Weed and Herbicide Considerations

Contributions from Dr Jack Desbiolles (Ashworth et al., 2010) and Baker et al., (2007)

Jack McHugh Jeff Esdaile

Tillage and weed germination

Residue effect on weeds

Herbicide in no till

Guidelines for pre emergent herbicide

Tillage and weed germination

Germination affected by:

Tillage, Climate & Soil type

Tillage effects microenvironment;

Light, O2, CO2 diffusion, Temp, N and Soil structure

– breaks down dormancy and stimulates germination.

Low soil disturbance disc seeding systems = a shallow weed

seed bank – affected by herbicides, low germination rates &

predation

Deep seed bank - long term seed viability, weed reinfestation

if brought to the surface.

Soil Facts – Saskatchewan soil conservation association 2001

Residue effect on weeds

Crop residue retention influences weed germination –

temperature, moisture, chemical interactions.

Affects growth of both crop and weeds – soil to seed contact, N tie up,

reduced soil temp, increased root diseases, release of phytotoxins

Effect on weed emergence is complex:

Type and quantity of residue

Allelopathy

Weed species

Seed depth

Soil type

Environmental conditions

Interaction with herbicides

The efficacy or pre emergent

herbicides improves under higher

disturbance disc seeding systems

due to increased incorporation

Herbicide in no till

Pre-emergent herbicides

were introduced to

combat resistance in post

emergence herbicides.

Trifluralin (treflan) is

incorporated

(photodecompostion) by

soil throw near the

surface. Other chemicals

can be toxic to crop seeds

at high rates

Furrow free of herbicide – treated soil is thrown

and mixed between furrows

Minimum disturbance, minimises mixing of soil

and weeds to depth - Small amount of soil

throw.

Efficacy controlled by positional selectivity – seed

& herbicide separation

Herbicide in no till

Performance is a function of amount and

pattern of soil throw

Leading fluted coulters are effective (dbl discs

drag herbicide into furrow)

Triple discs appear to perform the best (speed

and layout is a factor)

Hair pinning a significant issue + soft wet clay

Guidelines for pre emergent herbicide

Pre plant knockdown – emerging weeds

Optimise application rates – with separation 1.5-2 L/ha in cultivation

systems 0.8L/ha = less effective

Travel speed – Disturbance and throw affected by speed, depth, soil

texture and moisture – avoid throw to adjacent seeding rows –

monitor closely

Stubble – herbicide efficacy reduced by 40-50% if not on the soil

IWM – integrated weed management reduces weed burden and limits

evolution of resistance