Managing your understorey

That was the burning question researchers were keen to establish, during a three-year NZW funded project.

Led by Drs Mark Krasnow and Carmo Vasconcelos, the research focused on Merlot and Syrah blocks in Hawke's Bay and Pinot Noir and Sauvignon Blanc vineyards in Marlborough.

Krasnow told the Bragato conference that they went in with some fairly definite hypotheses prior to the research beginning.

"We thought that by changing just one parameter, (irrigation, plucking and crop management all remained the same), that mowing and cultivation would devigourate the vines. Mowing because you have actual vegetation under the vines, intercepting the water and using minerals that wouldn't be available to the vines. And perhaps cultivation would damage some of the shallow roots and impede water or nutrient uptake to the vines."

Because of that, the researchers also believed that yields would be reduced in the blocks where mowing and cultivation were trialed. They also believed that organic matter in the mowing trial would be higher than the herbicide control.

"On some of these (hypotheses) we were right, some though we were way off," he said.

Looking at the results, mowing did in fact devigourate the vines. On closer analysis it became apparent that there were far more gaps within the canopy of the mowed vines, than there were in the herbicide control and the cultivation trial.

"We looked at leaf area per shoot and when we looked at canopy gaps we saw some very clear differences, particularly in the Merlot vineyards. As you walk among the vines you can tell whether you are in the mowing treatment or the others, because the vines are much shorter."
The picture below illustrates where the herbicide ends and the mowing begins. Shorter shoots are obvious, there are more gaps and less leaf area per shoot.

In each of the three years of the trial, the story was the same, the Merlot mowing treatment had more gaps than the herbicide and the cultivation. On one day's Merlot measurements, those gaps equated to 18.2% for herbicide, the same for cultivation and a massive 36.4% for the mowing.

But when year three of the trial came around, it all changed. All of a sudden, the cultivation treatment showed signs of devigouration – they were now on a par with the mowing treatment compared to the herbicide control. In terms of the Pinot Noir trial, the devigouration was even higher in the cultivation treatment than the mowing.

As the canopy was reduced, so too were yields for the mowing trials.

"We saw yield reduction from the mowing treatment, which made sense. In the Merlot we saw yield reduction every single year, when compared with the herbicide and the cultivated. However it was only the third year that we saw yield reduction in Sauvignon Blanc from mowing."
Interestingly, the yield reduction wasn't associated with fewer berries. Instead in the case of Merlot it was consistently due to smaller berries. In year three, the Sauvignon Blanc yield reduction was also associated with smaller berries. (See figure 1 below).

Grape composition was the next parameter tested. Krasnow said they expected to see some big differences between the herbicide control and the mowing and cultivation treatments.

"But what we saw were very few differences in thinks like pH, TA, Malic acid and Brix.

"One significant difference we did see though was the reduction in YAN in the mowing treatment. We found this in all four varieties in all years when it was measured. Mowing had significant lower YAN than herbicide and cultivation.

Then in the past season, year three of the trial, again things started to change. All of a sudden the cultivation treatment was showing devigouration, more gaps in the canopy, less yield and interestingly, less YAN.

"In the 2014/15 year the cultivation treatment is now suddenly grouping with the mowing treatment in as far as YAN goes. Herbicide has the most, which makes sense as nothing is competing with the vine for nitrogen. But cultivation is now grouping with mowing, which shows something is happening here with the vines in the third year. We were beginning to see the same effects (in the cultivation treatment) that we had been seeing in the mowing treatment."

As a caveat, Krasnow said that lower YAN doesn't necessarily signify lower fruit quality.

Then it came down to basic composition of the wines made from the fruit. While the third year of analyses has yet to be completed, he said the first and second year Sauvignon Blanc wines showed up some interesting differences.

"We found there was significantly less isobutyl methoxypyrazines – that green capsicum aroma, in the mowing treatment. In the first year the herbicide had the highest. In the second year the cultivation had the highest, which may have been due to higher crops in that treatment in that year."

In terms of whether the mowing treatment would mean more organic matter under the vines, Krasnow says they went into the trial thinking this would be a "slam dunk" yes.

"When we looked at the numbers, what we saw was only about half of the time did the mowing treatment have a higher organic matter than the cultivation or the herbicide. And the only statistically significant difference was in the final season in our Sauvignon Blanc vineyard. That was a surprise."

Krasnow believes the results from the three-year trial provide important information for growers who are contemplating moving away from herbicide treatments to either mowing or cultivation.

"Mowing devigourated consistently, cultivation devigourated suddenly in the third year. When we saw yield reduction which was every year in Merlot, it was due to smaller berries. That may be a positive in Merlot, providing more skin to pulp ratio.

"We did not see huge differences in grape and wine composition, which is great news for people who want to move away from herbicide – you are not going to sabotage your brix. Your basic grape chemistry is going to remain pretty much the same as what you are used to.

"We did see some drops in methoxypyrazines in the mowing treatment with Sauvignon Blanc. This could be important for winemaker's gearing towards different markets, ie; those that don't want the renowned vegetal characteristics Marlborough Sauvignon Blanc is renowned for.

"And while half the time the mowing treatment did have the highest organic matter, it wasn't the trend we had expected."

The full report of the three-year study will be available later this year.