Planting year herbicide trial for Blackcurrants

The success of weed control during the establishment year has in the past been variable and the cost of weed control failures is high. It was decided to test five products/combinations, and to evaluate for weed control efficacy and plant safety.

Methods

NZ Arable staff applying herbicide treatments over top of Ben Ard cuttings. No weeds had emerged at the time of the herbicide applications.

Trial was hosted by Mike Read at Hinds, on Kaiapoi soil which is classed as a deep imperfectly drained silt.  NZ Arable was contracted to apply the herbicides with their variable rate small plot sprayer.  Applications were made on 31 August 2023, in fine weather.  There were two fields of cuttings used for the trial; one was Blackadder at green tip stage, the other was Ben Ard, fully dormant.  The Blackadder plot was slightly heavier textured soil and had no stones, while the Ben Ard plot had stones.

Paddocks were worked up with a rotary hoe 4–5 weeks prior to planting cuttings.  20cm cuttings were stuck 10cm deep, soil on either side of the cuttings was pressed with a roller, then pre-emergents were sprayed over top the cuttings in a 1.2m width.  Herbicides were applied with the equivalent of 250L/ha of water, using 4 x 110 02 flat fan nozzles at 40cm spacing.  A 30cm wide strip of burlap protected the soil in the middle of the label-rate plot, showing what weed pressure was there without any herbicide applied. 

The Blackadder plot (cuttings at green tip stage) was watered in immediately with a gun irrigator, 30mm of water applied. The Ben Ard plot (cuttings still dormant) was sprayed just after the irrigator had passed, and there was no rain afterwards for about a week.  The grower’s standard for the rest of the field was Foresite 4L/Simazine 1L. 

Results

Weed control over all was poor, including in the grower’s standard treatment.  Note that the first weeds to emerge are in the pressed strips adjacent to the cuttings.

Note the first weeds to emerge are in the pressed strips adjacent to the cuttings, Blackadder block (17 October 2023).  The grass pressure was so high at the field edge where the trial was located that the grower came back and sprayed with Gallant Ultra (haloxyfop) to eliminate grasses and allow us to assess the control of other species of weeds.

We determined that the cause of the poor control was the long lag time between rotary hoeing and herbicide application.  Although no weeds were visible above the soil, the 4–5 weeks post hoeing and relatively warm spring weather had allowed the weed seeds to germinate and they were working with a head start, allowing them to successfully break through the herbicide layer.

The grower noted that he tends to get the best weed control in springs where the weather constraints force the rotary hoeing to be close in timing to the planting and herbicide application, adding weight to this explanation. 

Still, we were able to assess differences between product performance on different weed species. 

Weed species prevalent were as follows:

1. field pansy (Viola arvensis)
2. wireweed (Polygonum aviculare)
3. scrambling speedwell (Veronica persica)
4. radish – ‘choi sum’ (only on Ben Ard field)
5. willow weed (Persicaria maculosa).
6. a brome grass species or wild oats

Weed pressure in the plots six weeks after herbicide applications (on 17 October), and after Galant had eliminated the grass. Photo on the far right shows weed pressure with no herbicide application.

The weed emergence was so massive that it is easy to miss what the herbicides were good at. 

Firstly, there was no damage to the blackcurrant cuttings under any treatment at any rate, which was surprising.  Blackadder was sprayed when cuttings were at green tip.  The Chateau treatments had the cuttings sealed with wound sealer.  We were concerned about plant damage but none was observed. 

Weed species present and how each herbicide treatment rated for control (x= poor, xxxx = good).  Ratings were done on the label rate treatments

The standout treatments were Terb/Simazine and Chateau.  Chateau was the best on willowweed, wireweed, pansy and the grass, while Terb/Simazine was reasonably strong on those same species plus the speedwell. 

Forsite/Simazine combination was relatively good on willow weed and pansy, but weak on wireweed, speedwell, and grass. 

Alion, Stomp and Foresite/Simiazine were the weediest plots overall. 

Herbicide longevity

We had wanted to test how long-lasting the various herbicides were, but were unable to complete this evaluation.  The weed control failure necessitated a further shielded application of Terb/Buster to rescue the plantings, confounding a later longevity assessment.

Lovett Farms trialed similar treatments on one-year-old plants (year after establishment) at the standard labeled rates (applied with 300L/ha of water in August). 

Additional considerations:
While herbicides provide an important function, there are some down sides to their use.  In 2004 Crop and Food Research did a study on soil quality in blackcurrants, and in collaboration with Landcare Research they tested the toxicity of terbuthylazine and glyphosate on earthworms, showing that terbuthylazine significantly reduced earthworm growth.  

The table below shows that all the herbicides tested are toxic to earthworms.  This direct toxicity combined with low organic matter inputs in the herbicide strip under the bushes goes a long way to explaining why soil health in blackcurrant plantings declines over the life of the planting.

Costs for the herbides tested are indicated below. Surprisingly, though Chateau is reputed to be expensive, it is no more costly than Exadia/Simazine or Foresite/Simazine.