Thinning of less than 30% gives rise to compensation phenomena. These phenomena result in an increase in the size of the bunches and/or berry weight, but have little or no impact on the quality of the remaining grapes. It is advisable to thin the bunches by 30% or more to overcome these compensation phenomena and to significantly improve the ripening process. Thinning 50% of the bunches will result in a yield reduction of around 30 to 35%. However, it is important to remember that each vineyard is different and that 30% can be very variable from one plot to another, as well as from one grape variety to another, etc.
2.3.9.d How aggressive should thinning be?
What is the appropriate number of bunches to leave per plant?
The number of bunches to be removed per vine is determined by the yield objective, the microclimate, the vine's vigour and its compensation. It is necessary to remove at least 30% of the bunches to overcome the compensation reaction. Thinning should be carried out when the vine is too vigorous, when the estimated yield is too high and when the load is likely to have a negative impact on the quality and/or the vines' health. Yield estimates are a reliable way to assess the plot's theoretical yield.
Which bunches should be kept?
In the case of thinning, it is preferable to remove bunches affected by cryptogamic attacks (downy mildew, botrytis, etc.), and those damaged by herbaceous projections and passing machinary, in particular during the leaf removal and the passage of the leaf shredders (continuous air flow which shreds the leaves). If all the bunches are healthy, leaf removal should be implemented to provide better ventilation of the vine's base and to reduce the compactness and overcrowding of bunches. Thinning is also carried out in order to distribute the load more evenly. The bunches located higher on the branches should be removed first.