Cracking the Code: Factors Shaping Herbicide Performance
- Herbicide selection
- The target weed species,
- Application timing
- Environmental considerations.
- How are you applying it?
- Environmental considerations.
- Herbicide resistance
- Turfgrass tolerance
- herbicide formulation and
- Planning ahead
Herbicide Selection
Target weeds
Ways in which the target weeds impact herbicide performance:
Sensitivity:
Different weed species have varying levels of sensitivity to herbicides. Some weeds are susceptible to a particular herbicide, while others are more tolerant. For example, Pigweed can be tolerant to 2,4-D compared to other broadleaf weeds.
Mode of action:
Some herbicides may target specific biological processes or enzymes that are unique to certain weed species. If the target weed species lacks the target site or mechanism of action, the herbicide may not work against that particular weed.
Dicamba is a broadleaf herbicide that primarily targets sensitive broadleaf weeds by disrupting the growth and development of plant tissues. However, oxalis may exhibit tolerance to dicamba, so you get poor control when you use this.
Growth stage:
How well a herbicide works against a target weed is also a result of its growth stage. Certain herbicides are most effective when applied at specific stages of weed development.
For example, quinclorac works well vs juvenile and mature summer grass weed but not so well when it’s at the 2-4 tiller stage. So applying a herbicide at the wrong growth stage may result in poor results.
Glufosinate is another example. When you use this as a non-selective herbicide for broad-spectrum weed control, it works best on young, actively growing weeds.
Weed size:
The size of the target weed can also influence herbicide performance. Larger and more developed weeds tend to have thicker cuticles and extensive root systems. These affect herbicide absorption and translocation within the plant. Some herbicides may be less effective on larger weeds due to reduced uptake or limited movement within the plant.
Resistance:
Weed species that have developed resistance to specific herbicides pose a significant challenge.
Application Timing
Some examples of application timing impacting herbicide performance:
Weed growth stage:
Different herbicides are more effective at specific stages of weed growth. Timing the herbicide application when the target weeds are in their most susceptible growth stages can lead to better control. For example, some herbicides are more effective when applied to young, actively growing weeds, while others may be more effective on mature weeds. Applying herbicides too early or too late in the weed’s growth cycle can result in reduced efficacy.
Turfgrass Growth stage:
Certain herbicides may cause damage or stunting to turfgrass if you use them at the wrong time.
Environmental conditions:
Environmental conditions at the time of herbicide application can also impact performance. Factors such as temperature, humidity, and rainfall can influence herbicide absorption, translocation, and degradation within the target weeds. Applying herbicides during unfavorable weather conditions, such as high temperatures, drought, or heavy rainfall, may result in reduced herbicide efficacy or potential off-target movement.
Weed size:
The size of the target weeds can affect herbicide performance. Some herbicides are more effective on small, actively growing weeds, while others may require larger weed sizes for optimal absorption and translocation within the plant. Applying herbicides too early when the weeds are too small may result in inadequate control, while applying them too late when the weeds are too large can reduce the herbicide’s effectiveness.
Repeat applications:
Some herbicides like MSMA or Poacure require multiple applications or follow-up treatments to achieve effective control. The timing of these repeat applications is critical. Herbicides often have specific intervals between applications to target weeds at different growth stages or to control emerging weeds.
Environmental Factors
Environmental conditions affecting herbicide performance:
Temperature.
Temperature has a big influence on herbicide performance. Some herbicides require a specific temperature range for optimal performance. A great example of this is with diclofop methyl to control crowsfoot grass.
The Destro label specifically states to not use this if temperatures are above 25C. High temperatures speed up the breakdown rate of some herbicides, and so stop them from working as well.
Conversely, low temperatures tend to slow down herbicide activity. This is the case with the sulfonylureas which includes Indigo Duke herbicide, Indigo Recondo herbicide and Bayer Tribute herbicide. When you use any herbicide, make sure it is within the recommended temperature range to get the best results.
Humidity.
Higher humidity helps herbicides stay on the plant surface longer, and enhances uptake. This is especially important for foliar herbicides. During hot, humid conditions spray droplets evaporate slower. This means any spray takes longer to dry and so increases herbicide absorption.
Absorption and translocation of bentazone in turfgrass increases in humid conditions1Kudsk, P.; Kristensen, J. Effect of environmental factors on herbicide performance. In Proceedings of the First International Weed Control Congress, Melbourne, Australia, 17–21 February 1992; Weed Science Society of Victoria: Victoria, Australia, 1992; pp. 173–186. This is due to the cuticle being hydrated, which encourages herbicide absorption2Matzenbacher, F.O.; Vidal, R.A.; Merotto, A.; Trezzi, M.M. Environmental and physiological factors that affect the efficacy of herbicides that inhibit the enzyme protoporphyrinogen oxidase: A literature review. Planta Daninha 2014, 32, 457–463..
Hot, dry conditions have a greater impact on systemic herbicides than contact type products. However, excessively high humidity or prolonged periods of leaf wetness can lead to herbicide runoff or wash-off before uptake can occur. This means that in hot and dry conditions the use of the right spray adjuvant and surfactant is important.
Adjuvants and surfactants enhance spray droplet retention on the leaf surface and increase herbicide uptake. They dissolve wax cuticles, and hold spray droplets on the leaf for longer by reducing the evaporation rate.
Rainfall.
Rainfall or irrigation shortly after a herbicide application impacts herbicide performance. Heavy rainfall or irrigation causes herbicide runoff, dilution, or leaching, and reduces the concentration of herbicide available for weed control.
- S-metalochlor present Pennmag (Syngenta) can quickly move out of a sandy soil profile after heavy rain, due to its relatively high solubility in comparison to pendimethalin.
Wind.
Wind affects the spray pattern and distribution of herbicides. Strong winds can cause herbicide drift, that leads to non target deposition of herbicide droplets or particles.
Herbicide drift can result in damage to desirable plants, crops, or adjacent sensitive areas. That’s why you use drift control agents to reduce spray drift. These work by increasing the average droplet size by binding with water molecules to form larger spray droplets.
Soil conditions.
Soil conditions have a big impact on herbicide performance. This is especially the case with soil-applied herbicides such as pre-emergent herbicides. Factors such as soil moisture, organic matter content, and pH can impact herbicide availability, absorption, and degradation in the soil. Additionally, soil texture and structure can affect herbicide movement.
- Foramsulfuron (Tribute Selective herbicide) works best on Crowsfoot grass under conditions of high soil moisture and high evaporation, and may be reduced in sandy soils.
Correctly Applying
- The spray volume that you use.
- The spray droplet size, and
- The nozzle selection,
These are all important to achieve thorough coverage and minimize drift. A good example of this is the use of dicamba and herbicide formulations that contain this active, like Methar Tri Kombi.
You use the selective herbicide Dicamba for broadleaf weed control in turf grass. Drift onto non-target areas, such as ornamental plants or sensitive vegetation, causes non-target damage or injury.
Herbicide Resistance.
Turfgrass Tolerance
How Turfgrass Tolerance effects Herbicide Performance:
Selectivity:
Selective lawn herbicides control specific weed species while minimizing damage to desirable turfgrass. Take for example the selectivity of the sulfonylurea herbicides on warm season turf. This selectivity enables you to use many of these to remove turf type ryegrass from oversown sports grounds when you transition back to couch in the Spring.
Safety margin:
Herbicide labels often provide specific application rates for different turfgrass species or cultivars. These rates are a direct result of turfgrass tolerance and are the range of herbicide rates that will not cause turfgrass injury. The herbicide formulation can also play a role with SC formulations generally regarded as being safer than EC formulations.
Recovery and regrowth:
Tolerant turfgrass has the ability to recover after herbicide application. Even if some injury occurs, tolerant turfgrass recovers and resumes normal growth.
Herbicide options:
Turfgrass tolerance to herbicides affects what you can use. Certain turfgrass species or cultivars may have higher tolerance to specific herbicides, allowing for more effective control of target weeds without causing significant damage to the turf. Understanding the tolerance of your turfgrass to different herbicides can help in selecting the most appropriate herbicide for your specific weed control needs. Examples of this are:
- Bispiribac sodium. There is a big difference between the tolerance of turf type perennial ryegrass cultivars.
- Oxadiazon. You can’t use this on Santa Anna couch but It is safe on all other hybrid couch varieties.
Herbicide Formulation
- A good example of this is Onset 10GR a pre emergent granule herbicide formulation of prodiamine and the liquid herbicide formulation called Barricade herbicide.
Planning ahead
FAQ
What are the main factors which affect herbicide selectivity?
- Plant species may differ in their growth habits.
- Herbicides may be applied at different times by different methods.
- Use of antidotes and adsorbents to prevent herbicide uptake by non target plants.
- Herbicide formulations may differ in their ability to contact with non target plants.
- Even though equal amounts of herbicide may be taken up by plants and weeds the rate they move inside the plant (translocate) may be different.
- Faster translocation does not necessarily mean it kills the plant quickly. In some instances it can mean it is an escape mechanism to avoid herbicides.
- A change in the molecular structure of a herbicide inside a plants.
- Intermediate compounds are more phytotoxic than the original herbicide that was applied.
- The herbicide binds with plant cell constituents inside the living plant
What are the reasons for poor herbicide performance?
There are several factors that can affect herbicide performance. These can be classed as environmental factors, plant factors and management factors
Environmental factors like sunlight, temperature, water stress and quality, humidity, temperature and sunlight all impact on herbicide performance. In general conditions that favour plant growth also favour herbicide performance.
- Sunlight. Plant cuticles (the outermost protective waxy layer of a leaf) thicken when plants are subjected to continous high light levels. This in turn makes the absorption of some water-soluble herbicides more difficult.
- Temperature. Assuming turf is not under any moisture stress, temperatures below 35 °C, increase the uptake and activity of many herbicides. The actual amount absorbed is generally not affected but the rate of uptake increases. Warm temperatures also speed up the activity of a herbicide.
- Humidity. High humidity reduces spray drift and slows the droplet drying time. This means it takes longer for a herbicide to enter a plant. Plants that grow in high humidity tend to have thinner cuticles, which causes a much faster absorption of chemicals. High humidity also enables many plants to grow faster, so herbicides are moved to their activity sites faster.
- Moisture stress. A lack of moisture reduces the movement of nutrients and water in the plant, which reduces the effectiveness of systemic herbicides. Plants growing under moisture stress tend to have thicker cuticles which reduces the absorption of herbicides.
- Physical barriers. In dry conditions there may be a layer of dust or dirt on the leaves. This can form a physical barrier that stops a herbicide entering into a leaf. The nature of the soil can affect the performance of pre-emergent herbicides.
- Rainfall/Irrigation. When rainfall or irrigation washes dust or dirt off a leaf its gives a herbicide a better target surface. It also is vital for pre-emergent herbicides to work as they need to be watered into the soil. The rain fastness of post emergent herbicides also varies greatly, and changes with weed species.
- Water Quality. Water quality affects how well a spray works. Factors such as pH, whether its hard or soft, and its clarity (turbidity) have a big impact on herbicide performance.
When should I apply my lawn pre emergent?
The best time to apply these products is in the early spring as this is before broadleaf weeds begin to grow. For grass weeds like summer grass apply later say in late spring or early summer. This weeds germinate later, so by applying your pre emergent later you will get longer control.
What is a turf herbicide?
Turf herbicides, also known as weedkillers are pesticides used to control and manage undesirable plants. It is important that when you use a turf herbicide that you precisely follow the label directions. If you don’t do this then the herbicide may fail to control the target weed, damage the turf grass or even prevent you from being able to reseed or re-turf damaged areas.
Turf herbicides can be:
- Selective or non selective. Selective turf herbicides are able to specifically target the weed without damaging the desirable turf grass. Non selective turf herbicides kill everything.
- Pre emergent or post emergent. Pre emergent turf herbicides prevent weeds from growing and becoming and are used before weeds grow. Post emergent kill weeds after they have emerged.
- Work against broadleaf weeds or grass weeds.
References
Graduated from Newcastle University with an Hons Degree in Soil Science in 1988, Jerry then worked for the Sports Turf Research Institute (STRI) as a turf agronomist before emigrating to Australia in 1993.
He followed this by gaining a Grad Dip in Business Management from UTS. He has worked in a number of management roles for companies as diverse as Samsung Australia, Arthur Yates and Paton Fertilizers.
He has always had a strong affinity with the Australian sports turf industry and as a result he established Gilba Solutions as an independent sports turf consultancy in 1993. Jerry has written over 100 articles and two books on a wide range of topics such as Turf Pesticides and Nutrition which have been published in Australia and overseas.