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http://www.vinesforwines.co.uk/learn/vineyard-management/can...

Canopy Management
Laterals or side shoots
Once a shoot starts to develop it may produce additional shoots from each leaf axil. These are lateral or side shoots and generally these must be managed to prevent overcrowding.
Commercial growers cannot usually afford the time to tend every single shoot and one of the main aims of the trellising method, soil and canopy management is to produce the maximum grape yield for the minimum cost - including labour cost. This means limiting side shoots and the rule of thumb is that the fewer nodes per vine that are left after pruning, the more they will tend to produce laterals. Unpruned vines produce the fewest side shoots and, indeed, this is one training method, but not one recommended for amateur growers.
The amateur is not so constrained and can pinch out or leave selected side shoots as required. With a newly planted vine it is customary to remove all side shoots in order to build up a strong shoot that will form the main stem of the vine. With a mature vine it is for the grower to decide how to treat the side shoots.
One year I returned from a brief holiday to find the leaves on my vines looking as though they had been machine gunned. I discovered that whilst I was away there had been ‘hailstones as big as golf balls’. To compensate, I allowed the laterals to develop and stopped these (i.e. pinched out the lateral leader) at just one leaf.
Another important factor to remember is that leaves are the ‘factories’ that produce the wherewithal for the plant to grow – including the sugars for the grape. A young leaf is created either from reserves stored in the plant or from products ‘exported’ from mature leaves. It is a waste of the vine’s efforts to create lots of side shoots (if these are not allowed to bear fruit). Even worse, the unwanted foliage may screen sunlight from the bunches and discourage ripening.
What is clear is that the lower the leaf is on the stem (i.e. the earlier it was created) the sooner it becomes a net exporter rather than an importer of photosynthesis products. Moreover, a leaf below the first bunch cannot shade the bunch (assuming the shoot is growing upwards), so itshould be beneficial to allow one or at the most two leaves on the laterals below the first bunch. In practice, these get in the way of the developing bunches and if not removed can make harvesting more difficult. Above the bunch it is a matter of compromise – shaded bunches versus exporting leaves. The side shoot at the top of the full length shoot is the least likely to become a net exporter, but it is also the least likely to shade the bunches; so it is really a matter of experiment to see what produces the best results with your vines on your plot using your method of trellising. According to Smart a leaf needs about 30 days to become a net exporter – and that in warmer climates than the UK. Remember that untended laterals can very quickly cause shading and overcrowding that makes canopy management difficult and time consuming, so the simplest rule is to remove all laterals.
My personal practice is to remove all side shoots except the ones at the top which I pinch back to one or two leaves depending on where they are. New side shoots will develop and these are again pinched back to one or two leaves. This practice causes a ‘flower’ of leaves to develop at the top where they are exposed to the maximum sunlight without any risk of shading the bunches.
Some methods grow shoots downwards. In this case the above needs to be modified as appropriate.
Laterals do have one very useful property, however. Should the tip of a growing stem break, the top lateral will take over as the leader; this sets the shoot back, but the stem is not wasted.