WORDS & IMAGES PROVIDED BY FAR
A technique which significantly reduces the spread of weed seeds during harvest is being demonstrated by the Foundation for Arable Research in a bid to generate a conversation around whether it is time it was adopted in New Zealand.
Chaff decks and other similar technology, attached to the back of a combine harvester to reduce the weed seed burden, are widely used in Australia.
FAR technology manager Chris Smith says FAR wants to kick-start the conversation. “This is to help New Zealand arable growers better understand what it’s about, and the pros and cons, to help them decide if it’s the right fit for them.”
Known as Harvest Weed Seed Control (HWSC), the techniques are all based around keeping the chaff separate from the straw during combine harvesting. This is because nearly all the weed seed is in the chaff, which is of much smaller volume than straw.
Chaff can be spread up to the width of the combine front (9-10 metres in some cases) during harvest, so technology which limits its spread, or crushes seeds, can substantially reduce the amount returning to the weed seed bank in the soil for following crops.
FAR senior field researcher Charles Merfield (Merf) says HWSC is proving a very valuable technique, now used by more than two-thirds of Western Australian growers. Australian research shows it is highly effective, with a 95% reduction in annual ryegrass plants in crop in three years.
“However, it has not made its way across the Tasman, despite herbicide resistance becoming an increasing risk, particularly in ryegrass, which is also the main problem in Australia.
“In arable systems, a large proportion of the weed seed does flow through the combine. So, by stopping that, we can actually achieve considerable amounts of weed control without using herbicides.”
There are six HWSC techniques: narrow windrow burning, chaff lining, chaff carts, bale direct, impact mills and chaff tramlining. All have pros and cons regarding: capital cost, mechanical complexity, running costs, maintenance, power required from the combine and nutrient removal from the paddock.
Merf says a misconception exists that many HWSC techniques won’t work in New Zealand, because of our much larger tonnages a hectare compared with Western Australia, where 2-3 tonne/ha is regarded as a good yield. “It was believed that the likes of impact mills won’t cope with the volumes of chaff flowing through our headers.
“However, it is not the tonnages/ha that is the correct measure, it is the tonnes per hour going through the header. As Australian growers have bigger combines and faster driving speeds of up to 15 km/hr they can harvest up to 70 tonnes of grain an hour. So, there is actually less chaff, straw and grain per hour running through most of our headers than in Australia, so all their systems will work here, likely with greater headroom.”
To demonstrate chaff tramlining, FAR purchased a pair of chaff decks from Australia and fitted these to a New Holland twin-rotor combine in Canterbury during the 2025 harvest to gauge its performance under New Zealand conditions. The material comes off the sieves and instead of spreading the chaff, it puts it in two narrow rows behind the wheels. This concentrates weed seeds into tramlines for easy follow-up, if needed.
“So far, they have been coping very easily with the amounts of chaff being produced and putting it in the wheelings with no problems.” Seed germination in the tramlines and its management will be monitored, particularly as the farm is on controlled traffic.
Ideally the chaff deck is used all the time during harvest, with different kits for different makes and models.
This is a much cheaper option than an impact mill, probably the most well known of HWSC techniques. All originating in Australia, these include the Seed Terminator and Harrington Seed Destructor. Impact mills grind up the chaff and weed seeds, killing them. Impact mills are integrated into the back of a combine, with manufacturers increasingly suppling them as an option at purchase. However, impact mills are complex and expensive, costing at least $120,000 and requiring a lot of power, up to 100hp, from the combine.