Management of hard to control weeds
Author: Christopher Preston (University of Adelaide) | Date: 13 Aug 2019
Take home messages
- Management of hard to control weeds requires an understanding of when the weeds germinate.
- High efficacy is needed with the first herbicide mixture to achieve good double knock control of fleabane in summer.
- Windmill grass needs to be controlled as soon as it is noticed, as the old plants are much harder to control.
- Early autumn applications are better for getting glyphosate into the root system of silverleaf nightshade than summer applications.
Hard to control weeds
The main reason there are hard to control weeds in farming systems is that the current weed management practices that are used do not control them. This may be because there are no effective herbicides available for that weed, the germination pattern of the weed means it avoids current control methods or for a number of other reasons. Many of these weeds tend to be weeds that have come into prominence due to changes in farming systems, and therefore, we have limited experience with these weeds.
In order to develop better management strategies for these weeds it is necessary to understand the biology and ecology of the weeds, so that the management strategies can be better targeted. Understanding the time of emergence of the weeds can be particularly important, in order that control practices are implemented at the time of or shortly after emergence when weeds are small.
Fleabane has been present for a long time in Australia but it has only become a weed of cropping systems in recent years. Fleabane is a small seeded weed that requires several days of moist soil on the surface to germinate. Therefore, it is favoured by no-till, stubble-retention farming systems. It germinates primarily in spring, but if water is available, it can germinate through summer into early autumn provided temperatures are not too hot. Many fleabane populations in Australia are resistant to glyphosate, which makes them harder to control in the summer fallow period.
The best time to control fleabane is in late winter and early spring as it is germinating. Amicide® Advance and FallowBoss® Tordon® can be used in cereals. However, the latter has a 20-month plant back to pulse crops, cotton and pastures and a 12-month plant back to canola, so care needs to be taken when it is used in crop.
If control is left until after harvest, a double knock approach must be used. This is normally glyphosate plus a Group I herbicide, followed at least two weeks later by paraquat. However, the first application needs to provide at least 60% control of fleabane on its own to get effective fleabane control with the double knock (Figure 1).
Figure 1. Fleabane control in fallow of standalone herbicide treatment (1st knock) compared with control from the double knock with paraquat for each treatment at two trial sites (Bute and Pinnaroo, SA). Data from Ben Fleet, University of Adelaide.
Crop competition can also be used to reduce establishment of fleabane. Fleabane occurs most commonly following poorly competitive cereals, pulse crops and pastures. Increasing the amount of competition in cereal crops will limit the number of fleabane plants that survive through to crop harvest.
Windmill grass is another surface-germinating weed species like fleabane. Windmill grass is an Australian native and has been sown widely in areas with summer rainfall as a pasture grass. Unlike fleabane, windmill grass is a short-term perennial, with plants surviving for several years. Some windmill grass populations have evolved resistance to glyphosate, making them harder to control in the summer fallow period.
Windmill grass germinates in both spring and autumn. The optimum temperature for germination is 25oC but it has a broad range for germination from 15oC to 30oC (Figure 2). Light is required for germination, which is why windmill grass is increasing as a problem in no-till systems. Windmill grass seed fails to germinate from even as little as 0.5cm below the surface, so is unlikely to be a problem in cultivated cropping systems. Cultivation can also be used to control perennial plants, but a follow up treatment for new seedlings is required.
Figure 2. The effect of temperature in the light on germination of windmill grass seeds from two different populations (CT2 and CT3). Data from The Duc Ngo, University of Adelaide.
The challenge to managing windmill grass is that there are almost no herbicides registered for its control. Butroxydim in summer-growing broadleaf crops is the only registered in-crop herbicide. In summer fallows, some glyphosate products are registered for control. There is also a permit for the use of quizalofop-ethyl in fallows, provided it is double-knocked within seven days with paraquat. Particularly challenging to control are the older established plants from previous years, which often have dead leaves around the base. Getting good herbicide coverage on these older plants is challenging. Consequently it is important to control windmill grass as soon as it is noticed.
Silverleaf nightshade is a summer growing perennial weed with a large root system. The root system may grow more than 3m deep and 10m or more across. Silverleaf nightshade has the ability to grow new stems from small root pieces. Controlling the shoots of silverleaf nightshade does not necessarily control the root system and control of the root system is necessary to achieve long-term control.
Silverleaf nightshade seedlings are rarely seen. Silverleaf nightshade seeds are covered by a mucilaginous coating that prevents germination. As this needs to be removed before germination can occur, germination typically only occurs after very wet conditions. Most often this will happen with wet spring and early summer conditions, such as in 2016. This is why silverleaf nightshade patches tend to appear out of nowhere. Control of seedlings is easy but finding them is difficult. This means control tends to focus on managing established plants.
Silverleaf nightshade seeds readily survive passage through stock. Sheep are the main cause of silverleaf nightshade spread in Australian agriculture. While initially stock may avoid eating the berries, sheep can get a taste for them and actively consume the berries. Birds, farm machinery and fodder are also likely vectors of spread of seed but are of much less importance than sheep. As silverleaf nightshade can grow new shoots from root fragments as small as 5cm in length, cultivation does not control silverleaf nightshade and can help to spread patches of the weed.
Work to introduce a biological control of silverleaf nightshade is underway; however, in the meantime herbicides are the only effective control practice. There are few herbicides with any efficacy against silverleaf nightshade. Some like 2,4-D simply kill the shoots without reducing root growth. With others, timing is everything in terms of getting the herbicide into the root system. Research work looking at glyphosate movement in silverleaf nightshade plants showed that application of glyphosate after flowering moved the most herbicide into the root system, whereas application at flowering tended to have little movement (Figure 3). This is because flowering plants are moving photosynthate (and glyphosate) into the developing flowers and fruits, rather than into the roots.
Figure 3. Translocation of glyphosate into roots of silverleaf nightshade plants at different times of the growing season. Silverleaf nightshade tends to flower in the period December to February and glyphosate translocation to the roots is reduced in this period. Data from Kerensa Greenfield, University of Adelaide.
Adding 2,4-D to glyphosate is counter-productive to getting glyphosate into the roots (Figure 4). This is because the addition of 2,4-D tends to kill the shoots quickly and limits the movement of glyphosate into the root system. The best approach for glyphosate application to silverleaf nightshade is to apply the herbicide immediately after harvest, but before flowering of the weed, and then to have a second application in early autumn. Due to the extensive root system, it will take five to ten years to see a noticeable decline in the density of silverleaf nightshade patches.
Figure 4. Effect of adding 2,4-D on translocation of glyphosate into roots of silverleaf nightshade plants. Data from Kerensa Greenfield, University of Adelaide.
The main problem with wild oats is that it has resistance to both the Group A and Group B herbicides. What is more of a concern is the increasing resistance to Axial®, which is used for late season control of wild oat seed set. Our data suggests that resistance across the fop herbicides in wild oats can be variable, so getting a resistance test to determine whether any of the products still work can be useful.
Control of wild oats with pre-emergent herbicides has been challenging in the past, but new products and a move to no-till have offered new approaches to controlling wild oats. Data has shown that Trifluralin + Avadex®Xtra or Sakura® + Avadex® Xtra consistently provide control of wild oats in wheat. Due to the ability of seeds to bury themselves in soil and the more extended emergence of wild oats, crop competition in combination with pre-emergent herbicides is an essential component of reducing wild oat seed set of late emerging plants.
The challenge comes in the pulse phase of rotations where due to lower competition, later emerging wild oat seedlings can produce a lot of seed. Wild oat tends to mature earlier than ryegrass and seed can shed well before harvest. This makes crop topping and harvest weed seed control practices less effective on wild oats. Getting the rotation right and using all the practices that are available can keep wild oats under control.
The research undertaken as part of this project is made possible by the significant contributions of growers through both trial cooperation and the support of the GRDC, the author would like to thank them for their continued support.
University of Adelaide
0488 404 120
GRDC Project code: UA00149, UA00158, UCS00020
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