Back Post emergence weed control and nutrient management for maize

Date: 21 October 2016

While weather plays a critical role in determining yields, growers should largely focus on factors they can control. Prior to planting focus is usually on, among other things, establishing a perfect seedbed to allow for good uniform plant establishment and the right population. Post emergence, a proper plan for disease, pest, weed and nutrient management is important to get the most out of these plants otherwise the gains from a previous good practice might quickly evaporate.

This article assumes that the best management practices have been followed up to planting and insect, pest and disease control have also been taken care of through selection of the right hybrid and relevant seed treatments. Focus is therefore on post weed and nutrient management.

A weed free paddock is the first step towards maximising yields. Weeds impact crop growth and yield primarily by competing with the crop for light, nutrients and water. In certain situations they even harbour undesirable pests.

Knowledge of the weed spectrum common in your paddock and/or location is critical to help select the best herbicide for the job as most chemicals are weed specific. A robust weed control option generally involves pre and post emergence strategies. Post emergence weed control is usually easier where a preemergence herbicide has been used as the latter helps increase the consistency of weed control and flexibility in the timing of post-emergence applications. While some growers may choose not to apply a post emergence herbicide if they have premerged their crop it is worth pausing and considering how quickly some weeds may grow and how many seeds they can produce.

A postemergence herbicide with some residual weed control up until the crop has canopied would help minimise weed growth and hence competition. Depending on environment and weed density post emergence control generally occurs 3 – 5 weeks after emergence. Where preemergence herbicides have not been applied, post applications should however be much sooner (if significant weed populations are present) because yield can still be impacted if weeds are present as early as within a week of crop emergence.

In addition to good weed control, a sound fertiliser program that considers the environmental impact yet provide sufficient nutrients at the right time must be in place. Adequate nutrients are needed throughout the season because deficiency at any stage could significantly affect yield. Even though N, P and K are the nutrients required in large amounts, others such as Cu, Zn, S, Mg etc are equally important and must be available in sufficient quantities because yield is determined by the most limiting factor. For instance, on average a maize crop would require approximately 250 kg N and less than 1 kg Zn. If Zn was insufficient, it wouldn’t matter if N was applied in the right amounts as maximum yield will be influenced by Zn availability. To allow for accurate assessment of the right nutrient and amount always consider 1) the soil test value and 2) the paddock yield potential. Nutrient application should match potential crop removal, considering what’s already in the soil.

Some growers may choose to apply all N by planting time. While this might save application costs always consider the potential risks associated with N loss for your particular situation and the effect to the environment. Nitrogen can be easily lost from the system through volatilisation, denitrification or leaching, potentially polluting waterways. Applying too much N too early when the crop does not need it increases chances of N sitting in the soil in a form it can be easily lost from the system. For instance, if soil saturation occurs after N fertiliser has been applied, a large amount of N could be potentially lost through leaching or denitrification. In general, the shorter the time interval between N application and crop uptake, the less likely the potential of N loss from the soil.

While some nutrients are used in larger quantities at the front end of the season less than 20 % and 70% of the total N requirement is been taken up by V6 or flowering, respectively. N should hence be split applied to minimise potential losses and environmental pollution.

Quite often people ask whether there is a benefit in applying N after flowering. There is no right or wrong answer because it depends on your situation. If the application is to correct N deficiency it is a no brainer because >30% of the total N requirement is still to be taken up. If the reason is to drip feed the crop for the sake of it consider the extra cost and balance that against the potential risk of losing N if it was applied earlier.

Even though lime has not been discussed in this article please note that for the plant to effectively utilise the available nutrients soil pH has to be around 6.3. Since most fertilisers tend to acidify the soil, a liming program is always important to maintain the correct pH.

Successful cropping is not necessarily achieving the highest possible yields but rather, practicing cost effective management strategies that are environmentally friendly. Nutrient application timing is critical because if too early, the nutrients may be lost before plant uptake and if too late, the crop may not need it, eventually ending up in waterways.

For more information, contact your local Pioneer representative


Dr Rowland Tsimba

Dr Rowland Tsimba

Agronomy Manager