Staying on top of weeds in potatoes

Potato farmers are having one of the worst years in living memory. Their potato stores are full and their bank accounts are empty. If they are to continue in potato production, this is the year where every cost must be examined and reduced if possible, while yield and quality must be maintained and improved. So everything must go under the microscope.

Weeds compete with potatoes for light, water and nutrients. They can increase the humidity in the crop, thus increasing disease risk. Many weeds can carry diseases to which potatoes are susceptible, (e.g. Solanum dulcamara can carry Ralastonia). Still, the biggest fear farmers have is that weeds will make harvesting the crop more difficult. Bindweed and knotgrass come to mind immediately in this regard. Also, a carpet of annual meadow grass growing over the drill at harvest can increase the level of bruising/damage because of the increased levels of agitation necessary to separate potatoes from clods/scraws.

Combinations of herbicides are needed to kill all the weeds present and to seal the soil against those which will emerge later. And this must be done while at the same time not causing a residual problem for the following crop.

The Sustainable Use Directive (SUD) directs us to use the least amount of herbicide necessary and to justify our choice of herbicide and the dose rate while considering IPM in our assessment and product choices.

Field assessment

Weed control should really begin before a field is even rented. The more you can learn about the field at that point, the more you will know about your growing costs.

Know the weeds:

1. Knowing the rotation is vital. It could help you to avoid serious problems; e.g. very short potato rotations bring PCN to mind.

2. Rotations with high levels of maize can lead one to expect black nightshade (Solanum nigrum). This is a problem weed because it germinates late, after you have applied herbicide (Figure 1).

3. Establishment systems that used minimum cultivations can lead to grass problems, with sterile brome and annual meadow grass among the usual suspects.

4. Fields that had oilseed rape in the rotation are likely to have cleavers and oilseed rape volunteers in the following crops.

5. Peaty soils usually have high levels of polygonium weeds and sometimes silver weed can be present in high levels.

6. Old ley fields usually have high levels of scutch, docks, nettles and yarrow.

7. Even a drive along the road to inspect the field before the previous crop is harvested can be valuable. The presence of wild oats, cleavers and poppies send a certain message.

Eliminate as many weeds as possible before ploughing with products such as Roundup/Rosette/Clinic, sprayed onto green foliage. This is especially important in the case of perennial weeds.

Soil type matters: Simple basics about the type of soil in the field, and its condition, should also be known.

1. The soil type and the organic matter level determine at what rate the soil residual herbicides should be applied.

2. It should be noted that some soil residuals cannot be applied to sands.

3. Seedbeds should be fine, clod free and stable but capable of holding adequate moisture levels on the surface for soil residual herbicides to work.

(a) Remember that one acre in drills is almost 1.5ac in surface area.

(b) Drills with steep sides are prone to soil slips along their edges, therefore herbicide application should be applied after this has taken place, i.e. when the drill is stable.

(c) Fine seedbeds – soil residuals do not work well in cloddy seedbeds and weeds usually emerge from around the edges of clods.

(d) Soil moisture is vital for soil residuals to perform to their potential.

Herbicide timing

Potatoes should be sprayed pre-emergence. The potato shoot should be at least 2in below the surface. The lighter the soil, the earlier they should be sprayed. Soil moisture is essential at the time of spraying. If there is no surface moisture, the soil residual component will not work. If heavy rain falls just after the herbicide application, it can wash down to the emerging shoots and cause phytotoxicity.

As many weeds as possible should have emerged at the time of spraying, especially those weeds which were problems in the past. The contact portion of the herbicide mixture will destroy the aerial portion of the weeds.

Herbicide Application

Compete coverage of the soil is essential. The use of fan nozzles and adequate water are important. There are a few things to keep in mind.

(a) Very high pressures and small droplet sizes can give very good coverage but excess drift.

(b) If there is wind at the time of spraying one should change to anti-drift nozzles.

(c) The boom should be at the correct height to hit the target -- the target is the top of the drill.

(d) There should be at least one hour of drying post spraying. Remember if a product is soluble in water in the sprayer it is also soluble in rain. If this occurs you may have no contact action at all. The soil residual portion of the mixture should be OK.

The products

Successful weed control generally involves a combination of different targeted activities. This is generally a mix of a contact active to kill the weeds emerged at the time of application in combination with a residual product to control weeds that may germinate later.

Fully translocated

Roundup Gold (PCS No: 2624)

Roundup Gold 450 (PCS No: 2623)

These are two total translocated herbicides. Both products have recommendations for use pre-emergence of the potato crop, but post-emergence of weeks. They can only kill what is present at the time of application but they can be used in conjunction with soil residual herbicides to seal the soil against future weed emergence.

However, if any form of Roundup comes in contact with potato shoots it can do considerable damage, both to the crop itself and the seed crop coming off that crop. Roundup should not be considered if the emerging potato shoots are within 3in of the surface of the drill. And remember that the potato shoots may not be emerging in line with the maximum height of the drill but rather through the side and could therefore easily pick up the Roundup.

And remember the absolute necessity to clean out the sprayer thoroughly after use to remove all traces of Roundup.

The rate at which Roundup Gold 450 should be applied is 350g of active per hectare (0.78 l/ha). Its particular strength is where there are perennial grasses emerged at the time of spraying.

Contact Products

Diquat: a broad spectrum (see Table 1 on P50-51) contact herbicide sold as a number of different brands but the ‘original of the species’ was Reglone. It is from the same herbicide group as Paraquat. Diquat, when used with a wetter, behaves almost exactly the same as Paraquat did on all weeds.

Diquat has no residual properties and is de-activated on contact with soil. It is therefore an ideal partner with soil residual herbicides. Good target coverage is absolutely essential when spraying this product alone or with partners. The Diquat is the stale seedbed portion of any mixture. Adequate water must be used to ensure complete weed coverage.

Diquat should be used pre-emergence of the crop but is frequently used at emergence, resulting in subsequent crop scorch. The rate of application is usually from 2.0 l/ha down to 1.0 l/ha. The rate will depend on the timing and the tank mix partners.

Carfentrazone-Ethyl (Spot light Plus – PCS No: 3294): This is a contact herbicide with a very broad spectrum of weeds controlled (see Table 1). It is usually applied in mixtures with soil residual herbicides. The rate of application is 0.33 l/ha and it can be applied up to 5% emergence of the potato crop. As with all contact herbicides, coverage is the key.

This product will be of interest to growers wishing to reduce their pesticide footprint but, as with all things, it comes at a price.

Residual Herbicides

Metribuzin: This active ingredient is the most widely used soil residual herbicide. It is sold under a variety of trade names – (Sencorex Flow, PCS No. 4361, is the latest and only liquid formulation).

Metribuzin has also considerable contact action and can be used post emergence on a number of varieties. However, because of its weed spectrum and residual properties, it is usually mixed with Diquat and applied just before emergence of the crop.

This active has several traits that might be described as unusual. Black bindweed is poorly controlled pre-emergence of the weed but it is extremely well controlled post-emergence (see Table 1). The rate of application should not be reduced below 0.6kg/ha when applied pre-emergence, as the residual life of the herbicide will be compromised.

The same rule applies to all residual herbicides. They need soil moisture, a stable drill, a fine seedbed – clod-free – and, most important of all, good even coverage on the soil.

Linuron (Afalon liquid PCS No. 2839): This is quite an old active. It is always used in mixtures with Diquat. Where it is used with Diquat and metribuzin it gives pre-emergence control of black bindweed. Linuron is quite soluble and so can be washed into the soil to scorch the emerging shoots. The rate most frequently used is 1.0-1.5 l/ha when used in mixtures with Diquat and metribuzin. Linuron should be used pre-emergence only.

Pendimethalin (PDM): There are many suppliers and formulations of PDM available – Stomp, Stomp Aqua and Most Micro to mention but a few. PDM is an extremely persistent and insoluble residual herbicide. Its uptake by the weeds is by cotyledon transfer from soil, so weeds emerge, turn purple and then die. The strength of this product is the control of black nightshade and annual meadow grass. Its weakness is its lack of control of groundsel.

This product has some contact action but its strength is its persistence (it has a half-life of 210 days @ 4oC which drops to 30 days @ 10oC) and a solubility of 0.2ppm.

The product should be applied pre-emergence only. The emerging potatoes need to be at least 2in away from the soil surface.

The product should be applied at no less than 1,300g of a.i./ha, otherwise persistence will be affected.

Defy (Prosulfocarb 80%. PCS No. 2706): This product is relatively new. It has an optimised wetting system and has both contact and residual activity. The strength of this product is in the control of cleavers, but annual meadow grass, black nightshade, groundsel and fumitory are also well controlled.

When applied in mixtures with Diquat and metribuzin, Defy appears to have better weed control than one would expect from the spectrum provided by the individual ingredients.

Pre-emergence timing is best but application can be delayed up to 10% crop emergence. Toxicity can be expected when application is delayed.

The rate of application in mixtures is 3.5-5.0 l/ha. Higher rates are usually associated with black nightshade and cleaver control situations.

Syngenta has developed nozzles which are optimised for the application of Defy.

Lingo (Linuron 250g/l + Clomazone 45g/l PCS No: 3660): Lingo is also a relatively new herbicide for potatoes and some other crops. The two active ingredients complement each other but it still needs a contact herbicide and probably metribuzin in most situations. Its strengths include cleaver and knotgrass control when used in mixtures.

Timing of application should be seven days pre-emergence of the crop.

Rate of application is 2.0 l/ha, with metribuzin @ 0.5 kg/ha and Spotlight @ 0.33 l/ha.

Post emergence herbicides

There are three herbicides used on potatoes post-emergence – metribuzin, Titus + Wetter and Basagran. Each of these products suffers a number of limitations.

1. The weeds tend to be hardened from the pre-emergence herbicide.

2. It is more difficult to apply adequate a.i. to the weed in rapidly growing crops.

3. Post-emergence weed control is usually an emergency situation. Weeds tend to be beyond the growth stage where the product gives satisfactory control by the time the problem is observed.

Basagran (Bentazone PCS No: 2341): Timing – potatoes should be sprayed before they are 15cm high. Split applications of 1.5 l/ha are usually recommended to help reduce phytotoxicity to the crop. Actipron @ 1.5 l/ha should be added to the Basagran to help improve contact action on weeds.

Only certain varieties can be treated with Basagran.

Titus (Ramsulfuron 250g/kg PCS No: 3992): The product application rate is 50 g/ha + Wetter (Agral at 1% in the spray solution. But it cannot be used on potatoes that are grown for seed or on some varieties used for processing. It can be used on Maris Piper. The latest timing is when the crop is 25cm high. It is excellent on cleavers.

Metribuzin: Follow-up treatments with metribuzin can be used on some varieties (do not use on Maris Piper) to control volunteer oilseed rape, knotgrass, black bindweed and orache. Rates of 0.25 kg/ha, followed up if necessary with 0.25 kg/ha, can give very good results. Where cleavers are present along with the aforementioned weeds, the addition of Titus at 35g/ha, with a wetter, can be extremely useful.

Table 1 shows that Black bindweed is not controlled by metribuzin when applied pre-emergence but still a much lower rate applied post-emergence will control it.

Master mixtures

Over the past number of years several herbicide mixtures have earned a reputation for consistent performance. As stated at the onset, prior knowledge of the field will help to target weeds more accurately. But most fields fall into one of the three main categories in Table 2 and mixtures can be chosen accordingly.

Helping the herbicides

It is not enough to control weeds by applying herbicides -- we should also be applying sulphur or N16 early in the season to promote foliage growth in the potatoes to help smother any uncontrolled weeds. The application of any product at the rosette stage, which helps the crop close the drills before 21 June, is to be encouraged.

Using herbicides in combination at lower than normal rates should be considered as the rate of degradation of each herbicide is independent of each other and so lower soil residues will result.