Assessing herbicide performance
Herbicide failure can occur for reasons other than herbicide resistance, including application error, adverse environmental conditions and plant stress. Understanding how different herbicides work helps when assessing herbicide performance.
Factors to assess and improve herbicide performance
- The rate at which plants die after spraying depends on the herbicide and rate applied as well as the weather conditions following application.
- Assess the ‘claims’ made by the herbicide manufacturer. Some products registered for the control of weeds do not claim to kill the weed but rather, ‘suppress’ growth, reducing seed-set and competition against the crop.
Evaluate the likelihood of application error by asking:
- Has the target weed been accurately identified?
- What product was used and was it a correct choice for the target weed?
- Was the correct product rate used for the weed growth stages present?
- Were appropriate adjuvants used at the correct rates?
- Did the product reach the target? Certain herbicides may be intercepted and bound to other plant material (for example, stubble) or soil and thus not reach the target weed.
- Was the product measured accurately when making up the spray-tank mix?
- Was the quality of the water used satisfactory? The performance of some pesticides is affected by water quality characteristics such as hardness, pH, salinity and clay content.
- Was the water volume per hectare appropriate?
- Was the boomspray accurately calibrated?
- Were there equipment problems, for example, blocked nozzle, erratic pump performance?
- Were the correct nozzles, pressure settings, boom height and boom speed used to achieve the desired uniform coverage?
- Were label directions regarding environmental spray conditions observed?
- What else was added to the tank mix? Some pesticide mixtures can be mix together, but may be biologically incompatible. This can result in reduced weed control and/or increased crop damage. Performance may also be reduced if insufficient time has been left between separate applications of antagonistic products.
- Was the tank solution mixed properly and agitation adequate to keep it mixed?
Environmental conditions at time of spraying can influence herbicide performance. When assessing performance problems, records of the conditions at the time of spraying are important. Herbicide labels provide some guidance as to desired conditions or, alternatively, conditions to avoid when spraying weeds. Unfortunately, due to the nature of weather, the number of ‘ideal’ spray days during a season is limited. Important environmental conditions to record include:
- Time of day applied
- Presence of heavy dew
- Temperature at time of application and up to 10 days before or after application
- Clear skies versus heavy clouds or overcast conditions
- Rainfall events, for example, before application and before the rain-fast period of the post-emergent herbicide has elapsed. Heavy rain shortly after use of soil-applied herbicides can move them into the crop root zone, increasing crop damage
- Stressed weeds due to many factors, including, too dry or wet, frosts before or after application, poor nutrition, disease, insect attack and/or competition
- Soil pH affecting herbicide availability to weeds or the crop
- Whether the product leached or was otherwise destroyed so that uptake by target weed was limited.