Understanding Herbicides

This course will give participants a good grounding in the principles of using herbicides to control weeds.

Key facts

Start date: 1 May 2024

Cost: The cost is $1500 + GST.

Delivery mode: Online


This course is designed primarily as a stand-alone course to give participants a good grounding in the principles of using herbicides to control weeds.

Person spraying thistles

Who this course is for

People working in the herbicide industry within New Zealand. But is also useful for people working in agriculture, horticulture and regional councils who need to control weeds with herbicides as part of their job.

What you'll get

Successful participants will receive an official certificate of completion and the material will count as 5 credits at the 300 level towards a university qualification.

Learning outcomes

On successful completion a participant should be able to understand herbicides well enough to be able to select the most appropriate compound for any weed situation and obtain maximum effectiveness from that herbicide without causing damage to desirable plants.

Course content and structure

This course is online and uses course material and recorded lectures. Participants study at a time convenient for them between May and July, and an online, open-book exam is in July.

Topic 1 – Introduction

This topic will cover:

  • an explanation of various things about the course
  • introduce participants to various terminology about herbicides
  • herbicide mode of action.

Topic 2 – Herbicide toxicity and legislation

This topic will cover:

  • safety testing of herbicides
  • relative toxicity of herbicides
  • New Zealand legislation
  • withholding periods
  • the GROWSAFE scheme
  • the 2,4,5-T and glyphosate controversies
  • herbicide residues in food.

Topic 3 – Herbicide formulation

This topic will cover:

  • solutions
  • emulsions
  • wettable powders
  • granules
  • dusts
  • surfactants
  • chemical drift through the air
  • common and trade names
  • calculations.

Topic 4 – Behaviour of herbicides in plants

This topic will cover:

  • interception and uptake of foliage-applied herbicides
  • availability and uptake of soil-applied herbicides
  • transport of herbicides in the plant
  • factors affecting these processes
  • herbicide selectivity.

Topic 5 – Behaviour of herbicides in the soil

This topic will cover:

  • adsorption
  • volatilization
  • leaching
  • photochemical degradation
  • chemical decomposition
  • microbial decomposition
  • rates of degradation of herbicides
  • bioassays
  • activity of soil-applied herbicides
  • selectivity of soil-applied herbicides
  • effects of weather on herbicides.

Topic 6 – Herbicide application

This topic will cover:

  • Conventional spraying equipment
  • calibration of a sprayer
  • back-pack sprayers
  • problems with conventional sprayers
  • improvements in spraying equipment
  • wiper applicators.

Topic 7 – Knockdown herbicides

This topic will cover:

  • characteristics of importance in decision making
  • broad-spectrum herbicides
  • translocated knockdown of dicots in grass crops
  • contact knockdown of dicots in grass crops
  • selective knockdown of grasses in dicot crops
  • sulfonylureas
  • other knockdown herbicides.

Topic 8 – Residual herbicides

This topic will cover:

  • Residual herbicides used primarily for non-selective control
  • herbicides used primarily for depth protection
  • selective residual herbicides which primarily control dicots
  • selective residual herbicides which primarily control grasses.

Topic 9 – Economics of weed control

This topic will cover:

  • when weeds reduce yields through competition
  • when weeds exert indirect effects in pasture
  • when weeds cause other effects difficult to measure
  • how most weed control decisions are made.

Topic 10 – Weed control in pastures and lucerne

This topic will cover:

  • problems caused by weeds
  • keeping pastures competitive
  • discouraging weeds that establish
  • weed control in new pastures
  • weed control in established pastures
  • control of specific problem weeds
  • clearing scrub weeds
  • weed control in riparian plantings
  • weed control in lucerne using management
  • weed control in lucerne using herbicides.

Topic 11 – Weed control in annual crops or controlling environmental weeds, forestry weeds and urban weeds

This topic will cover:

  • Obtaining weed control information
  • cereals
  • maize and sweet-corn
  • forage brassicas
  • fodder beet
  • peas.

An alternative module will cover control of weeds in:

  • forestry
  • natural environments
  • conservation estates
  • aquatic environments
  • urban environments.

Topic 12 – Herbicide resistance

This topic will cover:

  • sexual vs vegetative reproduction
  • advantage of genetic variability
  • development of ecotypes
  • development of herbicide resistance
  • rate of herbicide resistance development
  • using herbicide resistance
  • examples of herbicide resistance problems in New Zealand.

Meet your facilitator

Adjunct Associate Professor Kerry Harrington

Adjunct Associate Professor Kerry Harrington

PhD, MAgrSc(Hons), BAgrSc(Hons)
Associate Professor in Weed Science
School of Agriculture and Environment
College of Sciences

Until his retirement in late 2023, Kerry was the weed science specialist at Massey University. As well as teaching aspects of weed biology, identification and control, he supervised postgraduate students and undertook his own research.


Registrations for the 2024 course are open, and close on 20 April. To register, please email Kerry Harrington.


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