Weed and Herbicide Considerations Tillage and weed germinationResidue effect on weeds Herbicide in no till Guidelines for pre emergent herbicide Jack McHugh Jeff Esdaile Contributions from Dr Jack Desbiolles (Ashworth et al., 2010) and Baker et al., (2007)
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 bothcrop 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
Herbicide in no till 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 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