Read time: 5 minutes

Is it possible to apply additional competition to inter-row weeds?

with Hanwen Wu, Principal Research Scientist, NSW DPI

The inter-row space provides an ideal environment for weeds to grow, particularly if pre-emergent herbicides are not applied or are less effective than expected.

NSW Department of Primary Industries principal research scientist, Dr Hanwen Wu says filling the inter-row space with a productive species might be another way to suppress weed growth and reduce seed production of herbicide resistant weeds.

Dr Hanwen Wu, NSW DPI is investigating new ways to increase crop competition, particularly in the crop inter-row space.

“There is very strong evidence that narrower rows are an excellent way to increase crop competitiveness but there are some practical limitations,” he says. “We have looked at a combination approach, of planting most of the seed in rows and the rest broadcast to ‘fill in the gaps’. We have called this the ‘compound sowing technique’.”

In 2016 Hanwen set up two field trials at different locations near Wagga Wagga, NSW to assess the effect of this system on weeds and crop yield in both narrow (22.5 cm) and wider (45 cm) row spacings, with and without IBS trifluralin. Three broadcast species were evaluated – wheat, gland clover and French serradella. The two sites were assessed to have an initial annual ryegrass density of 48 and 25 plants/m2.

“We sprayed out the broadcast legumes in early September to prevent them competing with the crop for moisture,” says Hanwen. “Further trials are needed to test a range of parameters such as suitable legume species, optimal seeding rates, proportion of seed broadcast, row spacing for the conventional seeding and different pre-emergent herbicide options.”

Although the 2017 season did not allow Hanwen to replicate this trial he is keen to do more trials in 2018.

“We think this technique has merit and our initial trial suggested that weed suppression can be achieved without any yield penalty,” he says. “There even seems to be situations where a yield increase can be achieved in response to reduced weed pressure.”

Crop competition is rightfully attracting more attention from farmers and researchers in the war on herbicide resistant weeds. It is a numbers game and crop competition can play an important role in vastly reducing weed seed set.

Which was the most competitive broadcast species?
Short answer: Wheat.

Longer answer: At the weediest site the broadcast wheat treatment, without IBS trifluralin reduced annual ryegrass biomass by 71–77 per cent at both the narrow and wider row spacings. In the presence of less weeds the broadcast wheat still reduced weed biomass by 50 per cent in the narrow rows and 27 per cent in the wider row configuration.

IBS trifluralin further increased weed suppression at both sites and both row spacings. At the weedier site, annual ryegrass biomass was suppressed by 88–90 per cent. Where there were less weeds present the addition of IBS trifluralin increased biomass suppression from 27 to 70 per cent at the wider row spacing.

Compound sowing technique (conventional + broadcast sowing) dramatically increases crop competition in the inter-row compared to conventional sowing. Of the three broadcast species tested, wheat provided the strongest suppression on weed biomass.

What was the effect on yield?

Short answer: The wheat yield increased by 15–22 per cent at the weediest site when wheat was used as the broadcast species.

Longer answer: In the favourable season of 2016, only broadcast wheat generated a yield increase, and only in the presence of higher weed pressure. None of the broadcast treatments caused a yield reduction at either site. Further trials are required to evaluate the impact of site and seasonal climatic conditions on the weed control and crop yield associated with the compound sowing technique.

The broadcast legumes may provide additional soil fertility and moisture retention benefits while maintaining crop yields. More work is needed to identify more competitive legume species to have a greater impact on weed biomass and to identify the optimal timing to kill broadcast legumes to maximise weed suppression and minimise yield loss.

Using a broadcast legume that is sprayed out in September could have additional soil health and moisture retention benefits and warrants further investigation.

Have any farmers tried this idea?

Short answer: Yes.

Longer answer: Leigh Bryan at Swan Hill has tested this idea on his farm – he calls it zero-row spacing. Also in 2016, a strip-trial in barley resulted in the zero row spacing strip yielding 4.994 t/ha compared to 4.889 t/ha in the conventionally sown crop at 37.5 cm spacing. This was achieved with no pre-emergent or in-crop herbicide applied.

Leigh has noticed that the random placement of stubble is easier to sow through the next year and it still provides trellising for pulse crops and shades the soil to conserve moisture and reduce soil surface temperatures.

Related Articles

View all
Ask an Expert

How do you manage summer weeds without spraying at night?

Concerns are being raised about the practical implications of this for summer weed control programs. Mary O’Brien, a private consultant with extensive experience in managing spray drift, is keen to see growers fully adopt spray application practices that maximise herbicide efficacy and minimise off-target drift.   Mary O’Brien says the ‘community drift’ that can occur when a number of applicators are each putting a small amount of product in the air at the same time can have very damaging effects on off-target sites. “The bottom line is that allowing spray to drift is like burning money,” she says. “Any product that doesn’t hit the target is wasted and the efficacy of the spray job is reduced, mildly resistant biotypes may survive as a result of low dose application and there is potential damage to sensitive crops and the environment.” “The difficulty is that many growers want to spray at night to cover more ground when conditions are cooler and potentially weeds are less stressed. Having a restriction on night spraying does restrict the time available to cover the areas required.” Having heard these concerns from growers across the country Mary keeps coming back to the fact that if there was a limitation to capacity at planting or at harvest, growers would scale up to get the job done in a timely manner. “Buying another spray rig or employing a contractor is an additional cost, especially after a couple of tough seasons, but I really think this is insignificant against the cost of losing key products and the resultant escalation in herbicide resistance to the remaining herbicides,” says Mary. “This problem is not confined to 2,4-D or even to herbicides. I recently spoke to a stone fruit grower who was forced to dump his whole crop after a positive MRL return for a fungicide he had never even heard of, let alone used.” What about just slowing down and lowering the boom during night spraying? Short answer: This, coupled with a good nozzle, will reduce drift but it will never eliminate it. Longer answer: The correct ground speed and boom height will have a large effect on the amount of product that remains in the air. The problem is that it only takes 1 per cent of the product remaining in the air to cause off-target damage. Once there are a few operators putting just 1 per cent of their product in the air at the same time, the amount of product quickly accumulates and can potentially be very damaging. Mary calls this ‘community drift’. Isn’t it better to spray weeds at night when it’s cooler? Short answer: Not really. Longer answer: Research by Bill Gordon showed that even if you keep everything else the same, night spraying can put at least three times more product in the air than daytime application, even if weather conditions are similar and there is no temperature inversion in place. The main difference between day and night is how the wind is moving across the landscape, rather than the wind speed. Under inversion conditions, the air moves parallel to the ground surface and this means that the product can move significant distances away from the target before coming to the ground. To achieve the best results through daytime spraying, applicators should focus on treating small, actively growing weeds. When there is good soil moisture, weeds are unlikely to be stressed even when the temperature is quite high. Temperature inversion conditions are more common at night and in the early morning. These conditions generate a laminar flow of air across the landscape allowing small droplets to travel many kilometres away from the target site before coming to ground. Can I use other products at night and just avoid using 2,4-D? Short answer: The current changes to 2,4-D labels has drawn a lot of attention but the problem is the same for all crop protection sprays – herbicides, fungicides and insecticides. Longer answer: Different products have different properties and some may work better at night but the problem is the sensitivity of some crops to certain products, such as 2,4-D. All products are tested for their efficacy and the label provides detailed information about the required spray quality and spray application conditions. Many products have explicit label instructions regarding wind speed, temperature inversions (or laminar flow) and night spraying. Given the high risk of drift at night, applicators need to be very confident that there is no inversion present, and weather conditions should be measured at least every 15 minutes to ensure wind speed remains above 11 kilometres per hour. An on-board weather station is the best way to monitor conditions. A visual demonstration using smoke to simulate the the lateral movement of small spray droplets when a temperature inversion is in place. What can I do to improve spray efficacy and avoid spray drift? Short answer: If you do just one thing – change your nozzle. Longer answer: All the factors that increase drift also reduce efficacy. To improve efficacy and reduce drift, use a better nozzle (larger spray quality) and appropriate water rates (matched to spray quality and stubble load), slow down and keep the boom low. Wind is required to push product downward and onto the target, and remember that the 3–15 km/h wind speed is for day time conditions only, this does not apply at night.
Ask an Expert

Does ambient temperature affect herbicide performance?

with Chris Preston, Associate Professor, Weed Management
The University of Adelaide Temperature affects the absorption, translocation and metabolic degradation of herbicides applied to plants. Herbicides applied under the wrong conditions can appear to fail, however the reason may not be herbicide resistance. Dr Chris Preston, Associate Professor, Weed Management
 at The University of Adelaide says most herbicides have a temperature range at which they are most effective in controlling target weeds. “Applying herbicides outside the optimal temperature range is likely to contribute to a spray failure, even in susceptible populations,” he says. “Alternatively, applying herbicides within the correct temperature range can improve the control in populations known to have a level of resistance to that herbicide.” Dr Chris Preston suggests testing whole plants rather than seed for responses to a range of post-emergent herbicides. The Quick-Test is conducted in the same growing season as herbicide will be applied so the testing will occur under similar conditions to field conditions. Dr Preston says the effect of frost on the efficacy of clethodim is a striking example. Spraying clethodim in non-frosty conditions achieves vastly better results than spraying after three days of frost, even on populations that are resistant to this chemical mode of action. “Combining the optimal temperature with optimal weed size will give the best results possible,” he says. “The current common practice of applying clethodim to tillered ryegrass in the coldest months is not making the best use of this herbicide.” As a general rule of thumb, Group A (fops), paraquat (Group L) and glyphosate (Group M) are more effective at lower temperatures while Group A (dims), atrazine (Group C) and glufosinate (Group N) are more effective at higher temperatures. However, weeds that are resistant to paraquat become less resistant in warmer temperatures. “The other implication of this research is the effect of ambient temperature on herbicide test results,” says Dr Preston. “Seed collected in winter and grown out in the glasshouse in summer will be tested for resistance in conditions that are not representative of field conditions when growers are next treating that weed species. The Quick-Test using whole plants overcomes this problem and improves the reliability of herbicide susceptibility testing.” How can I get the best performance out of clethodim? Short answer: Avoid applying clethodim during frosty periods. Longer answer: Twice as much clethodim is required to kill susceptible annual ryegrass if the product is applied after three days of frost. Even higher rates are required if the plants have resistance to clethodim. Planning to apply clethodim for grass control outside the coldest months of June and July, and avoiding night spraying in winter, will see better results in both resistant and susceptible populations, particularly in tillered plants. Clethodim is most active when temperatures are over 20 degrees C. Weed seed that is tested during summer may return false negative results, which could translate into spray failure in the field the next season. Twice as much clethodim is required to kill susceptible annual ryegrass if the product is applied after three days of frost. Even higher rates are required if the plants have resistance to clethodim. When it is it too hot for glyphosate? Short answer: Efficacy is much better at 20 degrees C than at 30 degrees C. Longer answer: Spraying glyphosate resistant barnyard grass at lower temperatures is more effective than under hotter conditions. If barnyard grass is tested for herbicide resistance during the cooler parts of the year it may appear susceptible to the field rate of glyphosate but then when this rate is applied to the population in summer there may be many survivors. When glyphosate is taken up rapidly it tends to limit its own translocation, which can mean that although symptoms may appear more rapidly in warmer temperatures, plant kill is less reliable. Which herbicide resistance test should I use? Short answer: The weed resistance Quick-Test for post-emergent herbicides. Longer answer: The Quick-Test involves testing whole plants rather than seed for responses to a range of herbicides and rates. The Quick-Test is conducted in the same growing season as herbicide will be applied so the testing will occur under similar conditions to field conditions. The results of the Quick-Test are available within the same season, potentially giving growers an opportunity to apply an effective weed control tactic before the end of the season. The Quick-Test is not available for many pre-emergent herbicides. The Quick-Test is available through Plant Science Consulting and results are normally available after four weeks. Relevant links Maximising clethodim performance and the impact of frost fact sheet Keeping clethodim working in broafleaf crops Plant Science Consulting herbicide resistance testing – Quick-Test GRDC Update Paper – New developments and understanding in resistance mechanisms and management

Subscribe to the WeedSmart Newsletter