Though defined simply as how much fruit is on a vine, yield is a complex and controversial topic.
There is a long-standing idea in wine that lower yields lead to better wine quality, however, research and modem viticultural advances have shown that lower yields do not inherently mean higher quality. Low yields may be advisable where seasons are short or disease pressure high, but, determining appropriate yield levels depends on many interconnected factors between site, variety and cultural practices.
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Most viticulturists now embrace the idea of "vine balance", which considers these factors, but on the critic and consumer end, lower is still often equated with better. Lowering yields more than necessary is not only a less sustainable way to farm, but may lead to added cost in grapes and wine, putting industry members and consumers at a disadvantage. There's a lot tied to yield.
An idea that is key to understanding this article, summed up by Mark A Matthews PhD, a professor and plant physiologist at UC Davis, is that "the most consistent observation in viticulture is that if you hang enough fruit, ripening is delayed; and, if you reduce yield, ripening (sugar accumulation) is advanced". Everything below ties back into this simple observation.
Yields are measured in terms of tons per acre or kilograms or hectoliters per hectare. Two vineyards may both produce three tons per acre, the former planted with 1000 vines per acre, the latter with 4000, meaning there is much more fruit on each vine in the first vineyard than the second. As vine planting densities vary from a few hundred vines per acre to more than 5000, a note about the vine density – and often spacing – is usually given; for example, three tons per acre with 1700 vines per acre on 5'x5' spacing.
What size crop a site can produce depends on vineyard and vintage characteristics; most importantly water availability and soil water holding capacity, as well as climate, vine size and the grape variety's genetic potential with regard to yield. A more vigorous site can be planted to fewer, larger vines, each producing more fruit, or more smaller vines, each producing less fruit; in most cases more vines, each with less fruit, give higher overall yields.
The closer that vines are together, the more they compete for resources – water and nutrients below ground, sunlight above. This is why vines on dry-farmed sites in drier areas are spaced farther apart from one another. Most vineyards in the New World, however, are irrigated, allowing for much denser plantings and larger yield potentials. In irrigated vineyards, or dry-farmed vineyards with sufficient rainfall, the limiting factor becomes access to sunlight. Dense canopies from excess vigor can block airflow and lead to disease, as well as create shading that inhibits yield the following year.
Although we measure yields on a per acre/hectare basis, the important thing to remember for understanding the implications of yield is that each individual vine, based on the resources available to it and its genetics, has a certain amount of fruit it can ripen before the season ends.
Regardless of what growers do or the environment encourages, everything on the vine has to pass through the lens of genetics. Planted in the same vineyard and treated in the same way, different varieties can support different crop loads without a risky delay in ripening.
Genetics result in differing yield potentials because of differences in berry size/weight, the number of berries per cluster, and typical amount of clusters per shoot. Berry weights, for instance, range from half a gram to two grams. Berry size not only differs between varieties, but between clones of the same variety. Berry size is affected by water availability and can be manipulated with irrigation.
Different varieties produce wildly different amounts of flowers (potential berries). Note the extreme difference in the amount of berries between Chardonnay and Palomino Fino. Further, varieties differ in their typical fruit set percentage – that is, how many flowers actually become grapes (Keller 2015, 195).
Growers aim to set crop levels by different means and at different points during the year, but it is not entirely up to them, as the vintage's unknown environmental factors play a large role as well. "I find it a constant challenge to anticipate the season and the yield levels and pre-empt how the vines will finish off, where that maturity level will be, and what crop level is possible each season," says Dylan Grigg PhD, of Meristem Viticulture in the Barossa Valley, Australia. "This is especially important in non-irrigated situations."
Grigg says: "A good canopy of adequate shoot length and spacing is the first prerequisite before any yield discussion. This starts in winter [with pruning]." Every cluster and shoot on a vine has to share limited resources, so by knowing their sites, growers aim to leave a quantity of buds per vine that will lead to a crop load that vine can ripen on time, and to create an appropriately sized canopy to ripen fruit, without causing too much shading or blocking airflow. Too few buds means excess canopy growth and few clusters; too many means too small a canopy with too many clusters.
This is the idea behind "vine balance". Some argue that vine balance is a misleading concept, as it implies a particular balance point that is ideal for each situation, when in reality, there is a fairly large range of yields and canopy sizes that lead to fruit with similar characteristics (Matthews 2015, Chapter 2).
Growers may remove clusters during the season if they are concerned for ripening delays or that a congested fruit zone might lead to disease or shading. Robert Jordan, owner of Grand Crew Vineyard Management in Napa, California points out that things like "fruit dropping and cutting wings reduce yield and lead to quality improvements. However, this is because you're creating a better environment for fruit, not because you have a lower yield."
Jose Luis Beade of Adega Beade farms six acres of vines in Betanzos, a small wine producing area in northern Galicia, Spain, growing Branco Lexitimo and Mencia. Betanzos is one of the most difficult grapegrowing environments in Europe, owing to extreme humidity, high rainfall, and cool temperatures – typically 61-81F (16-27C) around harvest. At these temperatures sugars struggle to accumulate and temperatures are in the sweet spot for disease growth 24 hours a day. "We lower yields to reach a good level of maturity as early as possible, most importantly to have healthy grapes and to minimize botrytis damage," he says. "We're authorized to grow two tons per acre, but we keep it to under one ton to guarantee good maturity. Some of the higher, cooler plots, we go down to nearly a half ton per acre."
It is by maintaining such low yields that Jose Luis can reach his quality goals. However, it's important to note that although the low yields enable him to get the quality he wants, it is not the low yields themselves resulting in better quality, it is the fact that vines can ripen grapes faster with less fruit, and in his environment less fruit allows him to get desirable sugar levels and to harvest early enough to avoid disease. In a more forgiving climate, that could be done with a larger crop.
It should be clear that what might be a high yield for one site or variety may be low for others. Though the days of extreme low yields may be 10 years behind us, in some wine circles the concept remains deeply ingrained that lower yields result in better wines, or are necessary for quality. It's apparent in endless examples of wine marketing and critics' claims, some more vague and others giving blanket statements about crop loads not to exceed, regardless of site or variety.
On how the idea got started (it's been around since the Romans), Matthews comments: "In short, because it was true. In the long arc of wine growing, there has been one primary concern: to obtain grapes with higher sugar. Thus sugar accumulation was tantamount to ripening, perhaps initially to get...more stable wines; 12 percent was a blessing." Lowering yields so sugar accumulated faster would be a means of ensuring this happened in more years than higher yields would allow.
The reason this was more of a concern throughout history than today is thanks to the advances we've made in viticulture, like understanding and avoiding disease, water relations and irrigation, canopy management, plant nutrition and better clones and rootstocks. Germany, for example, saw a fourfold increase in yield over the 20th Century, "without comparable changes in ripeness" (Wine Science 2014, 124). More recently, climate change is also a factor, and much of Europe is harvesting earlier than in recent centuries, allowing for acceptable ripeness on a more constant basis, without as much risk of the season's end.
You might be thinking "yes, yes, you need reasonable sugar levels and healthy fruit to make great wine, so don't hang too much fruit – but what about quality?" These facts don't rule out that the possibility that lower yields might make for better wines, perhaps more "concentrated", "intense", "complex", which is often the rational given for lowering yields. In his essential book Terroir and Other Myths of Winegrowing – whose chapter on yields inspired this article – Matthews gives an exhaustive overview of the last 60 years of studies done on the relationship between yield and quality, many comparing wines from the same vineyard, either pruned, irrigated or treated in differing ways to give different yields.
The different studies each look at the relationship between yield and varying quality attributes, such as overall sensorial quality, color density or aroma compound concentrations. Nearly all of these studies – which Matthews interprets, contextualizes, and explains related plant functions – show that there is no direct connection between yield and quality. In fact, tasters in many studies could either not distinguish between wines of differing yields, or preferred those from the higher yielding vines.
This is not to say that wine character does not change at all when yield is different, but again, the differences result from various factors, not directly yield itself. For example, where yield is reduced by water restriction, more fruit character is noted. This is caused by the effects of water stress; yield reduction is a symptom of that stress, just as with the increased fruit character.
What's the closest we can get to differences that yield itself makes on wine character? It's established that lowering or raising yield will speed up or delay ripening, and the amount of time a grape is on the vine has implications for its character. Here, the changes result from how long the grapes are on the vine, which is also regulated by environment.
Differing "hang times" have an impact on aroma profiles, perhaps most obviously with methoxypyrazines, which decline the longer grapes hang, but with other aromas as well. Earlier picked whites tend to be more floral, citric and "mineral", reds more earthy and herbal. Further into ripening, both whites and reds tend to become increasingly dominated by fruit aromas. The pace of ripening affects this; slower ripening, like longer hang time, will push wine aromatics toward the fruitier end of the aroma spectrum for a given sugar level, and vice versa. Other factors contribute to these differences as well; sunlight usually increases fruitiness.
The longer the grapes are on the vine, the more malic acid is metabolized and lost (again, temperature also affects the rate of acid loss). As we've seen, higher yields cause sugar to accumulate slower, leading to lower sugar concentrations relative to other ripeness aspects.
So, although yield itself may not directly affect wine character or quality, changes in yield level can indirectly cause character differences, and crop level decisions can allow for fine tuning of wine chemistry, aromatic character, and ultimately, style. But, "it's simplistic to call for 'one bunch per shoot' or x kg/m unless you know the site really, really well", says Grigg. While fine tuning one's stylistic goal may sound like "quality", style preferences are highly subjective.
Napa is historically associated with seeking and advertising low yields, however Robert Jordan, who farms some of the valley's biggest names, says that's not the case these days – in most cases. "Nobody says 'I want one to two tons of Cabernet Sauvignon. Instead of five or six, we want four to 4.3, because the fruit is better spaced with better exposure.”
That's not to say people don't intentionally lower yields in the name of quality, but Jordan says this is usually market driven. "Maybe they have too many contracts and don't want as much fruit... Buyers always use the excuse that it's for quality, but that's not always the case."
Matthews feels similar motivations were behind historic movements to lower yield in the name of quality, but on a regional scale, and that these served to reinforce the high yield/low quality myth. "When limiting the acreage of an appellation was insufficient to secure a decent price, rules grew to also include crop yield." Today, if French growers exceed permitted yield levels in a vineyard, they only have to declassify the amount of wine that was in excess, not all the wine from that vineyard. This is clearly only to regulate supply and demand; if quality were a concern, all of the grapes that came from an overcropped vineyard would need to be declassified.
Thankfully, growers and winemakers do seem to be less strict about maintaining low yields than in past decades, but the press and public still often have the expectation that yields should be low.
But, one has to wonder, where yields remain unnecessarily low, even a small amount, is that not a disservice to the industry, consumer and the environment? Where yields can be safely raised, grape and wine prices could be lower. This would not just be easier on the consumers' pockets, but also lower risk and interest costs along the supply chain. Higher yields would allow for more efficient farming too; more grapes from less acreage mean fewer tractor miles, lower emissions, less chemicals sprayed and less natural land converted to farmland. To be clear, this is not to say that yields should be increased, only that where they are lower than necessary, allowing for more could help with these issues.
Grape prices in the US are already significantly higher priced than the vast majority of European fruit, where there is no shortage of inherited, inexpensive, or abandoned vineyards given away free by local governments. Contrast this to most quality-oriented parts of California, where unplanted land is prohibitively expensive, and planting a vineyard costs $30,000-$80,000 per acre – sometimes more – not including the land itself. Unnecessarily low yields just add to the cost of inherently expensive fruit.
Our understanding of yields has developed a great deal over the last century, and will surely continue to do so in the future. It's a welcome change that the practice of keeping yields unnecessary low is dissipating some, and hopefully the message reaches consumers soon. In an industry that often holds fiercely to traditions and historic notions, it's important to use science to help shed light on which of these notions to keep and which to let go. It's one of the delights of wine that by diving into a seemingly simple topic – like how much fruit to allow on a vine – we find a highly interconnected web of information from which to gain insights into this mysterious beverage.