Desiccants are designed to quickly dry down the crop, as well as any green weedy material growing in the crop that might otherwise hamper harvesting operations.
“It’s a common misconception that herbicides put on prior to harvest, whether it’s a desiccant or something like glyphosate, will hasten maturity — which is not the case,” says Clark Brenzil, provincial weed specialist with the Saskatchewan Ministry of Agriculture.
“In large part what you are doing is trying to address some of the harvest issues that occur when you have an indeterminate growth habit in a plant,” says Brenzil. “Typically that is going to be for a broadleaf crop, and pulse crops tend to be the most commonly desiccated.”
In indeterminate plants, such as pulses, flowers are produced at the bottom and continue to be produced all the way up as the plant grows. This results in mature pods at the bottom of the plant and greener material at the top. “The idea with desiccation is to dry out that green material very quickly so that you can get in there and harvest the mature pods down at the bottom,” says Brenzil.
Crop desiccants such as Reglone are contact herbicides that interfere with photosynthesis. This causes the plants’ cells to break down and release the liquid contents, allowing plant material to dry down rapidly. Water droplets can often be seen pooling on the leaf surfaces shortly after application of desiccants.
Although glyphosate products are not desiccants, it’s a common misconception that glyphosate applied prior to harvest will act as a desiccant. “There is often a blurring of the term,” says Brenzil. “Farmers will often say ‘we’re desiccating with glyphosate’ and that’s not the case. Glyphosate kills plants; then it’s left to Mother Nature to dry them down.”
More correctly, says Brenzil, farmers use a pre-harvest application of glyphosate to control perennial weeds. “The glyphosate circulates in the plant and gets down to the roots and controls that perennial weed,” he says. “Pre-harvest is a particularly good time of year to achieve that, particularly the further north you go.”
Incorrect timing of pre-harvest herbicides can actually have a negative impact on maturity, says Brenzil. “The maturation process is more than just the dry-down of the plant. The first step in maturation is the filling of the seed and then once the seed is filled, it starts going through that drying down process,” he explains.
Herbicides applied too early can interrupt the process of seed filling, resulting in yield loss. There is also a danger of herbicide residue ending up in the seed, a particular concern when using glyphosate, for which some European countries have set very low maximum residue limits in pulse and other crops.
“Glyphosate is a systemic product, which means that once it enters the plant it will get into the circulation system and move through the plant to the same places that the sugars are going, which are called sinks,” says Brenzil. “The sink at the pre-harvest timing is the seed. So basically what you are doing by applying early is taking what is applied to the surface of the leaf and putting it right into the seed.”
For this reason glyphosate should not be used as a pre-harvest application when growing pulse crops for seed the following year, because of an increased risk of poor emergence.
Desiccants and frost
Desiccants are contact herbicides which only have impact on the tissues they come into contact with. They do not move systemically through the plant.
Another myth about pre-harvest treatments, whether desiccants or glyphosates, is that they can protect a crop from the damage caused by a frost, similar to swathing. “When a crop is swathed there is still some subsequent maturation of the seed as the swath dries, but with herbicides you are simply killing the crop prematurely,” says Brenzil. “Desiccation could be seen as the chemical equivalent to frost and performs roughly the same process except it is ice crystals that form within the cells that puncture membranes and release the cell contents to the air. It is doubtful that a crop treated with glyphosate will be dry enough when a predicted frost materializes to protect it at all. Producers are just throwing away their money if they apply a day or two before the frost.”
When to apply
Once the crop is ready, desiccation allows farmers to control the timing of harvest within a relatively short window.
Deciding when to use desiccants will often depend upon the amount of variability in the field, says Brenzil, but farmers should time application for when they feel the majority of the crop will be ready.
“You might have a situation where you had a dry spring and only 20 per cent of your seed came up and then a rain came two or three weeks later and the other 80 per cent of the seed came up,” says Brenzil. “In this case you really should be timing the desiccant for the later crop because if you desiccate earlier to try and time it for that minority of plants that were early, you will sacrifice yield and quality and there could potentially be residues in the seed that could cause the crop to be rejected altogether.”
Robert Klewchuk, Syngenta’s technical lead for Western Canada suggests talking to your local agronomist for a second opinion about when your crop is ready for desiccation. Applying a desiccant too early can affect yield, harvestability and the development of immature seeds.
“I always term Reglone as a ‘finisher’, so you are taking a plant that is very mature and applying the product to dry up the rest of the green growth as a harvest aid so you can get in there with the combine,” says Klewchuk. “You can get a false sense of success if you apply way too early, because the crop looks dry but you can’t see the maturity of the plant. If the seeds are not ready, applying a desiccant makes them look ready but then you start harvesting and it’s not quite going so well.”
To decide when to apply a desiccant, farmers should look at the moisture content of the seed. A general rule of thumb is to desiccate when the seed has less than 30 per cent moisture. In the case of lentils and peas, if the bottom 10 to 30 per cent of the pods on the plant are brown and dry and rattle they are ready for desiccation. In peas, when the bottom pods rattle, meaning the seeds have become detached and the upper pods are turning yellow, the plant is ready for dry down. With beans, producers will often use pod colour and texture to determine timing for desiccation. Usually beans are mature when 80 to 90 per cent of the leaves have dropped off. To desiccate chickpeas, producers should wait until 80 per cent of the pods have turned brown.
In general, says Klewchuk, with all pulse crops, the field should have a colour and maturity change prior to applying Reglone. “I view the crop in three tiers,” he says. “The bottom grouping of pods have changed colour with seeds detached and rattling. The middle tier has seed color change and seeds split with no juice. The grower has to determine, if he is going to wait for the top grouping of pods, when and if they will mature in time for harvest.”
It’s important to achieve complete coverage of the crop to have a more complete effect upon a greater number of the plant cells. Applying in the evening, or preferably after dark, will help to give more efficient coverage by reducing evaporation of the water volume, and giving better droplet spread on the surface when the plant surfaces are not hot from direct sunlight.
“Most desiccation products are sun activated and the by products that they are producing are a result of the photosynthesis being interrupted. If you can put them on in the evening when the light levels are low, it allows a certain amount of time for the droplets that have landed on the plant to get into the plant and diffuse out from the impact point before the sun comes up the next day and starts activating the product,” says Brenzil.
Lots of water volume and using clean water that is free from particulates is also crucial to achieving good coverage of all the plant surfaces. Use the highest recommended water volumes for best results to ensure good penetration deep into the canopy.
Farmers with large acreages to harvest should try to spray in stages and only enough at a time so that they know they will be able to finish harvest within the window of opportunity.
When to harvest
Crops should be harvested as soon as they are ready after desiccation. The longer crops are left past the point when the desiccant has done its work, the more risk there is of pods on the bottom of the plant shattering during dry conditions. Or, crops can rot if wet weather occurs immediately afterwards or weeds can grow back from lateral buds that were not killed. The product label should carry a pre-harvest interval recommendation, but this should be viewed as a guideline as the actual time before the crop is completely ready to be harvested will vary depending on the product rate, how well it was applied and environmental conditions in the days following application.
The recommended interval for a desiccant like Reglone might be four to 10 days, but Klewchuk says there are a number of factors which affect the final harvest date. “With Reglone, since it’s just applied to the outside and doesn’t enter the seeds, if your crop is dry the day after you sprayed it you can harvest it,” he says. “That’s not very likely, but the point is if your crop was almost ready to harvest and you sprayed it and you achieved good coverage and weather conditions stay dry, you could be the guy harvesting in two days. If you want to cut corners on water volumes and spray far too early or it rains for five days afterwards you could be the guy waiting 10 to 14 days or longer.” †