Canola top dressing timing, maize herbicide options and seedbed preparation tips – our article this week is on a roll..

This week we answer following key questions: when is the optimum time to apply topdressing fertilizer to canola crops? Should we be rolling seedbeds to improve Fall Armyworm control? And a reminder of herbicide pro’s and con’s for maize crops…

Canola Top Dressing Timing

On farm trials and field experiments often throw up unintended lessons. And so it is with one of our latest collaborations between Crop Nutrition Laboratory Services Ltd. (CropNuts) and the Agventure Centre of Excellence for Crop Rotation.

Initially we set out to look at the optimum Nitrogen and Sulphur dose rates for canola crops; lots of work has been done in other countries but I have always felt that not only do our crops grow extremely quickly which can influence the efficiency with which Nitrogen is taken up by the plant, and behaviour of Sulphur in some soils can be difficult to predict and has an important interaction with Nitrogen.

We have collected valuable information on fertilizer responses on soils in both Nakuru and Timau from 9 different combinations of Nitrogen and Sulphur, but we also applied one of the applications several weeks later to look at the effect of timing.

As I have pointed out before, the European approach is to apply early in the crops life with the bulk of fertilizer applied by the 3 leaf stage. Here we applied the treatments at 4-5 leaves, except the late comparison which was applied at mid stem extension.

Effect of topdressing timing on canola yield

It is important to understand the practical implications of this research;

  • We can confidently apply top dressing (Urea especially) late on for optimum yields.
  • With a late planted crop or when the rains have finished early, we might still want to apply early so that we have fertilizer in the crop – the penalty for getting this wrong is far greater than the penalty for applying the fertilizer too early!
  • For large scale farmers during busy workload periods, we can confidently spread applications of fertilizer from rosette stage to green bud stage without concern.
  • Read the spreader manufacturers handbook when spreading in tall crops to ensure the spread pattern is correct. Some smaller Kuhn spreaders require the toplink to be shortened to throw the fertilizer above the crop, older Amazone spreaders (below) have vane-tips that need to be pointed upwards.

Amazone Spreader

Maize Herbicide Options – Some General Pointers

A reminder of the strengths and weaknesses of the different herbicides and how they fit into your own weed spectrum. Other products are available, and remember to always consult a professional agronomist before using any of the products below. Applied incorrectly or to inbreed male line of seed crops, some chemicals can cause serious crop damage.

Active ingredient Latest timing of application Strengths Weaknesses
Atrazine 7-8 leaf Wild Radish, Amaranthus, Blackjack, useful suppression of  Commelina. Mexican Marigold, Gallant Soldier. Variable on Eragrostis and persistent in the soil so limits rotational crop options.
Sencor Pre emergence Setaria, Eragrostis, Brome and volunteer wheat. Some activity on Devil Thorn Amaranthus can be variable. No activity on Black Nightshade or Cleavers.
S-metolachlor Early post emergence Good on a range of grasses, Commelina, some Watergrass activity. Poor persistence in dry conditions. Variable Amaranthus control
Acetochlor 6 leaf stage Safe in heavy rainfall – binds to soil and does not leach to seed layer.

Grass weeds, Blackjack, Marigold, Fathen, Commelina

Devil Thorn and Gallant Soldier
Saflufenacil + dimethanamid-P Pre emergence Blackjack, Amaranthus, Datura species are normally well controlled Needs bare soil – struggles with surface residue. Narrow spectrum of grass weeds
Thiencarbazone + isoxaflutole 3 leaf stage Amaranthus, and a wide range of grasses. Very weak on Commelina and Eragrostis. Poor persistence on Setaria.
Pendimethalin 4 leaf stage Good on Setaria, Chickweed and Erogrostis. Amaranthus and Gallant Soldier. Needs moisture. Can leach into seed layer in heavy rain.
Mesotrione + S-metolachlor + Terbuthylazine 7 leaf stage Very broad spectrum and flexible timing. Devil Thorn and established grass weeds are not well controlled.

Seedbeds – To Roll Or Not To Roll?

Seedbed preparation

This question often arises in respect of cloddy seedbeds where we want to ensure crop germination is good. But there are more reasons too which might surprise you….


  • An extra pass, more wheelings and compaction
  • Leaves soil surface finer and more prone to erosion
  • Some clods can help protect vulnerable seedlings from wind erosion


  • Level seedbed improves residual herbicide activity
  • Improved seed to soil contact means more even and quicker germination
  • Seals in moisture and firms the ground which can draw up moisture from depth by capillary action.
  • Allows higher planting speeds and less coulter bounce
  • A level seedbed is kinder on the sprayer during the growing season – important when lots of passes for Fall Armyworm are required. Less boom bounce equals better, more even application and control which for me is the most important aspect of good spraying.

Finally, a big thank you to the Agventure Center of Excellence for Crop Rotation and their assistance with our trials:

Read Part 1 and Part 2 of this series by clicking the links provided


Farming for the future requires a change of approach. Monoculture, soil degradation and climate change and soil degradation are threats to the future of how we feed the planet.Agventure Ltd set up the Center of Excellence for Crop Rotation to help farmers diversify cropping systems and introduce techniques which have a long-term outlook to improve soil health.

The Center of Excellence for Crop Rotation works extensively in partnership with Crop Nutrition Laboratory Services Ltd. (CropNuts), for trials and research.

Till next time,



About David

David Jones is the Broad Acre Specialist at Crop Nutrition Laboratory Services Ltd. (CROPNUTS). David has a keen interest in soils and no till farming systems where he has undertaken work looking into weed levels and changes in soil structure, and has extensive experience in field trials and in the development of precision farming techniques. In his spare time he enjoys playing rugby.