Sulfonylurea herbicides. 26

What are the sulfonylurea herbicides?

The SU’s are classed as Group 2 herbicides and have become widely used in the turf industry for three main reasons. First, due to their high selectivity. Second, due to their low level of mammalian toxicity and finally, because they have a very low impact on the environment. All SU’s have the same mode of action and they act by stopping acetolactate synthase (ALS). In short, once applied to a weed the plant begins to starve and later, as new growth occurs plants die as they cannot make proteins. They basically starve to death.

There is no “best” SU herbicide and they all have a roll to play in turf management.

Tips for using SU herbicides.

• Since all have the same mode of action, you should rotate with other herbicides from other groups;  

• Avoid spray drift due to the chance of non target damage;

• Some SU’s need watering in whilst others recommend the use of spray surfactants. Always read the label;

• Due to an increase in the chance of turf damage avoid using these within seven days of organophosphate insecticides;

• Their high specificity means that turfgrass tolerance varies. For example, you can safely use Prosedge® for sedge control in both cool- and warm-season turfgrass. In contrast, Monument® liquid turf herbicide also controls sedges but it will cause damage to some cool-season turfgrass;

Tank mixing the SU herbicides.

• Don’t mix these with other chemicals i.e graminicides. For example, Destro® in a tank mix with SU herbicides gives poor results;

• These work best at a tank pH of around 7.0 so don’t mix with acidifying chemicals or fertilizers;

Getting the Best results from the SU herbicides.

• With a high water pH they are very mobile and can cause carryover and injury. For example, Tribute®, Haloforce® and metsulfuron are 50 to 100 times more water-soluble when the water pH increases from 5 to 7. Due to this, ‘herbicide tracking‘ is possible for up to 20 hours after applying;

• The best result when transitioning ryegrass occurs when the couch is growing aggressively and the ryegrass is killed quickly. In this situation, the couch rapidly fills in the dead areas and there is minimal disruption. So aim to use these when the average air temperatures are 18°C or higher, and/or when average soil temperatures at a depth of 150mm are 21°C or higher. These target temperatures should ideally continue for the week following application. 

• You get the best results when weeds are actively growing. In contrast weeds that are under stress will not take up any chemical. 

• Despite having a short soil residual, once plants begin to grow, there can quite often be a small amount of chemicals left in the soil. This can impact on any reseeding work and is why there are witholding periods on many labels. However, these are not true pre emergent herbicides. 

Negatives of sulfonylurea herbicides.

In brief there are two major negatives of these:

• First, they are all prone to developing herbicide resistance and
• Second, all have some soil residual which impacts on the success of overseeding. Hence many labels have withholding periods before you can carry out seeding.

Signs of sulfonylurea herbicide damage

• First, growth stops;

• A few days later, target weed leaves become wilted, crinkled and yellow;

• Leaf margins take on a red colour as a result of anthocyanin build up and finally

• New growth is stunted and deformed.

The table below shows the tolerance of turf grass to SU herbicides.

Sulfonylurea herbicides for turf.

Tribute® – (22.5g/L Foramsulfuron).

General: Use Tribute® to control cool-season grasses and crowsfoot grass in couchgrass.

Tolerant Turfgrasses: In brief, apply to couch (common and hybrid) and Zoysia grass. However, don’t use on Queensland Blue Couch, Centipedegrass, and cool-season grasses, such as fescue, ryegrass, bentgrass, and bluegrass.

Use Pattern: Use Tribute® to control winter grass, crowsfoot grass, ryegrass, and intolerant warm-season turf grasses. It is also effective for the removal of clumpy ryegrass and fescue and also as a spring transition aid in overseeded couch.

Overseeding Restriction: You can overseed with ryegrass after one week while sprigging of couch grass should be left for two weeks after application.

Usage: Don’t immediately water after application and allow a 2 hour window to get the best results. Allow Tribute® to dry before traffic, as this avoids damage to cool-season turfgrass due to tracking. Despite this, tracking can occur after morning dew the day after treatment.  To reduce the potential for any issues, water the morning afterwards and try and leave a 3 metre buffer around areas of creeping bentgrass.

Weeds Controlled: Tribute® gives excellent control (>90%) of many very hard to control cool-season grasses like winter grass, ryegrass, and clump fescue.  Also, it is effective for the post-emergent control of crowsfoot grass.

The image below shows tracking of Tribute® on a mixed Poa/bentgrass green.

Prosedge® (750g/kg Halosulfuron).

General: Prosedge® is the only Group 2 herbicide that only controls yellow and purple nutsedge as well as Mullumbimby couch.

Tolerant Turfgrass: All cool and warm season grasses are tolerant.

Target weeds: In short, Prosedge® is used to control sedges with best results occuring when sedge plants are between the 3rd and 8th leaf stages. Also, with thick growth, it needs repeat applications, 6 to 10 weeks apart. Finally, use no more than 200g/Ha per season.

Overseeding Restriction: None.

Usage: Prosedge® is rainfast after 3 hours, so best results occur when no rainfall or irrigation happens within 4 hours of application. Usually, after 7 – 10 days you can see the first signs of damage and the full effects are seen after 4-6 weeks. To get the best results you need to use with a non-ionic surfactant such as BS1000.

Weeds Controlled: Depending on the age and size of the target, an 80 to 100% kill of yellow and purple nutsedge will occur. However, purple nutsedge is generally thought to be more sensitive to Prosedge® than yellow nutsedge.

Coliseum® (250g/kg Rimsulfuron).

General: Use to control winter grass and also as a spring transition aid in overseeded couch.

Tolerant Turfgrasses: Couch.

Use Pattern: Use Coliseum® on sod farms, professional sports fields, and golf courses. Both turf roots and leaves take up Coliseum® so watering 1 hour after application will move the herbicide into the soil and further increase herbicide performance. If watering, take care to avoid surface runoff as Coliseum® is able to move with water.

As a result, take great care when applying Coliseum® near, or on slopes that drain onto, cool-season grasses. In fact, short, frequent, irrigation cycles are the most effective to water it in without any runoff. 

Although tracking and runoff almost never result in the death of bentgrass, off-colour turf, and growth reduction will persist for 14 days, or longer, depending on weather and fertility.

Also always add a non-ionic surfactant (for example BS1000) at 250mL/100 L (0.25 % v/v) of final spray volume.

Overseeding Restriction:  Delay oversowing with cool-season turfgrass for 10 days after herbicide application.

Weeds Controlled: Coliseum® provides >90% control of ryegrass, fescue, and winter grass and broadleaf species such as common chickweed and spotted spurge.

Monument® Liquid herbicide (750g/kg Trifloxysulfuron).

General: In brief use Monument® Liquid herbicide for broadleaf weed, sedge, and winter grass control in couch. It is also used as a transition aid for removing overseeded ryegrass.

Tolerant Turfgrasses: Couch is tolerant to the Monument®

Overseeding Restriction:  Delay oversowing for 6 weeks after herbicide application while you can sprig couch or seed 4 weeks after herbicide application. This is an important if you have a limited amount of time for example on multiuse stadiums.

Use Pattern: Use Monument® on golf courses, sod farms, and other non-residential sites. What’s more, on juvenile turfg delay applying until there is 100% ground cover, and there is a 50mm root depth.

Use: Repeat applications are needed for control of very hard to control weeds such as sedges and tall fescue. Make the follow up application 4 to 6 weeks after the first application. Last of all, do not exceed an annual application total of 113g/Ha per year. Monument® is rainfast within 3 hours of application.

Duke®/Destiny® (100g/kg Iodosulfuron-methyl )

General: Use Duke® herbicide to suppress winter grass and reduce bahiagrass seed head. Duke® also controls ryegrass, as well as the weeds bindii, black thistle, cat’s ear, Cotula, cudweed, Guildford grass (Onion Grass), Medic, Mouse-ear chickweed, Oxalis, White Clover, and False onion weed/ fragrant false garlic/ onion weed.

Tolerant Turfgrass: Use on Kikuyu, buffalograss, or couch (common and hybrid couch) but NOT on Queensland Blue Couch.

Overseeding Restriction: Allow at least six weeks between the last application and overseeding with cool season grasses such as perennial ryegrass.

Use Pattern: After applying, growth slows within the first few days and chlorotic patches start to appear. Later on shoot death occurs with under good growing conditions death occuring 4 to 6 weeks later. 

Like many SU herbicides, Duke®can be transferred or ‘tracked’. So, to reduce this risk, allow at least 4 hours for uptake before watering. 

Application to very dry sandy soils followed by soaking rainfall as well as to alkaline soils (soil pH >8.5) may also cause turf damage.

In short, Duke® is a foliar herbicide with less activity via the soil. As a result, it is recommended to use a water rate of 200-500 L/ha.

Use: If the tank water pH is less than 5.5 raise the water pH to 7.0 and do not mix with acidic compounds. Do not make more than one application in a season except for Bahiagrass. 

Always apply with a non-ionic surfactant, for example BS1000 or Hasten® at the label rate.

Negate® (167 g/kg Rimsulfuron and 200 g/kg Metsulfuron-methyl)

General: In short Negate® contains two sulfonylurea herbicides and you use it to control winter grass, ryegrass, onion grass, and certain broadleaf weeds. These include bindii, caustic weed, white clover, Cobblers Peg, Creeping Indigo, Creeping Oxalis, Plantain, and Daisy.

Tolerant Turfgrass: Use Negate® on common and hybrid ouch, zoysia, and also Qld Blue Couch.

Overseeding Restriction:  6 weeks

Use Pattern:

Both shoots and roots take up Negate® and it is rainfast within 4 hours. It rapidly slows weed growth after the foliage take up the chemical. Leaves of susceptible weeds then bleach after 1-3 weeks before dying.

Herbicide performance increases in warm, moist conditions; however, in cold, dry conditions, herbicide activity slows down.

Root Absorption

As it moves downward through the soil, plant roots are able to take up any remaining herbicide.

Use: When treating annual ryegrass it often needs a second treatment after 4 to 6 weeks. The Negate® product label

Nominee® herbicide (100g/L Bispyribac-sodium)

General: Unlike other SU’s this is specifically used to control wintergrass (Poa annua) in creeping bentgrass, tall fescue, and also ryegrass. It also suppresses winter grass seed heads.

Tolerant Turfgrasses: You can safely use this on couch, Zoysia, and Qld Blue Couch.

Overseeding Restriction: 42 days is the figure to remember. You can’t seed bentgrass, perennial ryegrass or tall fescue after applying Nominee®. You also can’t use Nominee® for at least 42 days after seeding creeping bentgrass, perennial ryegrass, or tall fescue.

Use Pattern: Nominee® is taken up by roots and then moves to the shoots. Other points of note:

• Don’t mix with a surfactant or crop oil as it causes yellowing in tolerant turf;

• Nominee® claims to be slow acting. However, the image below shows this isn’t always the case. Be aware that the results can be “variable”;

• Don’t apply to turf under stress and

• If you expect rain within twelve hours of use don’t apply, irrigate or mow for 24 hours afterwards.

Research into bispyribac-sodium

• Nominee® can be used to control winter grass in creeping bentgrass at rates ranging between 60 and 140 g ha-1.
• Two applications of Nominee® on a creeping bentgrass fairway at 74 g a.i. /ha in the summer reduced Poa annua cover while also reducing creeping bentgrass injury.
• McCullough and Hart (2010) found the most successful rate for Poa annua control in creeping bentgrass was 24.6 g a.i. ha−1 applied weekly for up to 2 months. 8 weeks later winter grass was reduced by 90% coupled with acceptable levels of creeping bentgrass discoloration. However, after 8 weeks, all treatments reduced turf quality because of voids forming, with six applications reducing turf quality the most.

The future of the SU herbicides in the turf industry.

SU herbicides are useful as weed killers and as spring transition aids to remove cool season grass from couch oversown the previous autumn.

These all have the same mode of action and so the issue of herbicide resistance is a problem. Weeds have naturally-occurring biotypes that are resistant to these herbicides. Continuing to use this chemical group and not rotating to other groups means that at some stage sulfonylurea-resistant weeds are likely to develop.

Table showing the weeds controlled by SU herbicides.