Irrigation Scenario 1: Water supply sufficient to continue irrigation after harvest.
For high yielding vines (greater than 35 t/ha in Riverina) with a large permanent structure, six to eight weeks are required to fully replenish carbohydrate reserves after harvest. Irrigation therefore needs to be continued to maintain a healthy canopy for this length of time.
If there is a post-harvest root flush it may take several weeks to reach a maximum, so fertiliser does not necessarily need to be applied immediately after harvest.
However, for vines at intermediate cropping levels (~ 25 t/ha) there may be some option for saving water as reserves will be replenished in a shorter time and potentially prior harvest.
Irrigation Scenario 2: Sufficient water for one or two post-harvest irrigations.
For vineyards where a post-harvest recovery is thought to be necessary, and some water is still available, try to manage remaining irrigations so as to maintain a functional canopy for three to four weeks after harvest. This should be sufficient to replenish adequate reserves for the following season. An earlier loss of canopy may result in renewed shoot growth if rainfall occurs later in the season, but next year’s buds won’t burst in this situation, and the extra growth does not appear to be detrimental for the following seasons growth.
Post-harvest fertilser applications are still possible, but these need to be timed with irrigation. If not taken up prior to leaf-fall, mobile forms of nitrogen fertiliser may be lost with winter rainfall. If a lack of water is anticipated, it may be more effective to apply nutrients earlier in the season when root growth is at a maximum. Foliar nitrogen sprays can assist with building reserves for the next season, but beware of water quality used for the application.
A final consideration is the risk that leaving a dry soil profile over winter may lead to problems with restricted spring growth in the following season. Although the cause of this disorder is not well understood, young vines maybe more vulnerable in this situation.
Irrigation Scenario 3: No water remaining for irrigation after harvest.
Water stress during the post-harvest period for one season appears to have little impact on yield in the following year. Reserve accumulation may be reduced, but providing the vines have had reasonable irrigation or rainfall prior to harvest, the amount of reserves stored should still be adequate.
However, with successive seasons of poor post-harvest conditions, it would be expected that yield and vine health would start to decline. If comparable water shortages are expected in the medium-term, this essentially means that yields need to be reduced so that the vineyard is not in a situation where water is running out before harvest every year.
In the longer term, options such as alternative varieties or more water use efficient rootstocks need to be considered.
Stay with standard industry recommended practice of annual petiole testing combined with visual assessments, and build up a longer-term picture vineyard nutrient status. Determine application rates based on petiole results, removal of nutrients from the vineyard with fruit and past experience.
For the major nutrients, avoid any significant changes unless clearly justified as the soil and reserves will provide some buffering capacity. Continue with post-harvest applications in higher yielding vineyards, but if water is limited, keep in mind when roots are most active and time fertiliser with irrigation events.
If there is no water for postharvest irrigation then the nutritional status of the vines can be maintained with soil and foliar fertiliser applications earlier in the season.
Post-harvest Salinity Management
The salt content of water in the Murray Darling system increases as it moves downstream, but with precautionary monitoring of soil salinity and some leaching, it does not generally present a major issue to viticulture. Within regions, greater problems may arise if vineyards become dependent on poor quality groundwater.
In these situations it may be essential to leach salt from the soil to avoid excessive build-up. Approximate leaching fractions can be calculated using the conductivity of the irrigation water and the amount of irrigation applied during the season.
Typical leaching fractions may be in the range of 10 to 20%, and are best applied prior to bud-break if soil testing shows salinity levels were not reduced sufficiently by winter rainfall.
The importance of the post-harvest period is largely determined by climate, yield and management prior to harvest. If there is one situation to avoid, it is running out of water in the middle of ripening a heavy crop, as this will stress the vine exactly when the demand for carbohydrates is at a maximum.Vines will tolerate a season or two of poor post-harvest conditions, but productivity will eventually be reduced.
While beyond the scope of this paper, it highlights the importance of long term planning and understanding that yield, together with effective salinity management, is a key factor in determining the sustainability of vines with reduced water supply.
Jason Smith – Charles Sturt University
Stewart Field – National Wine & Grape Industry Centre
Leo Quirk – NSW Department of Primary Industries
Bruno Holzapfel – NSW Department of Primary Industries