What factors effect how well herbicides work?

2023 What influences herbicide performance?

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Adsorption: Some of the herbicide gets adsorbed onto the soil, making it less mobile.

Degradation: Over time, natural processes like photodegradation and, moisture, breaks down the herbicide.

Soil pH Temperature Rainfall Moisture Humidity

For a herbicide to work it needs three things: Good contact

Length of contact

Site of action

Poor contact will give poor results.

If it isn’t in contact for long. Poor results.

Must be able to move to where it actually works.

Hairy and waxy leaves can impact on herbicides

Rain after application

Actively growing

Challenges deep-dive Climatic Factors

Soil Factors

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CEC Organic matter Texture pH

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Humidity Temperature Amount of light Moisture

Soil Texture ● Most important for soil applied herbicides ● High levels of clay and OM > adsorption

CEC ● soils ability to adsorb positively charged compounds ● fine-textured, high-organic matter soils have larger CEC’s than coarse, low-organic matter soils

Injury symptoms caused by the residual effect of oxyfluorfen on sorghum. Injury symptoms caused by the residual effect of oxyfluorfen on sorghum at 0, 15, 30, 45, 60, 90, 120, 150, 180, and 200 days after application (DAA) of the herbicide in comparison with the control treatment (C), in soils in the region of Curitibanos-SC: Ultisol (clay), Rio Paranaíba-MG: Oxisol (clay), and Oratórios-MG, Brazil: Inceptisol (loamy sand).

pH ● influences water solubility, adsorption, and hydrolysis ●

triazines

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sulfonylureas

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Imidazolinones

● glyphosate

Temperature ● Couch grown at high temperatures retains 3x more glyphosate than at low temperatures. ● In general, uptake and translocation of foliar herbicides increases with increasing temperature

Active

Water Solubility (mg/L)

Soil Adsorption Capacity Koc

Soil Movement/ Leaching Rating

vary dramatically in how they

S-metolachlor

530

100

High

behave after rainfall

Pendimethalin

0.275

25455

Very Low

Prodiamine

0.013

13000

Very Low

Propyzamide

15

200

High

Rainfall ● Pre-emergent herbicides can

Soil moisture ● Crowsfoot control increases with increasing soil moisture ● Best at > 20% VMC and lowest at < 12% VMC. ● Diclofop activity reduced at low soil moisture

Humidity ● RH effects glufosinate ammonium ● Best results under high RH ● Use an adjuvant in low RH conditions. ● Potential for spray freight increases

Light ● A 44% loss in 3 days due to photodegradation of some pre emergent herbicides, when used on bare soil and not watered in. ● In a turf setting, 10% loss was within 7 days when not watered into the turf.

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