Incubator Initiative - Is organic weed control beneficial for winegrape production in the Limestone Coast?

Abstract

This trial was conducted to better understand how non-standard weed management practices (such as mulching, compost with mulch and mechanical weeding) compare to undervine herbicide application in Padthaway, South Australia. Across the 2020, vintage soil physicochemical properties, leaf blade nutrient composition, plant available nitrogen and yields were quantified. Results showed that immediate changes in canopy size (leaf area) were detectable, berry juice pH was significantly lower, titratable acidity and yeast assimilable nitrogen significantly higher for the mulch and compost treatment than for other treatments. These effects might be explained by significant increases in soil and leaf nitrogen or phosphorous, or significant differences in soil moisture.

Summary

Key Findings: In a single season, changes to berry chemistry and soil nutrient properties were achieved via the addition of mulch and compost. Mechanical weed control shows no significant advantages or disadvantages over traditional herbicide use based on this trial.

The use of herbicide in the undervine area of a vineyard to control the growth of competitive species is widespread. In recent years the use of mechanical weed control or mulching, with or without the addition of compost, has become increasingly popular. However, impacts on soil health and vine physiology associated with these new management practices are largely unknown and are dependent on many factors. This trial was designed to take in data from many facets of the vineyard to better understand how mulching and mechanical weed control (light surface tillage) might affect vine performance, and ultimately help determine whether these practices might be feasible for growers in the Padthaway region to adopt.

Two sites in Padthaway were chosen; one growing cabernet sauvignon (Site A), the other growing shiraz (Site B). Intensive data collection was carried out at Site A, while exclusively harvest and berry chemistry data were collected at Site B. The intensive data collection at Site A included soil physicochemical analysis, leaf blade analysis, soil moistures, plant available nitrogen, leaf area index as well as harvest and berry chemistry data. These data were collected at two time points; florescence (mid-November 2019) and pre-harvest (early March 2020).

Undervine weed cover was most effectively reduced by the mulch treatments, with mechanical weeding (light tillage) proving the least effective, but these differences were not significant by the end of the season. This means that if control of undervine competition is a viticulturalist’s high priority, any of the options trialled here are feasible solutions, though they may have differing effects on the vine’s nutrition and berry quality.

No significant differences in yield were detected at either Site A or Site B, however given the low yields across every South Australian region in 2020, effects may occur in future seasons.

Berry chemistry was influenced significantly by the treatments at Site B (though not at Site A). Titratable acidity (TA) and yeast assimilable nitrogen (YAN) were highest, and pH was lowest for the mulch and compost treatment. Low nutrient levels in the sandier soil at this site may help explain these differences, but soil data was not collected at Site B. In surface soils at Site A, moisture content was significantly higher for the mulch treatment, though this effect was not observable at the harvest timepoint.

Preliminary results from this trial were disseminated in early January 2020 at a field day. High levels of interest were shown by viticulturalists of the limestone coast wine region. To provide the complete data set and results to these groups, a presentation will be made during an upcoming meeting of the Limestone Coast Grape and Wine Council. Another field day may also be beneficial.

To summarise, the trialled undervine weed control measures are all effective at reducing weed coverage, however differing effects were observed in the vines over this season. Changes of note were the large differences present in berry juice quality for the shiraz cultivar when treated with mulch and compost. Further study is recommended to determine the persistence and long-term impacts of these treatments.

This research trial was carried out by staff from The University of Adelaide over the 2020 vintage, and gained substantial economic advantage through their other work for Wine Australia; exploring the effect of undervine cover crops on grape performance.