Potato growers are facing into this season without a tool which many would have regarded as critical to the operation of their businesses. From 4 February this year, the use of diquat is no longer permitted on European farms for the desiccation of potato haulms. This loss is one of the most significant challenges to face potato growers in recent years.
The magnitude of the decision by the EU not to renew the authorisation of diquat has been exasperated by the fact that there is no other readily available alternative on the market which achieves the same results.
Furthermore, many of the options which are available to growers this season are generally suited to countries with earlier seasons and drier conditions. As no emergency authorisation for the active has been granted for this season, growers will have to plan ahead for a life without diquat.
In this article, I look into the background to the decision on the active to help us understand how the Commission reached its decision. I also take a look at some of the options available to growers for this season and explore how these tie together.
What’s the problem with diquat?
Diquat or diquat dibromide falls under an old family of chemistry. Other well-known herbicides in this family include paraquat.
Diquat is a non-selective, quick-acting herbicide, causing injury only to the parts of the plant to which it is applied. Diquat is referred to as a desiccant because it causes a leaf or an entire plant to dry out quickly.
The active was widely used to desiccate potato haulms and seed crops and was highly effective in allowing growers to manage their crop and control harvest times. As it is not a residual herbicide, it generally doesn’t leave any residues on plants, although tubers could have been damaged if applied in dry conditions.
Paraquat and diquat are poisonous by ingestion, inhalation and dermal exposure. Paraquat is among the most toxic herbicides currently in use in the world but has been banned for use in the EU since 2007.
Diquat is somewhat less toxic and does not specifically affect the lungs, but rather the gastrointestinal tract, liver, and kidneys.
In a paper published in the 2017 11th edition of the Veterinary Medicine book exploring systemic and multi-organ diseases, it outlines how the accidental poisoning of sheep as a result of contamination of pasture by diquat had been associated with widespread illness with signs of diarrhoea and mortality.
In cattle, accidental poisoning with diquat has been associated with fatal abomasitis and enteritis, hepatic and myocardial degeneration, and pulmonary emphysema.
EU decision
In the final renewal report for diquat published by the Standing Committee on Plants, Animals, Food and Feed, the key deciding factors leading to the non-renewal of the active were outlined.
The application for renewal of the approval of diquat was submitted by Syngenta Crop Protection AG and Sharda Cropchem Ltd. The period of approval for the active was extended until June 2019 to allow the completion of the committee’s review on the application.
The European Food Safety Authority (EFSA) published a report in November 2015 that concluded there were high risks to workers, bystanders and residents from the use of diquat, but added that the Commission might need to reassess the non-dietary exposure to it. In February 2018, the EFSA further concluded that it had a high risk under all scenarios to humans and birds.
The Commission proposed that concerns around diquat could only be managed by withdrawing the approval for the active.
Concerns largely centred on the fact that the estimated operator, bystander and resident exposure to diquat exceeded the acceptable operator exposure levels (AOEL) even when the use of PPE was considered. They also outlined a risk to birds and highlighted the risk of the potential long-term consequences of the use of diquat regarding groundwater exposure.
Based on the assessment the Commission had at its disposal, it decided to implement the proposal not to renew the active’s approval. From 4 February this year, the use of diquat is no longer allowed.
The challenge
For many growers, diquat has formed the main tool in potato haulm destruction for decades. The timely destruction of potato haulms is an important management strategy for a host of reasons. As well as helping to stop tuber bulking and optimise the marketable yield, it is also important for:
Skin-set and stolon separation: burning down the haulm helps to induce and speed up skin-set on tubers to reduce bruising and damage at harvest and assists stolon separation from the tuber.Reducing late blight infection and virus spread: in a high blight pressure season, fast destruction of the haulm can help to slow the spread of foliar blight to daughter tubers, particularly important for crops going into storage.Weed control: in situations where weed control was inadequate, desiccation can also kill weeds that are likely to affect the speed of harvest.Ease of harvest: haulm destruction improves harvest ease, particularly where crop senescence hasn’t taken place yet.The loss of this tool now poses a huge technical and operational challenge for potato growers. Now, growers only have a handful of options to destroy potato haulms, although none of them on their own is as efficient or as cost-effective as diquat.
Indeed, many of these measures will need to be used as part of an integrated approach, combining management decisions along with haulm destruction tools.
Variety choice
Planning your haulm desiccation strategy begins with choosing the right variety. Varieties are often grown to suit the target market but haulm characteristics will become an increasingly important factor in variety choice. The haulm destruction options outlined in this article will be much more effective if the haulm is already beginning to naturally senesce.
Table 1 outlines the classification of varieties based on their haulm longevity. For example, Rooster is the dominant variety grown in Ireland but it has a “long longevity” profile. This could pose problems for growing the variety in regions which have a later season.
Carfentrazone-ethyl
Carfentrazone-ethyl (commercial name Spotlight Plus) is the only remaining chemical option for haulm desiccation. While it does have its strengths, it also has many weaknesses when compared to diquat. The mode of action of carfentrazone-ethyl centres on the disruption of cell membranes, eventually leading to cell death.
Spotlight is good on haulm stems and does not translocate to the tubers, so applying in dry weather isn’t an issue. The product actually works best in direct sunlight. The spray is most effective when the crop is naturally senescing, again highlighting the importance of variety haulm characteristics.
The main challenge with carfentrazone-ethyl is its weakness on foliage and speed of action. Growers will likely have to develop a defoliation strategy, such as using a haulm topper, and follow up with an application of the product. Growers are also applying the spray along with Ranman Top to assist in defoliating the leaves.
The maximum individual dose of the product is 1.0l/ha, while the maximum total dose is 1.6l/ha. Growers must allow at least 14 to 21 days between the final application and harvesting to allow skins to set. That means growers will have to plan for a desiccation period of at least four weeks.
Other more unconventional methods being trialled by growers include inducing nutrient toxicity and using salt brines and other chemistry.
Haulm topping
Rapidly slowing or stopping growth is important with immature crops and indeterminate varieties. The only option left for growers to achieve this is through haulm topping.
With haulm topping comes added expense and risk. Last season was a prime example of this as good ground conditions are needed for topping. Even in sticky conditions, a topper would likely manage but could leave a mess for the harvester later in the season. Topping is slow work and can also leave a crop exposed to bird attack.
Crops need to be clean and free of disease to avoid the transmission of spores by the machine or mulch. In any case, a fungicide will need to be applied within the following 48 hours to combat this. That’s why it is important for growers to leave at least 20cm on the stem, as well as some leaves intact, to allow for fungicide and desiccant uptake. The quality of topping becomes really important here.
Other novel machines are also on the market such as haulm pullers, thermal haulm toppers and high-voltage electrical topping systems, although these are much less common.
Key considerations
Planning for haulm topping begins at the start of the season. Many destoners and planters operate on a single bed, but flails may operate over two or three beds. So it is important to match the topper to the rows when forming beds as a few centimetres out will make a huge difference.
Another consideration is leaving enough space to turn a flail at the edges of fields. This may mean you choose not to plant headlands this year, or at least you will need to consider where to leave breaks.
Nutrition management
The challenge of desiccation will bring growers’ nitrogen rates into sharp focus. The relationship between nitrogen rates, timing and uptake and the persistence of potato canopies will now become more important for growers and their use of nutrients.
Table 2 outlines the recommended nitrogen rates for maincrop potatoes based on their haulm longevity group outlined in table one. Note the variation in variety nitrogen recommendations. Furthermore, it may be sensible for growers to reduce nitrogen rates on the crops which they intend to harvest first to ensure the crop will be ready on time.
Tying it all together
This article is far from having all the answers for growers, which accurately depicts the current lie of the land across research and advisory.
The loss of diquat will represent a seismic shift in how growers plan, manage and desiccate their crops. Moreover, this will also increase costs for growers. A 2019 study by the Agriculture and Horticulture Development Board in the UK put the increased costs to growers from using a three-row topper plus an application of Spotlight increased desiccation cost by 210% per hectare.
Above all else, planning is now crucial. This means developing an integrated approach throughout the entire season using a combination of the options stated previously. For example, the use of Spotlight has proven quite effective once the haulms are defoliated with a topper.
Potato growers are facing into this season without a tool which many would have regarded as critical to the operation of their businesses. From 4 February this year, the use of diquat is no longer permitted on European farms for the desiccation of potato haulms. This loss is one of the most significant challenges to face potato growers in recent years.
The magnitude of the decision by the EU not to renew the authorisation of diquat has been exasperated by the fact that there is no other readily available alternative on the market which achieves the same results.
Furthermore, many of the options which are available to growers this season are generally suited to countries with earlier seasons and drier conditions. As no emergency authorisation for the active has been granted for this season, growers will have to plan ahead for a life without diquat.
In this article, I look into the background to the decision on the active to help us understand how the Commission reached its decision. I also take a look at some of the options available to growers for this season and explore how these tie together.
What’s the problem with diquat?
Diquat or diquat dibromide falls under an old family of chemistry. Other well-known herbicides in this family include paraquat.
Diquat is a non-selective, quick-acting herbicide, causing injury only to the parts of the plant to which it is applied. Diquat is referred to as a desiccant because it causes a leaf or an entire plant to dry out quickly.
The active was widely used to desiccate potato haulms and seed crops and was highly effective in allowing growers to manage their crop and control harvest times. As it is not a residual herbicide, it generally doesn’t leave any residues on plants, although tubers could have been damaged if applied in dry conditions.
Paraquat and diquat are poisonous by ingestion, inhalation and dermal exposure. Paraquat is among the most toxic herbicides currently in use in the world but has been banned for use in the EU since 2007.
Diquat is somewhat less toxic and does not specifically affect the lungs, but rather the gastrointestinal tract, liver, and kidneys.
In a paper published in the 2017 11th edition of the Veterinary Medicine book exploring systemic and multi-organ diseases, it outlines how the accidental poisoning of sheep as a result of contamination of pasture by diquat had been associated with widespread illness with signs of diarrhoea and mortality.
In cattle, accidental poisoning with diquat has been associated with fatal abomasitis and enteritis, hepatic and myocardial degeneration, and pulmonary emphysema.
EU decision
In the final renewal report for diquat published by the Standing Committee on Plants, Animals, Food and Feed, the key deciding factors leading to the non-renewal of the active were outlined.
The application for renewal of the approval of diquat was submitted by Syngenta Crop Protection AG and Sharda Cropchem Ltd. The period of approval for the active was extended until June 2019 to allow the completion of the committee’s review on the application.
The European Food Safety Authority (EFSA) published a report in November 2015 that concluded there were high risks to workers, bystanders and residents from the use of diquat, but added that the Commission might need to reassess the non-dietary exposure to it. In February 2018, the EFSA further concluded that it had a high risk under all scenarios to humans and birds.
The Commission proposed that concerns around diquat could only be managed by withdrawing the approval for the active.
Concerns largely centred on the fact that the estimated operator, bystander and resident exposure to diquat exceeded the acceptable operator exposure levels (AOEL) even when the use of PPE was considered. They also outlined a risk to birds and highlighted the risk of the potential long-term consequences of the use of diquat regarding groundwater exposure.
Based on the assessment the Commission had at its disposal, it decided to implement the proposal not to renew the active’s approval. From 4 February this year, the use of diquat is no longer allowed.
The challenge
For many growers, diquat has formed the main tool in potato haulm destruction for decades. The timely destruction of potato haulms is an important management strategy for a host of reasons. As well as helping to stop tuber bulking and optimise the marketable yield, it is also important for:
Skin-set and stolon separation: burning down the haulm helps to induce and speed up skin-set on tubers to reduce bruising and damage at harvest and assists stolon separation from the tuber.Reducing late blight infection and virus spread: in a high blight pressure season, fast destruction of the haulm can help to slow the spread of foliar blight to daughter tubers, particularly important for crops going into storage.Weed control: in situations where weed control was inadequate, desiccation can also kill weeds that are likely to affect the speed of harvest.Ease of harvest: haulm destruction improves harvest ease, particularly where crop senescence hasn’t taken place yet.The loss of this tool now poses a huge technical and operational challenge for potato growers. Now, growers only have a handful of options to destroy potato haulms, although none of them on their own is as efficient or as cost-effective as diquat.
Indeed, many of these measures will need to be used as part of an integrated approach, combining management decisions along with haulm destruction tools.
Variety choice
Planning your haulm desiccation strategy begins with choosing the right variety. Varieties are often grown to suit the target market but haulm characteristics will become an increasingly important factor in variety choice. The haulm destruction options outlined in this article will be much more effective if the haulm is already beginning to naturally senesce.
Table 1 outlines the classification of varieties based on their haulm longevity. For example, Rooster is the dominant variety grown in Ireland but it has a “long longevity” profile. This could pose problems for growing the variety in regions which have a later season.
Carfentrazone-ethyl
Carfentrazone-ethyl (commercial name Spotlight Plus) is the only remaining chemical option for haulm desiccation. While it does have its strengths, it also has many weaknesses when compared to diquat. The mode of action of carfentrazone-ethyl centres on the disruption of cell membranes, eventually leading to cell death.
Spotlight is good on haulm stems and does not translocate to the tubers, so applying in dry weather isn’t an issue. The product actually works best in direct sunlight. The spray is most effective when the crop is naturally senescing, again highlighting the importance of variety haulm characteristics.
The main challenge with carfentrazone-ethyl is its weakness on foliage and speed of action. Growers will likely have to develop a defoliation strategy, such as using a haulm topper, and follow up with an application of the product. Growers are also applying the spray along with Ranman Top to assist in defoliating the leaves.
The maximum individual dose of the product is 1.0l/ha, while the maximum total dose is 1.6l/ha. Growers must allow at least 14 to 21 days between the final application and harvesting to allow skins to set. That means growers will have to plan for a desiccation period of at least four weeks.
Other more unconventional methods being trialled by growers include inducing nutrient toxicity and using salt brines and other chemistry.
Haulm topping
Rapidly slowing or stopping growth is important with immature crops and indeterminate varieties. The only option left for growers to achieve this is through haulm topping.
With haulm topping comes added expense and risk. Last season was a prime example of this as good ground conditions are needed for topping. Even in sticky conditions, a topper would likely manage but could leave a mess for the harvester later in the season. Topping is slow work and can also leave a crop exposed to bird attack.
Crops need to be clean and free of disease to avoid the transmission of spores by the machine or mulch. In any case, a fungicide will need to be applied within the following 48 hours to combat this. That’s why it is important for growers to leave at least 20cm on the stem, as well as some leaves intact, to allow for fungicide and desiccant uptake. The quality of topping becomes really important here.
Other novel machines are also on the market such as haulm pullers, thermal haulm toppers and high-voltage electrical topping systems, although these are much less common.
Key considerations
Planning for haulm topping begins at the start of the season. Many destoners and planters operate on a single bed, but flails may operate over two or three beds. So it is important to match the topper to the rows when forming beds as a few centimetres out will make a huge difference.
Another consideration is leaving enough space to turn a flail at the edges of fields. This may mean you choose not to plant headlands this year, or at least you will need to consider where to leave breaks.
Nutrition management
The challenge of desiccation will bring growers’ nitrogen rates into sharp focus. The relationship between nitrogen rates, timing and uptake and the persistence of potato canopies will now become more important for growers and their use of nutrients.
Table 2 outlines the recommended nitrogen rates for maincrop potatoes based on their haulm longevity group outlined in table one. Note the variation in variety nitrogen recommendations. Furthermore, it may be sensible for growers to reduce nitrogen rates on the crops which they intend to harvest first to ensure the crop will be ready on time.
Tying it all together
This article is far from having all the answers for growers, which accurately depicts the current lie of the land across research and advisory.
The loss of diquat will represent a seismic shift in how growers plan, manage and desiccate their crops. Moreover, this will also increase costs for growers. A 2019 study by the Agriculture and Horticulture Development Board in the UK put the increased costs to growers from using a three-row topper plus an application of Spotlight increased desiccation cost by 210% per hectare.
Above all else, planning is now crucial. This means developing an integrated approach throughout the entire season using a combination of the options stated previously. For example, the use of Spotlight has proven quite effective once the haulms are defoliated with a topper.
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