantly. The peak in seed sugar concentration suggests a large assimilate pool being redirected to the remaining fruits, leading to better seed filling and higher quality.
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Yield: Yield reduction is slightly higher than in early-stage thinning.
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Vegetative Growth: Strong positive response.
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Key Takeaway: Mid-stage thinning enhances seed sugar concentration, seemingly correlating with improved cupping scores, and vegetative growth.
Late-Stage Thinning (BBCH 81)
Development Phase: By this stage, phenological development of seeds is complete, limiting further sugar translocation to the seeds. Mainly fruit flesh development.
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Quality: Fruit sugar concentration dramatically increases. Seed sugar concentration, fruit size, and seed size is not affected.
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Yield: Comes at the cost of significant yield losses due to reduced fruit and seed weight as no compensation has time to happen.
Vegetative Growth: Shows less improvement compared to earlier stages.
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Key Takeaway: While late-stage thinning boosts fruit sugar concentration, it results in substantial yield reductions and minimal vegetative benefits. Only relevant for lots that need high levels of fermentable sugars.
No thinning
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Quality: Results in the lowest cupping scores.
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Yield: Increases the likelihood of biennial bearing, negatively affecting long-term productivity.
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Vegetative Growth: Severely suppressed in high years, extensively exaggerated in low years.
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Key Takeaway: Failing to thin can have detrimental effects on both quality and plant health
Figure 1 – Tendencies between thinning timing (BBCH stage) and vegetative growth, seed sugar concentration, and fruit sugar concentration.
The results demonstrate that coffee growth—both vegetative and reproductive—is highly source-limited. Thinning effectively alleviates source-sink stress, offering a valuable tool for manipulating plant development. Additionally, seed sugar concentration shows promise as an indicator of green coffee quality. Sugars, along with other nutrients and minerals, likely serve as precursors for a wide array of aroma compounds, enhancing both the range and intensity of aromas in the final product.
At Poma, we prefer to time the thinning to coincide with seed ripening, which occurs before the actual fruit ripening stage. This ensures that we're focusing on improving the quality of the seeds themselves while getting a good vegetative response. If the goal is to increase fermentable sugars in the fruit flesh for processing, thinning can be done later, but it won't address the issue of biennial bearing. Our 3-year study on the nutrients and sugars in the seeds and its effect on coffee quality by using GC-MS has suggested that an increase in nutrients and sugars in the seeds is strictly correlated to a greater diversity and intensity of aroma compounds, resulting in a more complex and amplified coffee drinking experience.
