Mechanisms of weed suppression – a new weapon in the making?
This paper and presentation will highlight specific crop growth traits and their measurement techniques and results. Information and data will focus on wheat, canola and barley studies.
Research was conducted in Wagga Wagga New South Wales (NSW) in 2012, 2013, 2014 and 2015 using replicated trial designs. In all crop years from 2012-2015 we observed differences in weed infestation in crop and also in post-harvest crop fallows associated with grain crop cultivar and species evaluated. Crops were produced in soils with low to moderate weed infestation and in the absence of residual herbicides. Crops generally proved to be both competitive with weeds during their establishment and growth. In addition, remaining crop residues were suppressive to summer annual weed establishment, compared to borders without stubble. Some crops were clearly more suppressive of in-crop weeds including rye, barley, wheat and canola, likely due to reduced light at the soil surface and competitive canopy architectures. However, once harvest was performed, crop residues were all that remained on the soil surface, and amount remaining was crop/cultivar dependent (Weston et al. 2014).Some of the specific traits related to competiveness in crops are:
- Height—particularly early height over mature height.
- Early vigour.
- Canopy architecture:
- leaf area index, leaf size and growth structure
- Below ground traits—root competition can be stronger than competition for light, particularly for N, P and soil moisture:
- root length density, elongation rate, number of root tips, total root length
- Speed of development
- Partitioning of resources
- Biomass of stubble residue
- Carbon/nitrogen ratio in residue
These phenological traits, plant morphology, crop and weed biomass and growth stage developments were measured and analysed. In 2015, additional data collection measurements were taken including NDVI (normalized difference vegetative index), and PAR reading (photosynthetically active radiation) included both leaf area index and per cent light interception.
Two key aspects of weed suppression trials conducted were:
To eliminate the environmental differences due to seed sources grow-out location. Seed for the cereal trials was therefore grown for the last two years at the Wagga location (excluding canola open pollinated and hybrid seed, and the hard seeded legumes in the pasture trials). Note that in the wheat trial we also include some older cultivars including Federation (bred over a century ago) for its competitive effects. Cereal rye was used in the trials as a check since it is very competitive and known to exhibit allelopathic chemicals
To eliminate differences in plant populations among the cultivars or crop treatments so all treatments are not significantly different in plants/m2. Trials were planted to normal regional seeding rates using standard rates of the seed dressing of Advance and Gaucho®. Stand counts indicted there was no differences in final emerged plants populations. This allows for a good comparison of a particular variety such that its suppressive ability is based on above and below ground architecture and /or any potential allelopathic effects and not because there were more plants/m2 in any treatments which would create an unfair competitive advantage.
Table 1: Crops grown in weed suppressive trials.
|Crop||Type or use|
|Wheat||Grain and duel purpose graze|
|Canola||Grain and duel purpose graze|
|Barley||Grain (feed/malt) and duel purpose graze|
|Cereal Rye||Grain and duel purpose graze|
|Triticale||Grain and duel purpose graze|
|Oats||Grain and duel purpose graze|
The trial designs utilized a minimum of four replications with up to six replications for most studies. In weed suppressive trials this allows for greater confidence in having the inclusion of weeds within the trials areas as weed seeds are not over sown in these studies.
The long term rotational weed seed bank trials used standard herbicide practices while the cereal trials had no herbicides applications and the canola trials included ‘with and without’ trifluralin treatments.
Trials in 2015 were planted using a precision cone planter with 22cm row spacing, knife points and press wheels. A starter fertiliser of 60kg/ha of MAP (impregnated with flutriafol) was placed below and behind the seed furrow. Canola trials were planted on 2 May and 8 May 2015 and the cereals in all the trials were planted on 22 May 2015. A top dressing of 50 kg/ha N (urea) was applied on all trials and followed later by an application of Prosaro® for foliar disease control. Data collection was initiated at crop emergence DAE (days after emergence) and at critical growth stages of the crop at regular timed dates. Early plant vigour ratings were taken visually using a 1-10 rating system. All other measurements were collected with instrumentation or physical sample removal.
Weed suppressive wheat trial
Figure 1: Comparison of early to late crop biomass and final weed biomass.
Figure 1 shows marked variation between the treatments in regards to how much biomass was obtained at early, mid and late growth stage and final weed biomass. High biomass was typically a strong competitive feature against weeds. Those cultivars showing high biomass and weed suppression include Janz CL, Wedgetail, Condo and Whistler. Both the Wedgetail and Whistler are true winter wheats and serve as dual purpose wheats, therefore creating large biomasses for early grazing.
Figure 2: Differences in per cent light interception at two critical growth stages (Wagga Wagga 2015).
Figure 2 presents significant differences in the ability of selected cultivars to shade the soil surface at a critical early growth stage with Suntop, Scout, Corrack, Condo, Mace, Livingstone and Federation showing very high light interception. As to be expected at the later growth stage, cultivars exhibited over 90 per cent light interception but Condo, Livingstone and Federation were significantly better in reducing light penetration to the soil surface. Gregory followed a similar trend.
Figure 3. Differences in crop height at two growth stages (Wagga Wagga 2015).
Figure 3 shows early crop height for the Federation and the Condo cultivars, an important architectural trait in providing weed suppression through shading and competition for resources. Condo and Federation continued to exhibit greater heights with Suntop and Gregory also performing well at the later growth stage measurement.
Figure 4. Normalised difference vegetative index (NDVI) measurements at two growth stages.
Figure 4 indicates that the NDVI readings varied among the cultivars between the two growth stages. However, some cultivars maintained proportional increases in NDVI scores among the two growth stages. Federation showed the highest NDVI score at both the 41 days and the later growth stage (125 DAE). All of the NDVI scores at the later growth stage were >0.8, indicating high crop cover over the experimental plots.
Figure 5. Grain yield of different wheat cultivars in the weed suppressive wheat trial (Wagga Wagga 2015).
Grain yields differed significantly among the cultivars tested by up to two fold differences (Figure 5). The highest grain yield was obtained in the wheat cultivar Trojan which was statistically similar to that of Mace, Corrack, Condo and Scout. The lowest yield was obtained in the cultivar Federation with the control check of Grazer cereal rye having the lowest yield.
In this weed suppressive wheat trial the outcomes clearly showed that although Federation had very good growth traits that provide for weed suppression and competitiveness, it still exhibited reduced yields in comparison to other cultivars. Overall, the Condo cultivar with its architectural traits including crop canopy, ability to suppress weeds, and high yields, showed the most promise for weed suppression over two seasons along with strong performance in terms of yield.
Weed suppressive barley trial
Figure 6: Comparison of early to late crop biomass of barley.
Crop biomass of barley in two growth stages is presented in Figure 6. Initial growth was higher in Urambie which was followed by Navigator, Compass and the cereal rye check. Navigator produced the lowest biomass at late sampling. In contrast, Hindmarsh, La Trobe, Commander, Compass, Litmus and cereal rye did not show any difference in late biomass.
Figure 7. Differences in per cent light interception at two critical growth stages of barley.Litmus showed significantly greater light interception (Figure 7.) of nearly 80 per cent while all of the other cultivars had less than 60 per cent at the early growth stage. As to be expected at later light interception measurements, all the cultivars intercepted over 90 per cent in light reaching the soil surface.
Figure 8. Grain yield of different barley cultivars (Wagga Wagga 2015).
As shown in Figure 8, the genetically similar cultivars of Hindmarsh and La Trobe were the highest yielding barleys. The remaining cultivars yielded between 3.6-4 t/ha and depending on their use as either feed of malt, would vary in return values. The weed suppressive barley trial was similar to the wheat trial in its outcomes. This similarity is likely based on the differences in the cultivar architecture in exhibiting weed suppressive characteristics and yield components.
Weed suppressive canola trial
Figure 9: Comparison of canola biomass at two growth stages in relation to trifluralin (TRI) or untreated (UT) (Wagga Wagga 2015).
Early plant vigour is important for weed suppression as shown in Figure 9. GT50, Hyola®600RR and AV Opal had the highest early growth stage biomass. It appears that the use of trifluralin had no effect on production of early season biomass. Late season biomass was lower in untreated canola crops than the trifluralin-treated crops with the exception of CB™Taurus and Hyola®725RT.
Figure 10: Comparison of light interception in canola at two growth stages in relation to trifluralin (TRI) or untreated (UT).
As to be expected with the architecture of canola plants, light interception ranged from 65 per cent to over 80 per cent at the 92 DAE measurement with the exception of the variety CB™Taurus (Figure 10). At the 136 DAE, the percent light interception was over 75 per cent for all cultivars. Notably, the light interception was less in the untreated plots compared to the trifluralin-treated plots.
Figure 11. Comparison of grain yield of canola in relation to trifluralin (TRI) or untreated (UT) (Wagga Wagga 2015).
In all of the cultivars evaluated the trifluralin-treated canola outyielded the untreated canola (Figure 11). GT50 produced the highest yield (above 1.8 t/ha) among the cultivars evaluated, and yield was correlated with high biomass production and high light interception into the crop canopy. These characteristics coupled with high yield potential are promising for production of weed suppressive canolas.
Understanding specific crop traits that lead to competitiveness and weed suppression, along with the use of instrumentation and sampling techniques for assessment of crop growth proved to be important in evaluating crop performance with respect to weed suppression. A combination of key crop traits as well as a high yield index can result in crop cultivars that provide a cost-effective tool for integrated weed management (IWM) to address the issue of herbicide resistant weeds in Australia.
Funding for this work was provided through the GRDC Project UA00020 and their support is gratefully acknowledged.
Weston, L. A., R. Stanton, H. Wu, J. Mwendwa, P. A. Weston, J. Weidenhamer, and W. B. Brown. 2014. Comparison of grain crops and their associated residues for weed suppression in the Southern Australian mixed farming zone. Proc.of the 19th Australasian Weeds Conference. pp. 1-4.
GRDC Project code: UA00020
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