An excellent way to stop weeds in their tracks is to collect these weed seeds at harvest and either destroy them or deposit them in a known location where they can be monitored and controlled later. Soybean, wheat, and other crops harvested with a grain header are ideal choices for harvest weed seed control (HWSC). Other crops such as cotton and corn need further equipment development to make HWSC a viable option.
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 been initially developed by farmers.
What is Harvest Weed Seed Control?
At harvest time, weeds that have escaped season long management often have mature seed still attached to the parent plants. These weed seeds can enter the combine along with the cash crop and exit the back of the combine in the chaff portion (small plant pieces and weed seeds), separate from be spread across the field as well as from one field to another. It seems a waste to spend all year spraying weeds with expensive herbicides only to reward the survivors at harvest by spreading their weed seeds out for next year.
Research has demonstrated that all are very effective weed seedbank management tactics for a range of weed species, achieving over 80 percent control and for some nearly 100 percent.
Narrow windrow burning collects all of the crop chaff and straw residue, and funnels it into narrow rows in the field. These rows are burnt to destroy the weed seed. This method is effective but removes all of the crop residue from the field.
Destroying weed seeds with combine modifications, therefore reducing the return of seeds to the soil weed seedbank, is an effective way of reducing herbicide selection pressure and the resultant evolution of herbicide resistant weeds. But as they say, there is no free lunch. Early versions of chaff mills cost approximately $120,000, but people estimate that with mass production the cost could be reduced by at least 50%. In addition to the cost of the equipment, there are several factors that pose challenges to this technology. Current versions require approximately 80 to 100 horsepower, this power drain can result in a 12-20% reduction in combine capacity (Anonymous 2018). The increased power requirement associated with weed seed destruction results in an average increase in fuel consumption of 0.4 gallon per ton of grain.
Identifying weak links in a weed’s life cycle
Harvest weed seed control (HWSC)
Preventing seed produced by weed escapes from entering the seed bank is an effective approach to weed management. The first mechanized combines often were equipped with weed seed collectors to prevent weed seed from being returned to fields. Walking beans was a rite of passage for generations of Iowa farm kids. However, with the introduction of modern herbicides and increases in farm size, these tactics have fallen out of favor. HWSC is one of the few alternatives to herbicides that can be incorporated within Iowa’s production system without significantly increasing labor requirements.
Schwartz-Lazaro, L.M., J.K. Norsworthy, Walsh, M.J., and Bagavathinnan, M.V. 2017. Efficacy of the Integrated Harrington Seed Destructor on Weeds of Soybean and Rice Production Systems in the Southern United States. Crop Sci. 57:2812-2818.
The interest in HWSC has been driven by Australia’s struggle with herbicide resistant weeds. Western Australia is recognized as the herbicide resistant weed capital of the world due to the widespread occurrence of multiple resistant weeds. The loss of effective herbicides for several important weed species, especially annual ryegrass (Lolium rigidum) in wheat production, has forced the development of innovative approaches to managing weed seed before they enter the seedbank. A survey of Australian farmers in 2014 found that 82% of respondents expected to use some form of HWSC within 5 years (Walsh et al. 2017).
The HWSC methods commonly used were windrow burning, chaff carts, and chaff lining. Windrow burning involves placing all of the stubble into windrows at harvest and burning it. Having concentrated lines increases the temperature at which it burns, thus killing a higher percentage of seeds. Chaff carts are towed behind the header and collect the chaff directly from the header, which is dumped later to be burnt or fed to livestock. Chaff lining simply involves dumping the chaff onto the wheel tracks behind the header, which is a low-cost but still a relatively effective method.
Although a wide variety of strategies have shown success, once ryegrass numbers were brought low, farmers including Harvest Weed Seed Control in their regime have consistently kept weed numbers close to zero, while those without have experienced more volatility.
Some examples of IWM strategies are the double knockdown, which involves applying two generalist herbicides (usually glyphosate and paraquat) in quick succession before sowing; the rotation of herbicide groups to slow down the build-up of herbicide resistance; and the mixing of herbicides in a single application to further prevent resistance. Other strategies include crop-topping, crop rotation, chemical fallows, grazing management, mouldboard ploughing, and crop competition. Harvest weed seed control (HWSC) is another tool in the IWM toolbox, which has been used with great success by a number of growers in the Geraldton region.
This survey has allowed growers to see the level of control which can be gained by incorporating HWSC into their practice, and how a wide variety of weed management strategies combined can give growers the confidence that they can continue to successfully control weed numbers and to continue their cropping rotation long into the future.
As a way to combat this resistance, growers were experimenting with new and innovative ways to control numbers and creating a more comprehensive management approach than herbicides alone. This comprehensive management approach is known as integrated weed management (IWM), and has grown widely in popularity since then, due to its success at keeping weed numbers low and preventing further development of resistance.