Preventing Grape Surplus

Mechanical thinning of grapes to moderate production from high-yielding vines.

Crop reduction has become very topical in the Marlborough wine industry over recent years, particularly with recent oversupply issues of Sauvignon blanc. Plant & Food Research Scientists at the Marlborough Wine Research Centre are investigating the impact of mechanical thinning on yield, berry size and juice composition. The aim is to establish a mechanical systems that reduces (or eliminates) the need for expensive and time consuming manual thinning, without adversely affecting the quality of the berries.

The research, which began in 2009 and is funded by New Zealand Winegrowers (NZWG), is using a modified machine harvester to test the viability of thinning grapes in January. Trials in the first year examined both Sauvignon blanc and Riesling grapes, however the trial focused solely on Sauvignon blanc for the second harvest.

Key points:

Cost comparisons (by hand vs. by machine)

Grape quality

Disease rates

Berry size

Fruit thinning has traditionally been done by hand. It is time consuming and expensive. There had been some trial work done in Australia using machines but that trial provided little info on the set up of the machine harvesters, how to work out how much crop was removed and how this kind of thinning might influence fruit composition and maturity at harvest.

This trial has been investigating those issues as well as looking at the timing and degree of thinning in Marlborough. Theyve done mechanical thinning with harvesting machines in January. Mike Trought, Research Scientist, says one of the issues is trying to work out how much of the crop is removed. They now know that by changing the speed of the machine they can affect the rate that fruit comes off but theres no easy way of assessing how much fruit ends up on the ground.

Another issue is to what extent does mechanical thinning have an impact on the diseases resistance of the vine. Does mechanical thinning make the vine more disease prone.

They modify the harvester by removing belts and inserting wooden panels. Beaters were set up at between 1200 and 1400 mm from the ground. The machine travelled at 3.2km with beater speeds at 400 and 450rmp.

Fruit was collected on polythene sheets below the canopy and then weighed. The results indicate that machine thinning removes berries rather than bunches resulting in anything from 22% to 45% reucation in vine yield.