Wet years and late rain, big crops with large canopies and a seemingly endless growing season – these conditions in recent years might have tempted some lentil growers to desiccate their crops a little early in an effort to bring on harvest.
But patience is the key, according to long-time lentil growers, with recent research into desiccation strategies backing up their experience. There is little to be gained from spraying the crop too early.
Growers have, however, refined their desiccation strategies in these challenging conditions to more-evenly dry crops with a large biomass and target weeds.
Desiccation trials in SA
South Australian agronomist Sam Trengove helped to coordinate desiccation trials at Maitland and Mintaro in 2021, to assess how different spray combinations, including double-knock strategies, would affect the dry-down time of lentil crops in high-yielding environments.
The trials were part of the GRDC-supported ‘SA Grain Legume Validation’ project, with sites at Maitland and Mintaro. Treatments included glyphosate and paraquat, diquat (Reglone) and the addition of group 14 – saflufenacil (Sharpen) and pyraflufen-ethyl (Sledge) – products, different water rates and an evening application.
Mr Trengove is based at Bute on the Yorke Peninsula, where all of his clients grow lentils. For some growers, lentils are up to 50 per cent of their crops.
“Growers are pushed to harvest as soon as the crop is ready to avoid pod drop and pre-harvest losses,” he says. “Harvesting at the earliest possible timing also minimises the opportunity for weather damage and quality downgrades.
“The aim is to desiccate lentils as soon as grain fill is complete and the crop has matured, to allow harvesting to commence as soon as possible, after the seven-day harvest withholding period has been observed (two days for Reglone). However, in some scenarios when crop-topping targeting weed seed-set control, typically with the use of gramoxone, it may be necessary to spray earlier than the ideal timing for crop desiccation. There is an associated increases risk of reduced yield and seed quality.
“Sometimes, in years like the one we’ve just had (2022) where it’s been quite wet in the lead-up to desiccation, crops just hang on for a long time and take longer to mature. If they’re a bit too green and not quite ready, it’s actually difficult to get them to dry down quickly for an earlier harvest.
“That’s one of the key findings from the desiccation trials. Spraying too early did not accelerate drying time, measured as canopy moisture content. Waiting an extra week for the crop to commence natural senescence (starting to turn yellow) allowed treatments to work more rapidly once they were applied.”
Dry finish at Maitland
There were 13 treatments in trial plots at Maitland, which had 25 millimetres of rain in the month leading up to desiccation on 3 November 2021, and 39mm in the three weeks following. Starting canopy moisture was 48 per cent. This had reduced to 24 per cent seven days later, and to 15 per cent by day 13, across most treatments. However, for three treatments – the untreated control, paraquat plus Sledge and paraquat plus Sharpen – canopy moisture averaged 20 per cent by day 13.
“The crop was drying down quite well when the desiccation treatments were applied and they basically all worked; there was little difference between the treatments. So, in those conditions, any number of treatments were suitable for enabling timely harvest, including the standalone treatment of paraquat 250 applied at 800 millilitres/ha,” Mr Trengove says.
Early desiccation at Mintaro
However, the trial at Mintaro showed there was little to no acceleration in dry down by spraying a crop that was too green. At the commencement of the trial, up to 40 per cent of the crop was still green, with many green seeds in the top pods. This site received 30mm rainfall in the month prior to desiccation, but 78mm in the three weeks following.
It evaluated 18 treatments on a standing lentil crop, with desiccation beginning on 8 November with the application of glyphosate. Gramoxone, Sharpen and Sledge treatments were applied on 10 November, about a week ahead of what would have been optimum timing.
At trial initiation canopy moisture was 65 per cent. At the first assessment, eight days later, only three treatments had significantly reduced canopy moisture from 56 per cent in the untreated to 45 per cent. These treatments were mixtures of paraquat and Reglone.
Sometimes you just have to be patient. Desiccating a crop too early can work against you.
By day 21, all treatments reduced the moisture content of the crop to harvestable levels, with an average of less than 10 per cent. The untreated control still had a moisture content of 31 per cent. Paraquat alone, or applied with Sledge, or with a low rate of Reglone, was less-effective than other treatments, at 14 per cent moisture content.
Double-knock treatments, where glyphosate application was followed two days later by a second knock, did reduce canopy moisture by day 21, compared with the same treatment without glyphosate applied first, averaging eight and 14 per cent canopy moisture respectively. However, no benefit was observed at the eight or 14-day assessment that would have enabled earlier harvest.
A comparison of paraquat alone applied early, and seven days later, at closer to optimum timing, saw both treatments dry down to about 13 per cent by the same date. Confirming the early desiccation timing was too early, it yielded 3.49t/ha with lower grain weight and increased seed wrinkle, compared to 4.04t/ha for the timing seven days later.
“Sometimes you just have to be patient,” Mr Trengove says. “Desiccating a crop too early can work against you.” Accelerating uneven drying and increasing the risk of losses is one potential risk.
Measurements taken on day 14 in the Mintaro trial showed that moisture levels in the upper canopy in paraquat treatments had dropped to an average 29 per cent. However, moisture in the lower canopy remained at 40 per cent on average, and the crop was still unharvestable.
A risk with trying to fast-track an immature, large, bulky lentil crop to harvest using only contact desiccant herbicides, such as paraquat, is that the upper canopy desiccates quickly, and this is where the majority of pods are attached. Within days of treatment, these can be at greater risk of pod drop than they were pre-desiccation. However, if the lower canopy remains green and ropey and continues to delay harvest, these pods might actually be at risk for a longer period of time to weather damage.
These green lower vines continued to be an issue that challenged growers in 2022.
Paul Jarrett, based at Maitland, trialled a double-knock strategy on his 1400ha of PBA Highland XT and PBA Hurricane XT lentils in 2022.
The wet year produced a crop heavy with foliage that was still green well into October. He desiccated with glyphosate in mid-October, following up with paraquat five days later.
“Because of the wet soil and the amount of vine, we were just trying to make sure it dried down enough to harvest, which we started on Melbourne Cup Day,” he says.
Much of the crop was ready within seven days of the paraquat application, but where vines were thicker, it took another week.
“You just have to keep monitoring to get the timing right. It’s a bit of a juggling act between spraying at the right time to get ryegrass under control and cutting lentils at the right time,” Paul says.
The Maitland region is subject to surface temperature inversions, and Paul says he does not spray late in the day or at night because of this.
Across the Victorian border, at Kaniva, long-time grower Malcolm Eastwood managed last year’s harvest with just a single pass of paraquat (Gramoxone) plus Sledge, along with the adjuvant Hasten.
Malcolm says this was to boost the efficacy on weeds in the crop, specifically vetch, milk thistle and ryegrass.
His 200ha of PBA Kelpie lentils were late-sown into stubble, with wet conditions continuing into November – much like the previous year – producing another large-biomass crop. It was not until 28 December it had dried off enough to spray.
One change he did make to his desiccation strategy in 2022 was to spray in the evening. “In 2021, after spraying, the seed was ready to go but the plants were still too green to go through the header,” Malcolm says.
“The spray chemistry is activated by sunlight. So, spraying in the evening in 2022 allowed the plants to absorb more of it overnight before the drying began to take effect. The result was more-even drying than the year before,” he says.
Further east, at Wallup, Alexander McRae used a single pass of paraquat on his 600ha of PBA Jumbo2 and PBA Hallmark XT lentils in 2022.
“Because of the huge biomass last year, and a few bigger weeds such as sow thistle, we added Sharpen to the mix to help with those broadleaf weeds and a faster dry-down,” Alexander says.
He also takes his time when spraying and uses a high-water rate with either an early morning or evening application to slow the uptake of the chemistry and more-effectively sterilise weed seeds, particularly ryegrass.
With the long, wet growing season in 2022, it was not until 17 December that he desiccated his crop. It was another 11 days before he began harvesting.
While he expects he missed the timing for effective ryegrass control, he is confident that waiting was the right decision for the crop. “We harvested up to 4t/ha in our best areas, and all the crop was graded number one.”
Hazardous Surface Temperature Inversions
The spraying of contact crop desiccants, (i.e. gramoxone, Reglone Sharpen and Sledge) during early evening and at night is a common practice as highlighted by Malcolm and Alexander. However, this time of day/night is also highly conducive to the development of hazardous surface temperature inversions which can result in spray drift traveling many tens of kilometres before depositing onto other crops and vegetation.
Extra care is needed by spray applicators to ensure that hazardous surface temperature inversions are not present during spray application.
More information: Sam Trengove, [email protected]