Summary of key weed research activities in the northern grains region

Take home messages

• High amounts of chaff are required for suppression of weed emergence in chaff lines.
• ‘Weed chipper’ is a targeted tillage system for fallow weed control.
• Site-specific weed control creates the opportunity to use alternative physical weed control technologies.

Background

Chaff lining and chaff tramlining

With many annual weeds in Australian cropping systems retaining their seed at maturity, harvest weed seed control (HWSC) is an alternative approach to weed control that is now widely adopted by Australian grain growers (Walsh et al. 2017). Several similarly effective HWSC systems have been developed to suit a range of crop production practices (Walsh et al. 2013). Chaff lining and chaff tramlining have recently become very popular with growers due to the low cost and simplistic approach of these systems to targeting weed seeds at harvest. These techniques concentrate the chaff fraction into very narrow windrows (<30cm) directly behind the harvester or onto dedicated wheel tracks. The chaff environment is sub-optimal for seedling establishment and these practices can be as effective as other forms of HWSC (Broster et al. 2018).

Targeted tillage

Tillage can provide highly effective weed control, but when used conventionally (i.e. 100% soil disturbance), it cannot be routinely used in conservation cropping systems due to the impact on soil health. The development of weed detection technologies creates the opportunity to incorporate strategic tillage operations for targeting individual weeds or weed patches in fallow situations. The ’weed chipper’ is a targeted tillage system that incorporates weed detection with rapid response tynes for site-specific fallow weed control.

Alternative weed control technologies

Physical and thermal weed control techniques were in use well before herbicides were introduced and the development of new options has continued throughout the herbicide era. Most of these technologies have not been developed, primarily due to cost, speed of operation and fit within farming systems. The introduction of weed detection and actuation technologies creates the opportunity to target individual weeds. This greatly increases the cost-effectiveness of many directional alternative weed control techniques. The challenge now is to identify which of these research and development activities should focus on.

Materials and methods

Chaff lining and chaff tramlining

A series of pot experiments investigated the influence of increasing amounts of wheat, barley, canola and lupin chaff on the emergence of annual ryegrass. In trials at three locations — Toowoomba, Wagga Wagga and Narrabri, eight rates of chaff were spread over a known number of annual ryegrass seed (100 or 200) on the surface of potting mix filled trays or pots. Chaff amounts used (0, 3, 6, 12, 18, 24, 30, and 42t/ha) were calculated as amounts equivalent to those concentrated in a 30cm wide row during the harvest of 0, 0.25, 0.5, 1, 1.5, 2, 2.5 and 3.5t/ha yielding wheat crops using a 12m front. Once the chaff was evenly spread across the soil surface, the pots were watered thoroughly and maintained at or near field capacity for 28 days.

Chaff amount = 0.3 x grain yield (t/ha) x (harvester width (m)/tramline width (m)).

Note: Assuming chaff yield is 30% of grain yield.

Differences between chaff types and rates on annual ryegrass emergence over a 28 day period.

Targeted tillage

A rapid response tyne system has been developed with the operational specifications of being able to specifically cultivate targeted weeds when present in a field at densities of up to 1.0 plant/10m2 at an operational speed of 10km/h. The rapid response tyne was based on the retrofit of a Shearer Trashworker tyne with a hydraulic breakout system. The hydraulic breakout system is typical of many other manufacturers thus permitting a design approach which could be adapted to accommodate other arrangements. The design focused the engineering on minimising the number of additional components and keeping the design simple whilst achieving the chipping action similar to a conventional hoe in well under half a second. A modular approach to the design was taken so as to permit the system to be readily scaled.

Field testing using two prototype rigs at Narrabri and Toowoomba was conducted on a range of problematic fallow weed species. The targeted tillage system was evaluated in a series of field trials for efficacy on weeds of winter fallows (annual ryegrass, wild oats, sowthistle and wild turnip) and summer fallows (barnyard grass, feathertop Rhodes grass, fleabane and sowthistle). The efficacy of the response tyne on the targeted weeds species across a range of growth stages was also investigated.

Comparison of weed control technologies

There is a diverse array of alternative physical and thermal weed control technologies with a proven ability to control weeds. The majority of these have not been commercialised and evidence of their efficacy is from a range of strictly controlled laboratory trials or studies, making cost-effectiveness comparisons difficult. While inputs and control methods differ significantly between physical control options, all systems share an energy requirement value for activation and use. Therefore, the energy required for effective weed control can be a reasonably accurate approach to comparing the efficiency and efficacy of physical control systems on an energy consumed per weed or hectare basis.

The direct energy requirements for the control of two-leaf weed seedlings were estimated from published reports on the weed control efficacy of a comprehensive range of physical weed control techniques (Table 1). To determine the energy requirement per unit area, a weed density of 5.0 plants/m2 was chosen to represent a typical weed density in Australian grain fields (Llewellyn et al. 2016).

Results and discussion

Chaff lining and chaff tramlining

Preventing the emergence of annual ryegrass in chaff lining or chaff tramlining systems requires the concentration of very high rates (>42t/ha) of chaff material. The enhanced suppression of annual ryegrass emergence with increasing amounts of wheat chaff was clearly evident in pot trials conducted at three locations in 2018 (Figure 1). Regardless of location, there were no differences (P>0.05) in annual ryegrass emergence for chaff treatments between 3t/ha and 18t/ha (Figure 1). The 30t/ha and 42t/ha chaff treatments produced the lowest (P<0.05) emergence of just 49.5% and 20.7%, respectively. However, annual ryegrass emergence was not prevented in these studies, even at the highest wheat chaff rate of 42t/ha. At the lower chaff rates (3t/ha to 12t/ha), annual ryegrass emergence was consistently lower (P<0.05) at Wagga Wagga than the two other locations. Emergence in the Wagga Wagga trial was lower (P<0.05) than both sites at 3t/ha, 6t/ha and 12t/ha chaff rates and lower than one site at the 24t/ha chaff rate. However, at the 42t/ha chaff rate, emergence at Wagga Wagga was higher (P<0.05) than at the other two sites.

Figure 1. Influence of increasing amounts of wheat chaff on the emergence of annual ryegrass in pot trials conducted at three locations. Means with same letter are not significantly different (P=0.05.)

Barley chaff was generally more suppressive of annual ryegrass emergence at equivalent rates of wheat, canola and lupin chaff. When these four chaff types were compared in a single study at Wagga Wagga, there was generally lower annual ryegrass emergence through barley chaff, however these values were only significantly lower (P<0.05) at the highest chaff rates (30t/ha and 42t/ha) (Figure2).

Figure 2. Influence of increasing amounts of wheat, barley, lupin and canola chaff on the emergence of annual ryegrass in pot trials conducted at Wagga Wagga, NSW. Means with same letter are not significantly different (P=0.05).

Targeted tillage

Engineering research, development and testing were conducted on the University of Western Australia (UWA) test rig at (Figure 3A). As with any engineering design, the process involved iterative improvements to the design layout. Once the system was able to achieve a chipping cycle time of less than 400m from actuation to return to standby position and the design had been simplified and deemed reliable, the pre-commercial rig (Figure 3D) was designed and built.

Figure 3. Initial proof-of-concept rig, UWA (A), University of Sydney (USyd) trailer mounted self-powered rig (B), Qld Department of Agriculture and Food (QDAF) 3-point-linkage rig (C) and pre-commercial rig – the ‘Weed Chipper’ (D) used in the testing and validation of targeted tillage fallow weed control.

Weed kill field testing achieved very high efficacy on all targeted summer and winter annual weeds regardless of growth stage (Tables 1, 2 and 3). The survival of any weeds during testing was due to the design of current cultivator sweeps not being suitable for targeted tillage. Weed control was 100% effective when the weed was targeted by the point of the sweep, however there was high weed survival when the weed was hit by sweep side. There was also reduced efficacy when weeds were excessively large. When feathertop Rhodes grass was >70cm diameter, there was only poor control (Table 2). The system is highly effective on both broadleaf and grass weeds with potentially little resulting soil disturbance (Figure 4).

Table 1. Response tyne efficacy following direct or partial sweep impact on four winter and three summer weed species at eight growth stages, Narrabri, NSW, 2017 and 2018.

-   Indicates no treatments where there was partial contact of the tyne with the weed.