*Note: In Australia we call the whole machine a harvester, not just the cutting front.
At harvest time, many weeds that have grown in the crop still have seed held in the seed head. These seeds enter the harvester along with the grain and most exit the harvester and are spread across the paddock in the chaff and straw.
Collecting these weed seeds at harvest and either destroying them or depositing them in a known location where they can be monitored and controlled later, is an excellent way to stop weeds in their tracks.
Brome grass is the most costly weed for Mallee farmers to manage, even though herbicide resistance in brome grass is currently low in the region.
If you are considering adding harvest weed seed control (HWSC) to your weed control program there are excellent resources on the WeedSmart website to help guide you through the initial decisions and the implementation of this important weed control tool.
Decide on which system fits your farm best.
Get maximum weed seed into the header.
Know how to manage the collected weed seed.
Which system is best?
HWSC is being rapidly adopted in Australia and other countries around the world. There are six systems currently being used on Australian farms and they have all been developed by farmers. Research has demonstrated that all are very effective weed control tactics, achieving over 80 per cent control and for some nearly 100 per cent.
There are six systems currently used to collect and manage weed seed at harvest:
chaff decks (chaff tramlining)
narrow windrow burning
While they are all effective, they vary considerably in capital and ownership cost, nutrient removal costs, operational costs and labour costs. Some HWSC tactics involve the purchase of substantial machinery – such as an impact mill, chaff cart or chaff deck – but the operational and labour costs might be lower than methods such as narrow windrow burning, which involves low set-up costs but higher nutrient losses and labour costs associated with burning. Invariably narrow windrow burning is the most expensive option in the long-run due to the high nutrient removal cost.
The HWSC tools all involve some modification to the harvester. The simplest modification is for chaff lining and narrow windrow burning, where a simple chute is attached to the rear of the harvester to direct the residue into a band on the ground, running the same direction as the harvester has travelled. These chutes are often constructed and fitted on-farm.
All the other systems are commercial modifications that are fitted to the harvester – chaff decks and impact mills – or trail behind the harvester – chaff cart and Bale Direct.
Harvest weed seed control only works on weed seed that enters the header. Getting the weed seed into the header relies on the seed being held in the seed head at the time of harvest. The seed head must also be at harvestable height.
Consider the weed spectrum and the likelihood of seed capture. Even if some seed has shed, chances are there will be other seed heads that have not yet shed and even this will assist with reducing the amount of seed entering the seed bank.
There are four chaff-only systems and two all-residue systems.
The chaff-only systems – chaff carts, chaff lining, chaff decks and impact mills – require the harvester to be set up to separate chaff and straw, and to keep the weed seed in the chaff stream. This may require modifications to the harvester rotor and sieves and the installation of a baffle to keep the weed seed in the chaff stream.
If you choose the Bale Direct system or narrow windrow burning, all the straw and chaff ends up in the same place, so no other modification to the harvester is needed.
If you choose an impact mill as your HWSC tool then the tactic is completed in one pass at harvest, with nothing extra to do. All the residue is spread in the field and the weed seeds are rendered unviable.
All the other HWSC tools involve some action after harvest to remove or destroy the weed seed collected at harvest.
Chaff decks deposit the weed seed-laden chaff in one or both harvester tramlines or wheeltracks. Some growers find that the chaff rots and the weed seeds die, but in other environments growers find that it is necessary to control weeds that germinate in the tramlines using herbicide or non-herbicide tactics applied just to the tramlines.
Chaff carts can be emptied as they fill in the paddock or emptied at a central point. Many growers use chaff piles as a high nutrient value stockfeed, others burn the piles and others leave them unburned in the paddock and sow through them the following season.
Chaff lines are usually left unmanaged with the expectation that the following crop will provide adequate competition to the weeds to minimise weed growth and seed production.
The Bale Direct system results in large bales of crop residue that can be sold into suitable markets. Distance to market is usually an important factor in the success of this system for HWSC.
Narrow windrow burning uses fire to destroy the weed seed in the Autumn following harvest. There are significant labour costs and safety risks to consider along with the loss nutrients and ground cover.