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Understanding Herbicides

A short course for people using herbicides

The Understanding Herbicides course is designed primarily as a stand-alone course to give participants a good grounding in the principles of using herbicides to control weeds.  It has been designed to prepare participants for 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.

Upcoming course

  • The next course will be held during May and June 2023 with an online exam on 6 July. 

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.


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

Course outline

Topic 1 – Introduction

Explanation of various things about the course; introduce participants to various terminology about herbicides in case they know nothing; herbicide mode of action.

Topic 2 – Herbicide toxicity and legislation

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

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

Topic 4 – Behaviour of herbicides in plants

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

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

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

Topic 7 – Knockdown herbicides

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

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

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

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

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 and urban environments.

Topic 12 – Herbicide resistance

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.

Date and venue

The course is taught completely online and through use of material in a study guide that will be posted out in early May.  Recorded lectures will be watched when convenient for participants during May and June.  The exam will also be held online.


The cost in 2023 is $1500 + GST. 

Course co-ordinator

Assoc Prof Kerry C Harrington
School of Agriculture and Environment
Massey University
Private Bag 11-222
Palmerston North 4442

Phone:  (06) 951-7814


Full registrations for the 2023 course are now open.  Either email Kerry Harrington using the email address above or click on the registration button below if you wish to register.

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