Deficit irrigation and canopy management practices

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A field trial was carried out on 16 year-old Shiraz/R110 grapevines, between the 2011/12 and 2014/15 seasons in a semi-arid region. The objective was to determine the effect of different drip irrigation strategies and canopy manipulation combinations on vegetative growth, plant water potential, water usage, yield, overall wine quality and profitability.

Project layout
  • The field experiment was done on a fine sandy loam soil about 25 km Southwest of Robertson;
  • The soil was replenished back to field capacity by means of 3.5 L/h drippers once the total plant available water (PAW) within the root-zone (ca. 750 mm deep) depleted by respectively ca. 30%, ca. 60% and ca. 90%;
  • For each irrigation strategy grapevines were hand pruned and canopies were either left unsuckered and sprawling, shoots vertical positioned (VSP) with suckering (two shoots per spur) or VSP without the suckering of water shoots;
  • In addition, there was a further treatment which was irrigated at 90% PAW depletion and mechanically (boxed) pruned.

Results
A few highlights of the project findings include:
  • Higher irrigation frequencies resulted in higher evapotranspiration (ET) losses through higher evaporation losses from the more frequently wetted volume of soil.
  • The seasonal ET was more sensitive to irrigation frequency than to different canopy manipulations. Sprawling canopies of more frequently irrigated grapevines used only 10% more water than their VSP counterparts that were frequently irrigated, but only during the hottest period of the growing season.
  • Grapevines subjected to severe water constraints ripened their grapes more rapidly than those experiencing no or medium water constraints i.e. reaching the target juice sugar content of 24°B earlier by up to two weeks. Grapes of sprawling canopy grapevines tended to reach target sugar levels earlier than VSP grapevines within the same level of PAW depletion.
  • Lower frequency irrigation applications resulted in higher production water use efficiency (fresh mass of grapes produced per unit of water) compared to medium and high frequency ones. Even though the grapevines irrigated at ca. 90% PAW depletion had received ca. 81% less irrigation water than those irrigated at ca. 30% PAW depletion, they only had a ca. 47% lower yield than the high frequency irrigated grapevines.
  • Irrigation applied at a lower frequencies, improved overall wine quality compared to more frequent irrigation. Within the more frequently irrigated grapevines, sprawling canopies tended to produce wines with better overall quality, while those with VSP canopies left unsuckered producing the poorest overall wine quality.
  • The gross margin was strongly determined by the gross income, i.e. yield and price pay-out per tonne of grapes. Grapevines irrigated at ca. 60% PAW depletion with sprawling canopies and the irrigated at ca. 90% PAW depletion that was mechanically/box pruned, resulted with the highest gross profit margins

Significance of the study
Water is becoming an increasingly scarce resource. Responsible and effective use of water and more economically viable management practices are of critical importance to ensure sustainability of the wine industry.

Keep an eye on future editions of WineLand Technical for popular articles resulting from this project.


Researcher: Vink Lategan

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