These two operations have different objectives and are not always performed at the same time. They also have common objectives and this is one of the reasons why they can be confused depending on the stage in the growth cycle. The desired effects of these operations can be physiological, climatic or practical.
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Physiological effects: The main consequence of cutting branches by trimming and/or topping is to temporarily stop the branch growth and to move the flow of sap towards the inflorescences. This operation consists in limiting vigour and redirecting the flow of sap from the main branches and young leaves to the future bunches, thereby reducing the risk of coulure (grape shatter). This practice also reduces the stress that can affect younger vines during the summer, when their potential transpiration is greater than their root absorption capacity.
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Effects on the vine/bunch microclimate: Trimming/topping reduces the foliage load. By removing parts of the shoots and leaves, the amount of shade cast is reduced and the bunches get more air and sunlight.
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Practical effects: These operations have many practical advantages. First of all, they guide the vine horizontally and maintain uniform growth. The reduction of the foliage height also means that the vine is less likely to be damaged by wind. Moreover, the removal of cumbersome vegetation facilitates the passage of machinery between the rows and avoids any branches getting caught in machinery and broken. It also helps to reduce the risk of fungal diseases attacking new growth that is more sensitive to such diseases. Another advantage is that pesticides can penetrate better which reduces the need for repeated applications. It can therefore also represent an economic advantage.
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Effects on yield and quality: When well timed during the growth period, trimming can increase the yield by reducing the shatter rate. On the other hand, trimming too severely can have a negative impact on the yield and the quality of the grapes. Removing too much leaf area disrupts the ratio of ELA (effective leaf area) to grape volume and can contribute to delayed ripening, lower sugar levels and higher acidity.
These two operations can also have different objectives. This will be discussed in the following section.