Rotate crops and pastures

Now is the time to take pre-emptive action to reduce the incidence of glyphosate resistance in weeds. Eric Koetz, NSW DPI weeds research agronomist, says the majority of agronomists and a growing number of farmers recognise the need to implement management practices that help protect the efficacy of glyphosate in farming systems. “We know that it takes between 14 and 19 years of routine use of glyphosate to evolve resistance,” he says. “We have now had Roundup Ready cotton for 15 years and there are many instances of glyphosate resistant populations of weeds such as fleabane, windmill grass and barnyard grass.” “The 2 + 2 + 0 strategy developed in the cotton industry to protect glyphosate and Roundup Ready technology is also applicable to other farming systems,” says Mr Koetz. “When planning an integrated weed management program, the Cotton RDC recommends including two non-glyphosate tactics in-crop, two non-glyphosate tactics in the fallow and ensuring zero survivors.” This is where the use of residual herbicides can play a part, along with non-herbicide tactics, such as strategic tillage in both summer and winter crops and in fallow situations. Including residual herbicides in both the fallow and crop phases increases the diversity of herbicide modes of action and places downward pressure on the weed seed bank. Eric Koetz, NSW DPI weeds research agronomist says residual herbicides need to play a part in the control of glyphosate resistant weeds in crop and in the fallow. “Roundup Ready technology has been of great benefit to the cotton industry, and has a fit in other farming systems too, but it can not stand alone. It must be supported and protected through an integrated weed management strategy,” says Mr Koetz. “To preserve glyphosate it is necessary to reduce the total number of applications across the crop rotation.” What are the non-glyphosate options for in-crop weed control? Short answer: Residual herbicides applied at sowing, inter-row cultivation, crop rotation, maintaining ground cover and increasing crop competition. Longer answer: Residual herbicides that require incorporation are best applied at sowing. There are some older herbicides that have not been used for several years that are still quite effective and could make a come-back to farming systems that have come to over-rely on glyphosate. There are also new use patterns being registered for a number of different products that can be used to add diversity to control weeds at different growth stages or to control later in-crop germinations. Increasing crop competition and promoting rapid canopy closure has a significant impact on in-crop germination. Diversifying the crops grown automatically makes more herbicide options available to use against weed populations that may be evolving glyphosate resistance. Residual herbicides applied at seeding or soon after harvest help reduce the number of glyphosate applications across the cropping cycle. What are the non-glyphosate options for controlling weeds in the fallow? Short answer: Residual herbicides applied soon after harvest, strategic tillage, double knock, optical sprayer, cover cropping. Longer answer: Choose residual herbicides carefully. Some products have long plant back periods and will reduce the grower’s options for the following season. Tillage is widely practiced in irrigated cotton and is an effective way to eliminate weeds. In dryland systems tillage operations may be best suited to low crop residue situations such as following a chickpea crop. The cultivation operation may be done across the whole paddock or in small patches and can be coupled with paddock renovation, the incorporation of soil ameliorants or deep banding of nutrients. Several research projects are currently investigating the usefulness of cover cropping and brown manuring on weed numbers. What can be done to ensure there are no survivors? Short answer: Scouting and chipping, optical sprayer, patch tillage. Longer answer: Actively looking for survivors must become a key management practice. A few large plants that have survived all control tactics can generate a huge number of seeds that are very likely to carry some level of herbicide resistance. If these plants are physically removed before they set seed they will not contribute to the seed bank for future germinations.

Adding a broadleaf break crop to the cropping sequence helps keep wheat profitable in a sustainable cropping system. Tony Swan, CSIRO says their 5-year GRDC funded project illustrated that adding at least one break crop, and preferably two, to the crop rotation was beneficial for weed control and nitrogen management, and can be as profitable or more profitable than continuous wheat. A series of experiments were established to challenge the idea that break crops are risky and not profitable. “Many farmers in south-east Australia are sceptical about growing break crops such as pulses and canola,” says Tony. “The problem is, once high populations of herbicide resistant annual ryegrass become apparent, the profitability of continuous wheat significantly reduces.” “Rotations that include a break crop in paddocks with high populations of resistant annual ryegrass were more profitable than continuous wheat and had significantly less ryegrass numbers after three years, provided all the available tactics were used to reduce germination and prevent seed set,” he says. “Our experiments demonstrated that it is cheaper and more effective to control ryegrass using one of the many break crop options than attempting to achieve control in wheat using expensive herbicides.” What break crop options did you trial? Short answer: RR canola, TT canola, lupins for grain, field peas for brown manure, fallow and wheat cut for hay. Longer answer: The combination of a fallow or break crop in year 1 followed by a second break crop in year 2 resulted in the greatest reduction in annual ryegrass seed bank population and panicle number after 3 years. This sequence was significantly more profitable than continuous wheat, but not as profitable as a RR canola–wheat (high input) –wheat rotation. What was the most effective option in a weedy situation? Short answer: A 2-year break crop option. Longer answer: The double break rotations of lupins grown for grain followed by RR canola, or RR canola followed by wheat cut for hay provided a very high level of weed control while also generating high average annual 3-year gross margins of $790/ha/yr and $834/ha/yr, respectively. This compared to the most profitable 3-year sequence of RR canola followed by wheat (high input) / wheat of $883/ha/year. However, this sequence did not achieve the same reduction in annual ryegrass and grass herbicides cost over $140/ha in the wheat crops. Sequences that included fallow or brown manures followed by RR canola were extremely effective at reducing the annual ryegrass seed bank but were not as profitable as continuous cropping. Where herbicide resistant annual ryegrass is a major problem, an alternate three year sequence of wheat-hay (sprayed afterwards) in year 1, pulse-grain (spray topped) in year 2, and RR canola in year 3 can be profitable and also reduce the seed bank to extremely low levels. A two-year break crop can break the weed cycle without breaking the bank. What is the key recommendation from this trial work for annual ryegrass control? Short answer: Two consecutive years of total annual ryegrass control using break crops and implementing all available weed seed control options. Longer answer: Break crops work and can be profitable. Two or more years of effective ryegrass control using break crops and other management options including strongly competitive crops, rotating herbicide groups, pre and post emergent timing and prevention of seed set using crop topping, hay making and brown manuring along with fallow management and harvest weed seed control such as narrow windrow burning. How to ask WeedSmart a question Ask your questions about using break crops to manage annual ryegrass on Facebook or Twitter @WeedSmartAU or leave a comment below.

Peter Newman, Communication Leader with the Australian Herbicide Resistance Initiative says the real culprit is always a lack of diversity. He recalls a grain grower friend taking him to a paddock that had been in a lupin / wheat rotation for 30 years. “He said his wheat was always full of brome grass and his lupins were full of wild radish and blue lupins,” he says. “Then one year he decided to break the cycle and planted barley after wheat, instead of lupins, and the barley came up full of wild radish and blue lupins.” This grower concluded that his weeds were responding to what had become a predictable farming system. “Managing resistant weeds requires farming systems that are diverse—using different crops, different chemical modes of action and including non-herbicide control tactics,” Mr Newman says. The days of simply changing from one chemical to another when the first one becomes ineffective are gone. All chemical options need the support of other tactics and this is where longer crop rotations can provide much more diversity to confuse the enemy than short rotations.” “Diverse rotations can include longer rotations with different crop species, using varieties with different herbicide tolerance traits or sowing dates, and using an array of tactics to reduce weed numbers and prevent seed set.” “We are seeing growers win in the battle against herbicide resistance and that is great news,” he says. “It’s time to get serious and to start looking for ways to make every farming operation as diverse as possible.” Peter Newman, Communication Leader with the Australian Herbicide Resistance Initiative says the real culprit causing herbicide resistance is always a lack of diversity. How does lengthening a crop rotation help manage herbicide resistance? Short answer: By adding diversity. Longer answer: A longer crop rotation means having more tools in the toolbox—better herbicide rotation, a range of seed set control options, varied planting times, competitive crop species or varieties and the ability to implement a variety of harvest weed seed control options. What can I do if I need to keep a fairly tight rotation? Short answer: Still look to add diversity. Longer answer: If you can’t see your way clear to lengthen your crop rotation, look for ways to increase diversity within the crops you grow. Changing varieties may allow a different sowing time and in herbicide tolerant crops such as canola, you can rotate between the RR and TT hybrids. In a tight rotation harvest weed seed control and maximum crop competition are even more important. Peter Newman, Tom Murphy and Rob Hughes discuss the benefits of narrow row spacing for yield and weed suppression. What other benefits can I expect from longer crop rotations? Short answer: A more sustainable farming system. Longer answer: Increasing diversity in the crop rotation is good for pest and disease management as well as weed management. Adding a legume will also have a lasting impact on soil nutrition. In the longer term a diverse rotation reduces overall production risk. Are there limiting factors? Short answer: Yes. Longer answer: Crop sequencing can have impacts beyond weed management. A rotation should be planned and the use of residual herbicides must take the following crop into account.

For some weeds, the ability to survive herbicide treatment comes with a ‘fitness penalty’ that makes them less able to thrive when faced with strong competition.   Glasshouse experiments show that some herbicide resistant weeds suffer a ‘fitness penalty’ that reduces their ability to compete with other plants when no herbicide is applied. Unfortunately not all herbicide resistance causes a fitness penalty. AHRI research fellow, Dr Martin Vila-Aiub studies the ability of herbicide resistant weeds to survive when faced with other environmental pressures in the absence of herbicide application. “What we have found is that some weeds that are selected for their ability to survive herbicide treatment are less able to grow well and reproduce—this is called a ‘fitness penalty’,” says Dr Vila-Aiub. “However, some herbicide resistant weeds do not suffer any ‘fitness penalty’ and these ‘super weeds’ can adapt well to every control tactic applied to them.” For the weeds that do suffer a fitness penalty, Dr Vila-Aiub says there are opportunities for growers to exploit this new weakness and to greatly reduce seed production of these herbicide resistant weeds without using herbicides. There are many variables involved in this field of study that make it difficult for researchers to issue general recommendations. Importantly, it is not possible to observe resistant weeds in the field and make assumptions about what might be causing their apparent lack of fitness or otherwise. Dr Martin Vila-Aiub studies the ‘fitness penalty’ associated with herbicide resistance in some weeds to find ways to exploit this weakness using environmental factors. What does it mean if weeds survive a herbicide treatment but look sick? Short answer: There may be an opportunity to exploit this weakness. Longer answer: Sometimes herbicide resistant weeds can survive a treatment but are very damaged by the herbicide. These plants are less likely to compete well against a vigorous crop or pasture. In a competitive environment, these plants are less likely to set seed and so the population size can be significantly reduced. If I think my weeds might be herbicide resistant, what should I do? Short answer: Get samples tested to determine if they are resistant and to identify any herbicides that are still effective. Longer answer: Providing a competitive environment is usually a beneficial tactic in a weed management plan. Weed populations that suffer a fitness penalty associated with their herbicide resistance trait can be significantly reduced in size. Resistant weeds with no fitness penalty are likely to still grow well but may be inhibited by the rapid early growth of a suitable crop cultivar. What other fitness penalties can be inferred with herbicide resistance? Short answer: Increased susceptibility to insect and disease attack. Longer answer: The herbicide resistant biotypes of some weed species become more attractive to pests and diseases compared to plants that are susceptible to herbicides. Some species use lowered photosynthesis activity as a mechanism to survive herbicide treatment. This has a side effect of reduced seed production, which reduces the resistant population’s ability to thrive and spread. Do herbicide resistant weeds have other weaknesses that growers can exploit? Short answer: Probably, but more research is needed. Longer answer: Some herbicide resistant weeds may have a new ‘weak link’ in their ecology. For example, glyphosate resistant ryegrass and wild sorghum are susceptible to glyphosate applied at low temperatures. In another experiment, researchers showed that some herbicide resistant plants produce seed that does not germinate without light. Burying these seeds just a few millimetres was enough to prevent germination. These responses have been shown experimentally but can not be relied on as management tactics until the processes are better understood. How to ask a WeedSmart question Ask your questions about taking advantage of reduced fitness to help manage herbicide resistant weeds, using Twitter @WeedSmartAU or leave a comment below.