By: Jackie Dresser
The threat of cold winter temperatures and early season frost in the Lake Erie region has prompted growers to prune conservatively, leaving higher bud numbers as insurance against damage from these conditions. The downside to this practice is the potential for over-cropping if most or all of the buds survive and the shoots that emerge bear fruit as they mature. Shoot thinning is a tool that growers can use to bring crop potential into balance after the threat of spring frost wanes. Using variable rate technology, the CLEREL team has demonstrated that shoot thinning can be accomplished efficiently, taking variability in shoot number across a vineyard into account.
As with any project, having the right tools for the job is key. For variable rate shoot thinning, these tools include an NDVI (Normalized Difference Vegetation Index) sensor to collect spatial data relating to shoot count across a vineyard, some personnel that know their 1-2-3s and are willing to count shoots, software that facilitates spatial data processing and visualization, and variable rate shoot thinning equipment. The variable rate shoot thinning system includes a tractor with a field computer and software capable of controlling a PWM (pulse-width modulation) valve that regulates the flow of hydraulic fluid to the motors of the shoot thinner, increasing or decreasing their RPM. For more information on the costs of this system, check out this LERGP newsletter.
After assembling the appropriate tools, we selected three commercial Concord blocks to work in this season, traversing the Lake Erie grape belt end to end. With the intention of facilitating commercial adoption of variable rate shoot thinning, we selected both hand pruned and machine pruned vineyards to run our trials in and sought to keep our protocol within realistic boundaries of what commercial enterprises could be expected to implement. Starting with early season NDVI scanning, we efficiently assessed variability in shoot number in relative terms. With stratified manual sampling for shoot count, we translated NDVI information to shoot count on a whole vineyard scale. This information was used to create management classes, reducing variance within each class and maximizing variance between classes, setting the stage for variable rate shoot thinning.
We worked with growers to develop a management plan in each class, setting a desired target for shoot number after thinning. A digital prescription map of the management classes was fed into a field computer that controlled the rate of the shoot thinner, increasing paddle speed to remove more shoots in one management class, and decreasing paddle speed to remove less in another, based on the management plan that the grower decided on. Since the canopy architecture varied between the commercial vineyards in this trial, we performed an in-field calibration of our variable rate shoot thinning equipment in each vineyard. We recommend that early adopters make sure to calibrate equipment in each uniformly managed block, as canopy architecture (few long canes with many buds vs. many short canes with few buds), shoot growth stage, and variability in shoot count will all impact the shoot removal that corresponds to a particular RPM of the machine.
Once we dialed in the shoot removal algorithms based on vineyard-specific calibration at 4mph ground speed, all the operator had to do was drive and the variable rate equipment adjusted itself to the needs of each area of the vineyard. Overall, we thinned within 10% of the targeted average shoot removal in each management class and consistently took shoot numbers down to the desired ranges set by the growers. Management decisions made by the growers in each vineyard varied. Two out of three growers sought to create a more uniform shoot count across a vineyard, and one wanted to balance shoot number with the size of the vines. Low NDVI areas had smaller vines, so were thinned more severely than high NDVI areas where vines were larger and the grower had confidence in their ability to ripen a larger crop. The map shows the management classes with shoot count and thinning information. Stay tuned as we monitor the growth of the vineyards this season, the ripening rate of fruit in thinned vs. un-thinned areas of each vineyard, map the yield at harvest and look at the impact on vine size in the following seasons.