Too many grapes spoil the crop
Viticulture is a weird form of farming. It is perhaps the only one where the grower tries to restrict the amount of fruit that his crop produces.
Yield is a measure of that amount. It is often expressed in hectolitres per hectare but can also be measured in Kg/vine.
That is because the grapevine, left to its own devices, would produce more bunches of grapes than it could ripen to the level required to make wine. The vine doesn't care. It doesn't know we grow it to make wine. It is happy if birds eat the berries and spread its seeds.
Vignerons need to find a way to force the vine to over-ripen its fruit to a point where the sugar levels are so high and the acid levels so low that we can make wine from the grapes without having to add anything.
That is why we prune the vine in winter to restrict the number of new shoots it grows and, therefore, the number of bunches it can produce. Each shoot should only produce one or two bunches.
Sometimes pruning is not enough. Some soils and some regions will still have vines that produce too many bunches. These are removed as a 'green harvest' in summer.
Often, young vines tend to overcrop, whereas old vines tend to undercrop. This is why Vielles Vignes is seen as a term of quality and AOC wines cannot be made from vines under five years old.
The danger of having too high a yield is that the vine will ripen some bunches and some grapes more than others and, as the harvest is done in one go, the crop will contain more variation than desired. In red grapes this is particularly important because the skins all need to be ripe and dark too. Too many pale grapes will result in a thin, pale looking wine with no body.
It is not just the number of bunches that determine the yield. The size of the bunches and the size of the individual grapes also matters. Much of the flavour in the wine comes from the skins and the layer just under the skin. Therefore, a small grape gives more flavour than a large one.
This is why every Appellation in France has a limit on the yield-per-hectare to maintain a minimum quality level. Higher-quality independent producers will often have yield levels much lower than the AOC limit as they want their wines to stand out.
The terroir of the vineyard will have an impact on yield. Wine growers do not want rich soils, plots with damp soils or areas with heavy rainfall. Some will grow cover-crops or allow wild plants in the vineyard to introduce competition for water and nutrients.
However, heavy pruning can lead to more leafy growth, which will encourage fungal diseases and restrict skin-ripening through too much shade. Some vignerons may brag about 'ridiculously low yields' to suggest their wines are better when, in fact, the yield is artificially low because of dead and diseased vines.
As in many things, balance is the key word. As balanced vine will produce a homogenous, balanced crop that will make a wine where alcohol, acidity, tannin and concentration are all in balance.
Jonathan Hesford has a Postgraduate Diploma in Viticulture and Oenology from Lincoln University, New Zealand, and is the owner, vigneron and winemaker of Domaine Treloar in the Roussillon