Running a small farm, one question is always there. Which tree gives the most harvest and the best quality? I wonder every year, but once harvest starts there is no time for it. The data give a rough idea, but much remains unconfirmed. This time I wanted to check whether agronomy backs it up.
Overload a tree with fruit and, during the period when fruit fills, the fruits compete for the products of photosynthesis. Seen as leaf area per fruit (leaf-to-fruit ratio), heavily loaded trees have 6 to 8 cm² per fruit, trees with reduced load about 20 cm². When the share per fruit falls, beans are smaller and less filled (Vaast et al., 2006). Indeed, thinning fruit to lower the load raised the share of large beans (above the 6.75 mm screen) from 56% on heavily loaded trees to 67%.
The interesting part is where the cost appears. In a study that maximised fruit load, the cupping score (SCA, out of 100) of a carefully processed lot was about 80.5 and did not differ with load. On the other hand, compared with lightly loaded trees, the share of large beans and of sound beans fell, and the number of leaves fell by 32% (León-Burgos et al., 2024). Fewer leaves means next year's crop is being eaten in advance.
But the two are not impossible together. If you avoid the competition, both grow. In a study that concentrated fertilisation in the fruit-swelling period, compared with normal fertilisation, yield rose 8.86% while the cupping score (SCA, out of 100) rose from 81.13 to 82.63 (Li et al., 2023). On the variety side, too, there was no intrinsic negative correlation between yield and quality (Bertrand et al., 2021). The trade-off is not a law of nature but a mismatch of supply and demand.
"We do not judge a harvest by that year's kilograms alone. Will we get the same quality next year; is each bean properly filled. These three decide how fast we grow."
"In truth, we do not worry about it that much. Hardly at all, really. We make as much shade as we can, of course, but in practice nobody can go around measuring square centimetres per fruit. That is the gap between research and reality; what matters is understanding it in theory."
For why ripeness splits see Why fruit of different timing shares one tree; for designing records, What to record for each lot at harvest; for the whole picture, the overview of harvest.
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