Alternative methods to combat herbicide resistance

Herbicide resistance starts with the loss of one or two herbicides but rapidly escalates as the increasing use of a smaller pool of herbicide actives greatly increases the selection pressure for weeds to evolve resistance to the remaining active groups.

The loss of effective herbicidal weed control represents a major threat to food security, as weeds compete with agricultural crops for water, sunlight and nutrients. It can render crop production uneconomical because of its impact on yield and quality, and the higher costs of using non-chemical alternatives, such as tillage, to manage weeds.

In Australia, herbicide resistance and the subsequent loss of economic viability contributed to a substantial reduction in the number of Australian grain growers during the late 1990s and early 2000s, Dr Michael Walsh, professor of plant sciences at Charles Sturt University in Australia, said during a farmers’ day hosted by the Western Cape Department of Agriculture at Tygerhoek Research Farm near Riviersonderend in the Western Cape.

He pointed out that herbicide resistance was a more severe problem in Western Australia than in other parts of the Australian wheat belt, as farmers in this region were more focused on intensive crop production with reduced livestock.

In comparison, growers in the eastern wheat belt region typically have a mixed production focus with an integrated crop and livestock production system, which helped to slow herbicide resistance development.

According to herbicide resistance surveys that are conducted every five years, about 90% of the ryegrass populations in Western Australia have been herbicide resistant since 2010 onwards, with 95% of these populations showing resistance to more than one mode of action group.

About 70% of the ryegrass populations in South Australia, Queensland and New South Wales, and 46% in Tasmania, are herbicide resistant.

Weed seed control

Walsh said that herbicide resistance forced farmers in Western Australia to rethink their weed management strategies.

For one, it led to the development of harvest weed seed control (HWSC) systems, which include the use of chaff carts, narrow-windrow burning, direct baling, chaff-lining, chaff tramlining, and the use of impact mills.

Walsh said a solid understanding of the harvester set-up and operation is important for these systems to work properly: “HWSC systems have been developed to target the weed seed containing chaff fraction to prevent the return of weed seeds to the field and the soil seedbank during the harvesting process. With a proper harvester set-up, all the weed seed should exit the harvester in the chaff, and not the straw.”

Research by Walsh and his team showed that annual ryegrass, wild radish, brome grass and wild oat plants present in the crop at the start of wheat harvest had at least 75% of their total seed production retained at a height that ensured collection during harvest.

“If not managed properly, all this seed will be broadcasted back onto the land during the harvesting process, which will exacerbate problems,” he said.

Walsh’s studies also indicated that one ryegrass plant per square metre at harvest produced sufficient seed to plant a pasture at a seeding rate of 2kg/ha.

Grazing, baling and burning

The main difference between the HWSC systems is the way in which the chaff is treated. Narrow windrow burning involves the attachment of a chute to the back of the harvester that concentrates the straw and chaff fractions into narrow 50cm-wide windrows. These rows are burnt in autumn (March to April) to destroy all the weed seed.

While windrow burning is 100% efficient in destroying weed seeds, Walsh does not recommend it as a long-term option: “A cross wind is needed to get sufficient air into the windrow to fuel the fire and ensure the windrow is completely burnt down to the soil.

Climatic conditions need to be perfect for this management system to work and to ensure you don’t burn more than you planned for.”

He added that the straw fraction had to be included in the rows, as the fine size of the chaff prevented air movement and subsequently only smouldered when set alight.

Another drawback of narrow windrow burning and the concentration of straw and chaff material into narrow rows is that it results in the concentration of nutrients into these narrow rows.

“In Australia, this process was found to remove potassium from across the field and concentrate it into narrow strips. The accumulation of potassium in these strips resulted in improved growth in these areas in subsequent crops and clearly identified the lack of potassium in the unburnt areas,” Walsh said.

Chaff carts have been used to combat herbicide resistance in Western Australia since the 1980s. The carts were originally designed to collect chaff as feed for livestock in Canada, but were introduced to Australia as a means of targeting the weed seed bearing chaff fraction as an HWSC technique.

The collected chaff is strategically dumped into piles across the field for subsequent grazing, burning or baling.

Walsh pointed out that farmers were initially concerned that most of the seed would just move through the gut of the sheep and back onto the land, but research indicated that only 5% of annual ryegrass seed survived digestion.

With bale direct, the straw and the weed seed bearing chaff are baled and either fed to animals on the farm or sold as animal feed. While this is effective, it is not a highly favoured control method in Australia because of the limited market demand for feed bales, according to Walsh.

An unfavourable environment

The weed seed bearing chaff is concentrated into a 30cm wide windrow behind the harvester with chaff lining or onto the wheel tracks with chaff tramlining.

Walsh said that the chaff rows in both these systems were typically left undisturbed, as the chaff and compacted wheel tracts in the case of tramlining created an unfavourable growing environment for emerging weed seedlings.

As an added method to prevent seed germination, farmers might however send sheep into the land straight after the harvest.

Although not commonly practised, Walsh pointed out that the palatability of chaff material could be increased by spraying molasses onto the chaff lines.

In terms of efficiency, he said chaff lining and chaff tramlining were “messy”, as there typically was some emergence of weed seedlings in chaff lines, but ultimately about 95% effective in restricting weed seedling emergence across the whole field.

He pointed out that chaff was rarely moved by high winds: “Winds can be a problem in fields with little stubble, for instance where legumes were previously planted, but this is generally not a problem in fields with some stubble.”

Impact mills

The impact mill is the only HWSC system that fits in with conservation farming principles, as it returns and spreads all the harvest residues to the field during harvest.

This system has been around for 12 to 13 years and was originally developed as units that were towed behind harvesters, such as the Harrington Seed Destructor, and had their own motors along with chaff and straw transfer systems. Newer impact mill systems are incorporated into the rear of the harvester and are powered by it.

Walsh said a study evaluating all the harvest seed control systems found them equally effective. On average, each resulted in a 60% reduction in ryegrass emergence, compared with a 90% reduction in low-density and 30% reduction in high-density populations.

“Efficiencies declined with increasing moisture levels and higher chaff amounts, and there are concerns around the power requirements of the mill systems and the subsequent loss of harvester capacity,” he said.

The initial set-up costs vary greatly between the different HWSC systems, from a few hundred Australian dollars for something like chaff lining and narrow windrowing, to several thousands of dollars for an impact mill.

Walsh pointed out that farmers could use a calculator on the Australian Herbicide Resistance Initiative website (ahri.uwa.edu.au/podcast/harvest-weed-seed-control-model-explained/) to calculate the cost of using each of these systems over time.

According to calculations, looking at the purchase price, running costs, depreciation and nutrient replacement costs, narrow windrow burning and baling work out the most expensive at AU$29,90/ha (R373,13/ha at the time of writing) for windrow burning and roughly AU$64,88/ha (R759,73/ha) for bale direct.

The cost of the impact mills, on the other hand, ranged from AU$13,47/ha (R168,09/ ha) to AU$15,03/ha (R187,56/ha).

High residue cropping systems

To increase the amount of residue retained in crop production systems, farmers use harvesters with stripper fronts to leave cereal stubbles upright and intact, which when flattened during subsequent seeding helps to create a thick mulch that supresses weed growth.

Other benefits of the mulches include the addition of organic matter to the soil, improved water retention, and the buffering of the soil against extreme temperatures.

“Stripper fronts when combined with disc seeding systems allow the retention and maintenance of higher crop residues in the production system with the overall effect being a more resilient production system,” Walsh said.

Stripper fronts also improve harvest efficiency by only collecting the grain containing heads of cereal crops, thereby eliminating the need to process straw that is normally collected when harvesting with a draper front.

Walsh said that their research had found stripper fronts as effective as draper style harvester fronts in collecting annual ryegrass seed during harvest, but the efficiency of both these systems was influenced by header speed, height, setting, crop variety, row spacing, climatic conditions and the amount of seed shedding that occurred before harvest.

Stripper fronts, nevertheless, were unsuited to windrow burning and direct baling, and diminished the suppressive impact of chaff lining on weed seed emergence because of the reduced amount of chaff material produced during harvest.

Moving forward

Once weed populations have been reduced to low levels, for instance less than one weed per 10m2, farmers can switch from applying herbicides to the whole field to targeted application systems, according to Walsh.

The success of this switch is reliant on accurate weed recognition. Currently, the main systems used for targeted spot-spraying in fallow situations in Australia are WeedIT and WeedSeeker.

WeedIT and WeedSeeker use reflectance sensors to detect green plants that are weeds in fallows, which then triggers a spray nozzle to specifically target that weed with a herbicide treatment.

A non-chemical alternative to this approach is Weed Chipper, where the reflectance sensors trigger rapid response tines, which behave like mechanical hoes to remove weeds.

An even more advanced system is the SwarmFarm robot, which is fitted with WeedIT technology for the spot spraying of weed in fallow lands. Instead of doing two to three quick passes through fields, this autonomous system does six to eight slow passes a year with the aim of targeting smaller weeds.

Timing

Walsh said that many new software and hardware technologies had emerged over the past decade towards improving the efficiency of weed control techniques and technologies. Farmers should sift through what is available and identify which of these technologies will work best under their unique conditions.

To this end, he identified two critical periods of weed control that should be the focus for weed management planning: early post emergence, and late post emergence.
Early post emergence requires the eradication of weeds that can be readily targeted due to an open crop canopy over a large area in a short time, on average 3 000ha in three to four weeks.

Because of the time sensitivity of weed control during this stage, Walsh identified a combination of blanket and spot spraying as the most effective solution for site-specific weed control during this period.

“You need the capability to cover approximately 50ha per hour to spray 3 000ha in four weeks. This means that you would have to apply the herbicide at 15km/h when using a 36m boom sprayer,” he said.

Late post emergence, however, requires the eradication of weeds where there is likely to be a closed crop canopy, across a large area, but within a longer time frame of three to four months. This makes it ideal for the use of autonomous technologies.

“The solution would need the capability to recognise and target weeds but at a much slower speed of approximately 1,5ha per hour to cover 3 000ha in 12 weeks,” he said.

Besides tillage and spot spraying, these technologies might use flaming, stamping, electrical weeding, laser weeding, air blast, microwaves, waterjet cutting, heating, abrasive grit, or solar irradiation to destroy the weeds.

Walsh identified laser weeding, waterjet cutting and abrasive grit as the most promising new weed control technologies for development as precision weed targeting technologies during the later post-mergence stage.

Strategic tillage

Walsh pointed out that 90% of farmers in Western Australia were currently using conservation farming methods, which included the practices of minimum to no-tillage, and stubble retention.

Many of these farmers have introduced a strategic tillage to address issues that may arise with long-term use of conservation cropping practices, such as nutrient stratification, physical and chemical subsoil issues, non-wetting soil, and shallow weed seed banks.

He pointed out that soil inversion during mouldboard ploughing had been found to reduce weed seed emergence by more than 90% and to increase wheat production by up to 20% consistently for several years after this tillage treatment.

Walsh warned that weed seed control, or any other alternative methods, were not standalone solutions to weed problems: “Weeds like annual ryegrass can adapt to any kind of intervention, even burning and impact mills. The key to successful weed management, therefore, is to use a variety of management practices and to destroy all weeds to prevent those that have developed resistance to survive.”

Email Dr Michael Walsh at michwalsh@csu.edu.au.

Source: Farmersweekly.co.za