Why do crop estimates?
Crop estimates are important for many reasons for both grower and winemaker. Growers want to produce high quality fruit to get the best price for their grapes, or if it’s for their own wine, then to get the best quality wine. Ideally, the grower really wants to get the best quality grapes AND maximise their yield AND maintain the health of the vineyard. The crop load also affects harvest costs and logistics such as the timing of harvest, ensuring you have enough pickers etc. Of course crop load can also affect vine health.
As one researcher writes, “Overcropping a vine has many well documented negative impacts on fruit and vine quality including reduced and delayed fruit ripening, potential vine stress that may lead to increased susceptibility to winter injury, disease problems especially late season rots and other significant problems. Undercropping can also affect wine quality, as well as cheat you of valuable revenue.”
Vintner Will Davenport also pointed out “Knowing your (accurate) crop estimate can help to understand if fruit thinning is necessary and how much of the crop needs to be removed in order to keep the vine healthy and ripen the remaining grapes.”
Winemakers are also interested in crop estimates as they need to juggle tank space, and want to know which grapes are coming in when. Plus they need to know how many grapes they will be buying, and what they will then produce and have to sell.
Getting your initial crop estimate
Early in the season, often around flowering, you establish the number of clusters by going out and counting the number of flowers (we are referring to the whole inflorescence as a flower here) in each different block, or plot, of similar vines. This is a quick and easy way to give you an estimate of the number of bunches of grapes you will have, and therefore a general estimate of your yield.
There are a few things that can affect the accuracy of this part of the prediction significantly. If there is a frost, or a heavy rain/hail incident before fruit set and many flowers are lost, or if just before picking badgers come and eat all your fruit. But for the most part the majority of flowers will turn into bunches and bunch number is pretty predictable.
Using the Sectormentor for Vines app it’s very easy to collect flower counts on your phone and then the app automatically averages those counts for you, per block or variety. Our yield predictor tool is setup to assume 100% fruit set initially, but if there is some incident that changes this, it’s easy to go back and reduce the % fruit set for each block or variety.
Finding average bunch weights for your vineyard
The variable that is much less easy to anticipate in yield prediction is bunch weight. This varies every year depending on weather and the many other facets of the natural world that keep us on our toes. It is also often different for each variety or clone. It is only once you monitor bunch weights for each plot for a number of years that you start to have a pretty good idea of the average bunch weight for your vineyard, or at the very least the maximum and minimum average weights.
If you aren’t already monitoring this, then we definitely recommend recording average bunch weight for each block starting this harvest, this information is invaluable in the long term, we would consider it an asset to the vineyard! Of course this is very easily recorded and accessed with the app.
THE TRICK! How to get a more accurate crop prediction
To get an accurate yield prediction is part art, and part science. Knowing your average bunch weight stands you in good stead, but we have spoken to quite a few vineyards and all those that consistently get within 5% accuracy on their yield prediction use the beautifully simple art of going out and actually looking to judge (guestimate) what the average bunch weight will be this year, and then use that to adapt their yield prediction. This happens around the time of veraison, when vineyard managers will return to each plot in the vineyard and look at sample vines, visually estimating the eventual bunch weight for each bunch on the vine. This is the ‘art’ part and definitely gets easier the more experience you have.
How do other vineyards do it?
We have heard some clever ways of doing this:
Once at least 50% of the grapes have gone through veraison, Nigel Riddle at Wodetone Vineyard in Dorset picks a large bunch of grapes for each block and weighs it so that it’s 100g (take off extra grapes if it’s too heavy). He then goes to a series of sample vines and holds the 100g bunch in front of the bunches on the vine. By visually comparing with the 100g bunch in his hand, he can estimate the average weight of the bunches on the vine. After doing this for a while he doesn’t even need to carry around the 100g bunch with him as he can reasonably estimate the weight of each bunch just by eye. We chatted with Nigel and he agreed that to get started it might be best to compare the 100g bunch to each individual bunch on every vine at your sample site and record visually estimated weights on a bunch by bunch basis. But as you get better you can begin to record an estimated average bunch weight on a vine by vine basis.
At Davenport Vineyards the team have been working with the same vines for many years and they use a combination of historical data and visual surveying to optimise their prediction. They go out to their sample sites just as veraison begins and count the number of bunches on each vine. This gives them an accurate % flower set number which is helpful in future years, it also verifies the actual number of bunches on each vine. At this point they also do an estimate of the average bunch weight on each vine. The Davenport team do this purely by eye, they know what the maximum and minimum average bunch weight was for that variety and block in previous years. They use this range of numbers and their experience from past years to judge if the grapes on each vine are larger/smaller compared to previous years, and then estimate an average bunch weight for each vine at the sample site based on this. Will recommends making a best-case and worst-case scenario yield prediction for each variety based on the range of bunch weights you would expect for that variety. Luckily, this is easy to do using our yield predictor!
Another US-based vineyard told us how they use the lag weight method. Lag phase is a period of little or no growth in berries between two periods of rapid growth, it’s the point where the plant puts its energy into hardening the seeds and starts really building sugars shortly after. It happens just around veraison, and approximately 50-60 days after bloom. (See the graph below for a nice visual representation of lag phase). The vineyard manager will actually weigh the clusters on the sample vines at lag phase. It is estimated that grapes increase in size by 50% from lag phase to harvest, and therefore they multiply the lag cluster weight by approx 2 to estimate the final cluster weight and yield. Then in the weeks before harvest they will go out and look again and based on previous years experience they might increase or decrease the cluster weight multiplier (e.g 1.8 rather than 2).
Of course a vital part of this method is getting the multiplier correct for each block/variety based on how big the clusters look on the vine that year. It was also interesting to hear that Nigel at Wodetone doesn’t use a multiplier in his estimates when he weighs the 100g cluster a number of weeks before harvest, however he consistently gets good estimates. Nigel pointed out that most sparkling wine is harvested at 18 Brix, rather than 22 Brix, so we would expect a much lower weight increase between veraison and harvest for sparkling wines. We reckon it’s a combination of slightly smaller grapes at harvest for sparkling wine in the UK, and the fact that we humans are ever-optimistic in our estimations, which means that his method works perfectly without taking into account any increase in weight of individual bunches.
Do the same thing each year
A key to getting a good consistent yield prediction is to use the same method each year. It is also important to not be disheartened when you are starting out, as predictions may be up to 20% out, this is ok initially! As you get more experience and build up your numerical vineyard history it will all become more accurate and easier. If you are consistently getting it way out after a couple of years, then you probably need to sample more vines…or rethink your methodology.
How do you do your yield prediction? Do you have another method you think works well?
We are always learning what works and how people do crop estimates on their vineyard so please do email us if you have anything to add or any questions.
Using Sectormentor for Vines for your yield prediction
We have built Sectormentor for Vines to work with both the art and science of vineyard monitoring. The app makes it very easy to collect bunch counts and bunch weights out in the vineyard and then the yield predictor makes it easy to turn that information into a yield prediction for each block or area of vines you want (see diagram below). The tool is also flexible so that you can consider minimum and maximum predictions. Of course this is just one of the many tools the app provides, including a Ripeness Monitor, Soil Health Indicator and more. Please get in touch with any questions or if you would like to hear more about how the app might work for you.
Thank you to Nigel Riddle at Wodetone Vineyard, Will Davenport and Phil Harris at Davenport Vineyards and the following resources for help putting this article together.