Know your vines #7: How to optimise your yield prediction and the power of the Yield Predictor Tool – PART 2
In our Know your Vines blog series we share practical tips on what metrics to monitor in your vineyard. This is the seventh instalment, stay tuned for more as the seasons unfold! “If you can’t measure it, you can’t improve it.”
Use bunch weights at lag phase and veraison to optimise yield prediction
In part 1 we looked at optimising your yield prediction by getting accurate bunch counts. As we move through the season there is more information available to us, allowing for our yield predictions to be optimised even further. Part 2 of yield optimisation starts with weighing actual bunches in lag phase, or at veraison, and then again at regular intervals in the run up to harvest. Last-minute losses due to disease can also be taken into account at this stage. Many thanks to Luke Spalding at Chalk House Vineyard for sharing his tips and tricks for getting a good yield prediction.
Third prediction: Weigh bunches at lag phase or veraison
For the next step of your yield prediction it is time to start weighing your bunches! This can be done at lag phase, or veraison. Weighing bunches at this stage you get a better idea of what the bunch weight will be for each varietal this year. Theoretically lag phase is about 55 days after 50% flowering*. In fact, this is one reason why recording 50% flowering is important – so you can identify when lag phase will happen.
*This is based on studies done with Pinot Noir in Oregon State. In more variable cool climates, like the UK, due to the stop-start growing season impacting both flowering and grape berry development stage II (lag phase), the number of days can vary much more (It’s important to always be observing!).
The lag phase prediction method is often thought to be the most accurate when done correctly, however as Chalk House vineyard manager Luke Spalding explained, “For many people the best time to start weighing bunches is veraison – it’s easy to see when that is, so you don’t have to worry about counting the number of days since 50% flowering for each varietal. The issue is that for some vineyards, waiting until veraison is too late, they need to be able to get a more accurate yield prediction as early as possible.”
Whether you choose to start measuring actual bunch weights at lag phase or veraison, the method is very similar. You need to go out and randomly select as many bunches as is reasonable from each block (Luke recommends 15-25 bunches per 1000 vines for smaller vineyards). We have seen a number of different methods used by different vineyard managers to select random bunches in a bay. Two options are:
- Select a mix of 1st, 2nd (and 3rd bunches if you have them)
- Select every other bunch in one random bay
Once you have selected your bunches from the block, you are now ready to weigh your bunches. You just need an average bunch weight, so if you know how many bunches you picked, then you can weigh them all together and divide the total weight by the number picked to get the average.
Use the bunch weight multiplier
Yield prediction calculation using bunch weights and multiplier
The rule of thumb is that in a ‘normal’ year, your bunches will increase in weight by a factor of 1.65-1.9. We call that number the weight multiplier. The multiplier differs for lag phase and veraison.
At this point you start to really optimise your yield prediction to be much more accurate. This is the first time you have got a real idea of the bunch weights for this year. Go to the yield predictor tool, enter in the average bunch weights for each block, and adapt the multiplier to ~1.65 for veraison weights, or ~1.8 for lag phase weights. Of course getting the multiplier right is key, which is why it’s important to measure bunch weights at lag/veraison each year, and then again at harvest. This allows you to understand the weight multiplier for each of your varietals/clones. If you don’t have any previous data then using a weight multiplier of around 1.65 (veraison) or 1.8 (lag phase) initially is a good place to start.
Optimising yield prediction year on year
With the yield predictor tool you can now compare how this prediction compares to your previous prediction, and you can easily see the total predicted weight for the vineyard as well as the predicted yield for each block. In the previous prediction you were using average weights, now you are using actual weights from this year with a multiplier – does it look too high or too low? If things are looking exceedingly good this year, then maybe do another yield prediction with a weight multiplier of 2 – for example the harvest of 2019 in the UK people were seeing weight multipliers of 2 or more from veraison to harvest, but that was of course a very unusually prolific year. Also there can be considerable variation between different varietals when it comes to the weight multiplier, so if you do have past years data then it’s best to customise the weight multipliers for each block based on the history at your site.
Fourth prediction: Include last minute losses due to disease/pest pressures
Yield prediction calculation including potential loss from Botrytis
To see how things are progressing you can go out and weigh more bunches 4 weeks after veraison. How have the bunches developed? How much bigger will they get by harvest?
Of course there can be unforeseen issues with disease pressure in the final weeks. This is easily accounted for in the yield predictor by adding in what % of the crop you have lost due to the problem. This will update your yield prediction estimates in the final stages of your yield prediction journey.
Weigh bunches at harvest
It is key to measure the average bunch weight at harvest and enter that into Sectormentor too. You can either go and pick random bunches on the morning of harvesting a particular block, or you can harvest the sample bays from each block before harvesting the rest of each block (you have already counted the number of bunches for sample bays so you can easily work out the average bunch weight by weighing all the bunches that are harvested from that bay).
This number of average bunch weight at harvest is vital so that in future years you can figure out what the actual veraison-harvest weight multiplier is for each block in your vineyard. It also helps to get a very good idea of your average harvest bunch weight for a particular block over many years, so that those early predictions that rely on this weight become more and more accurate.
It’s an art and a science..
Yield estimation is a real art. The more in tune you are with your vineyard and the more you know your vines, the easier it will become – if you are consistently getting within 15% of the actual yield then that is considered good in cool climates. However, if you are very committed and refine the details, meticulously measuring bunch weights at lag/veraison and harvest each year, getting clear on your weight multipliers for each block, then many vineyard managers believe you should be able to get within 5% accuracy every time.
Check out 10 key metrics to monitor in your vineyard and find out how our app Sectormentor for Vines helps you record data & manage your vines for the best quality grapes