Introduction
Wild oat (weedy Avena spp.) is a troublesome winter-season weed having wide distribution across 55 countries and causing an enormous yield loss in more than 20 crop species (Holm et al. Reference Holm, Plucknett, Pancho and Herbeger1977; Sharma and Vanden Born Reference Sharma and Vanden Born1983). In Australia, three species, namely wild oat (Avena fatua L.), sterile oat [Avena sterilis ssp. ludoviciana (Durieu) Gillet & Magne; referred to as A. sterilis hereafter], and slender oat (Avena barbata Pott ex Link) occur. These three species altogether caused an annual revenue loss of A$28 million to Australian agriculture in terms of loss of crop yield and cost of control (Llewellyn et al. Reference Llewellyn, Ronning, Clarke, Mayfield, Walker and Ouzman2016). Avena fatua and A. sterilis are quite common in the cropping regions of Australia (Cousens Reference Cousens2002; Fernandez-Quintanilla et al. Reference Fernandez-Quintanilla, Gonzalez Andujar and Appleby1990; Storrie Reference Storrie2019). Avena fatua is mostly dominant in southern Australia; however, A. sterilis is mostly dominant in northern New South Wales and southern Queensland (Nugent et al. Reference Nugent, Storrie and Medd1999). In the eastern region (Queensland and New South Wales) of Australia, wild oat (Avena spp.) holds the top ranking in the regional ranking of residual winter weeds in all crops when assessed in terms of infested area (Llewellyn et al. Reference Llewellyn, Ronning, Clarke, Mayfield, Walker and Ouzman2016).
Avena fatua and A. sterilis have prolific seed production (Storrie Reference Storrie2007, Reference Storrie2019). A recent field study conducted by Mahajan and Chauhan (Reference Mahajan and Chauhan2021a) in Australia showed that A. sterilis could produce around 2,500 seeds plant−1 under a lack of competition when it emerged at the start of the winter season (May). However, plants that emerged in July were shorter than plants that emerged in May and yielded fewer seeds. In another pot study under well-watered conditions, A. fatua produced a higher seed number (480 seeds plant−1) than A. sterilis (417 seeds plant−1) (Sahil et al. Reference Sahil, Loura, Raymont and Chauhan2020). These studies suggest that A. fatua and A. sterilis have different seed production potential and that seed production may vary with environmental conditions. There are no reports showing seed production of A. fatua and A. sterilis when they are grown in competition with wheat (Triticum aestivum L.) at different infestation levels.
A herbicide-resistance study on Avena spp. in Australia revealed that populations that have experienced repeated use of acetolactate synthase–inhibiting herbicides over the last 15 yr have a high risk for evolving resistance to these herbicides (Storrie Reference Storrie2007, Reference Storrie2019). In such a scenario, a better understanding of weeds, competitiveness, and interference in crops could provide important information for strengthening integrated weed management (IWM) strategies (Lemerle et al. Reference Lemerle, Luckett, Lockley, Koetz and Wu2014; Reiss et al. Reference Reiss, Fomsgaard, Mathiassen and Kudsk2018).
Avena fatua and A. sterilis infestation may cause yield reductions (30% to 80%) in winter crops such as wheat, oat (Avena sativa L.), barley (Hordeum vulgare L.), rye (Secale cereale L.), pea (Pisum sativum L.), and canola (Brassica napus L.) (Beckie et al. Reference Beckie, Warwick and Sauder2012; Daugovish et al. Reference Daugovish, Thill and Shafii2002; Dew and Keys Reference Dew and Keyes1976; Torner et al. Reference Torner, Gonzalez-Andujar and Fernandez-Quintanilla1991; Walia et al. Reference Walia, Seema, Brar and Singh2001). The magnitude of yield loss in these crops depends on the weed and crop density, species, and environmental conditions. Weeds vary in their potential to compete with crops (Korres et al. Reference Korres, Norsworthy and Mauromoustakos2019; Soltani et al. Reference Soltani, Dille, Robinson, Sprague, Morishita, Lawrence, Kniss, Jha, Felix, Nurse and Sikkema2018). A previous study in Australia showed that wild oat caused a 78% yield reduction in a wheat crop (Martin et al. Reference Martin, Cullis and McNamara1987). This study was mainly focused on the prediction of wheat yield loss in response to A. fatua competition, and the weed seed production components were not evaluated.
Crop maturity time and seed production of weeds may vary under competition conditions at different weed densities and weather or environmental conditions. Such information may provide an opportunity for harvest weed seed control and timely weed management (Mahajan et al. Reference Mahajan, Walsh and Chauhan2020; Walsh and Powles Reference Walsh and Powles2014; Walsh et al. Reference Walsh, Broster, Schwartz-Lazaro, Norsworthy, Davis, Tidemann, Beckie, Lyon, Soni, Neve and Bagavathiannan2018). The quantity of weed seed production, maturity, and seed-shattering time of weeds in relation to the crop are major determinants affecting the success of harvest weed seed control such as row burning, the Harrington Seed Destructor, bale-direct systems, and other means of targeting the chaff during harvest (Schwartz et al. Reference Schwartz, Norsworthy, Young, Bradley, Kruger, Davis, Steckel and Walsh2016; Walsh et al. Reference Walsh, Harrington and Powles2012, Reference Walsh, Newman and Powles2013). The performance of the Harrington Seed Destructor can vary with weed species and with the maturity time (Walsh et al. Reference Walsh, Harrington and Powles2012). For example, in a wheat crop, more than 90% control was observed with the Harrington Seed Destructor for rigid ryegrass (Lolium rigidum Gaudin) due to high seed retention, while less than 50% control was observed for common lambsquarters (Chenopodium album L.) due to poor seed retention (Walsh et al. Reference Walsh, Harrington and Powles2012, Reference Walsh, Newman and Powles2013).
Information on seed-retention behavior of A. fatua and A. sterilis is very important when they compete with wheat, as seeds of A. sterilis shatter in pairs at plant maturity, while seeds of A. fatua shatter individually (Sahil et al. Reference Sahil, Loura, Raymont and Chauhan2020). The shattering behavior of these weed species may cause their reinfestation and impose competition to the crop in the next season. In the eastern region of Australia, knowledge gaps exist concerning the effect of interference levels on wheat grain yield, weed seed production, and weed seed retention at crop maturity. Therefore, a study was carried out in the winter seasons of 2019 and 2020 to evaluate the seed production, seed retention, and interference of A. fatua and A. sterilis in wheat.
Materials and Methods
Experimental Site and Treatments
Field experiments were conducted in 2019 and 2020 (from May to October) at the Research Farm of the University of Queensland, Gatton (27.5514°S, 152.3428°E), Australia. The study was conducted in two separate fields for A. fatua and A. sterilis, with six plant density levels (0, 3, 6, 12, 24, and 48 plants m−2) in wheat crops. All treatments were tested in a randomized complete block design, replicated three times. The soil type of the experimental site was clay with pH 7.1 and organic matter content of 1.12% (up to 10-cm depth). The field was cultivated twice before wheat planting using a rotary cultivator. The wheat cultivar ‘Spitfire’ was planted at 35-cm row spacing with a seeding rate of 100 kg ha−1. The crop was sown on May 8, 2019, and May 7, 2020.
Planting of wheat was done using a cone planter, and seeds were sown at a soil depth of 5 cm. The size of the individual plot was 1.4 by 1.0 m. Seeds of A. fatua and A. sterilis, as per respective infestation levels, were sown manually in each plot at a 5-cm depth immediately after wheat planting. Weed seeds were planted randomly in between wheat rows. Plots were surface irrigated immediately after sowing using an overhead sprinkler system. All plots were fertilized with urea at a dose of 92 kg N ha−1.
Seeds of A. fatua and A. sterilis used in this study were originally collected from Warialda (29.395°S, 50.620°E), NSW, with the permission of the property owner in October 2017, and multiplied in the field at the Research Farm of the University of Queensland, Gatton, in the winter season of 2018. Seeds were collected from 50 to 60 matured plants and stored in the laboratory at room temperature until used in the experiment. Seeds were 100% viable at the start of the experiment. For the viability test, 15 d before the start of the experiment, 20 seeds of A. fatua and A. sterilis were sown in pots replicated three times. All seeds germinated within 10 d of sowing.
Measurements and Data Collection
Seed production and biomass of A. fatua and A. sterilis were assessed at the wheat harvest. To estimate seed production of A. fatua and A. sterilis, seeds of all panicles from 1 m2 (center of the plot) were counted. Shattered seeds of A. fatua and A. sterilis were determined by counting empty florets on each panicle from 1 m2. Whole-plant samples were collected by cutting all A. fatua and A. sterilis plants at the ground level in each plot (1 m2). Then, each plant sample was oven-dried at 70 C for 72 h and weighed to determine weed dry matter.
At crop harvest, five wheat plants were selected randomly from each plot for height measurements and then averaged. Height was measured from the base of the plant to the tip of the plant. The number of wheat spikes per square meter was determined by counting the number of wheat spikes in a 1-m length of two center rows in each plot. Wheat grains per spike at crop maturity were recorded from five randomly selected plants from each plot. A 1,000-grain weight was obtained after threshing from a random sample of the bulk produce of each plot. The wheat crop was harvested manually, and grain yield was recorded from a harvested area of 1 m2 per plot. Grain yield was converted to kilograms per hectare and then adjusted to 12% moisture content.
Statistical Analyses
The 2-yr data were subjected to ANOVA using the software Elementary Designs Application (1.0 Beta; www.agristudy.com, published by Free Software Foundation; verified with Genstat 16th ed.; VSN International, Hemel Hempstead, UK) (see Supplementary Tables 1 and 2). No significant interaction was found between year and weed infestation level. Therefore, data were pooled across years. Treatment means were separated using Fischer’s protected LSD at the 5% level of significance. Before ANOVA, data were also validated for meeting the assumptions of normality. A three-parameter logarithm regression model was fit (as it was the best fit) to weed infestation level/weed biomass and weed seed production/wheat yield reduction (%) data (SigmaPlot v. 14.0, Systat Software, San Jose, CA, USA):
$$y = y_{0}+ a^{*}ln (x -x_{0})$$
where y is the wheat yield reduction/weed seed production, x is weed infestation/weed biomass level, y 0 is maximum crop yield, and a is a constant.
Results and Discussion
Wheat and weeds emerged 8 d after seeding in both years. Avena fatua and A. sterilis flowered 97 and 99 d after seeding in 2019 and 2020, respectively. The crop reached the final maturity stage at 132 and 135 d after seeding in 2019 and 2020, respectively.
Avena fatua Interference in Wheat
Avena fatua panicles per square meter increased from 9 to 57, with an increase in the infestation level from 3 to 24 plants m−2, respectively (Table 1). A similar trend was observed for biomass and seed production of A. fatua (Table 1). Avena fatua biomass reached 54 and 321 g m−2 at densities of 3 and 24 plants m−2, respectively. Avena fatua seed production reached 706 and 4,079 seeds m−2 at densities of 3 and 24 plants m−2, respectively. Avena fatua panicles, biomass, and seed production per square meter remained similar at infestation levels of 24 and 48 plants m−2. Seed retention of A. fatua varied from 17% to 38%, depending on the infestation level; it was the highest at the 3 plants m−2 infestation and lowest at 48 plants m−2.
Table 1. Effect of Avena fatua infestation levels on the weed and yield parameters in wheat.
In the weed-free environment, wheat plants produced 417 spikes m−2, which decreased by 22% and 42% at A. fatua infestation levels of 6 and 24 plants m−2, respectively (Table 1). Wheat grain number per spike in the weed-free environment was 36, which decreased by 11% and 22% at A. fatua infestation levels of 6 and 48 plants m−2, respectively (Table 1). The 1,000-grain weight of wheat in the weed-free environment was 40 g, which decreased by7% and 14% at A. fatua infestation levels of 12 and 48 plants m−2, respectively (Table 1). Likewise, a reduction trend similar to that of spikes per square meter was observed for grain yield. Grain yield in the weed-free environment was 6,600 kg ha−1, and it was reduced by 38% and 63% at weed infestation levels of 6 and 24 plants m−2, respectively (Table 1). Grain yield and spikes per square meter of wheat remained similar at A. fatua infestation levels of 24 and 48 plants m−2.
Avena sterilis Interference in Wheat
Avena sterilis panicles per square meter increased from 13 to 80 as the infestation level increased from 3 to 48 plants m−2 (Table 2). Avena sterilis biomass reached 83 and 302 g m−2 at densities of 3 and 24 plants m−2, respectively. Avena sterilis seed production reached 766 and 3,967 seeds m−2 at densities of 3 and 24 plants m−2, respectively. (Table 2). Avena sterilis biomass remained similar at infestation levels of 24 and 48 plants m−2. Seed retention of A. sterilis varied from 64% to 80%, depending on the infestation level; the lowest was at 3 plants m−2 and the highest at 48 plants m−2.
Table 2. Effect of Avena sterilis infestation levels on the weed and yield parameters in wheat.
a NS, nonsignificant.
Wheat height was not influenced by A. sterilis infestation levels (Table 2). In the weed-free plots, wheat plants produced 398 spikes m−2, which were decreased by 27% and 48% at infestation levels of 6 and 24 plants m−2, respectively (Table 2). A similar trend was observed for grain yield. Grain yield in a weed-free environment was 5,600 kg ha−1, and it was reduced by 36% and 60% at A. sterilis infestation levels of 6 and 24 plants m−2, respectively. Grain yield and spikes per square meter of wheat remained similar at A. sterilis infestation levels of 24 and 48 plants m−2 (Table 2). Wheat grain numbers per spike were not influenced by A. sterilis infestation levels. However, the 1,000-grain weight of wheat was 40 g in the weed-free environment, reduced by 7% and 21% at A. sterilis infestation levels of 6 and 48 plants m−2 (Table 2).
This study reports the interference of A. fatua and A. sterilis in wheat at various infestation levels. Results revealed that both A. fatua and A. sterilis behaved similarly for yield reduction in wheat (Figure 1). Grain yield of the weed-free plot in the A. sterilis experiment was lower than in the A. fatua experiment, due to site difference. Based on the three-parameter logarithmic model, A. fatua and A. sterilis densities corresponding to 50% yield reduction were 15 and 16 plants m−2, respectively (Figure 1). Avena fatua and A. sterilis caused yield reductions of 75% and 71%, respectively, at the infestation level of 48 plants m−2. The yield reduction in wheat was primarily due to a lower number of spikes per unit area and reduced 1,000-grain weight because of weed–crop competition. This suggests that early competition of A. fatua and A. sterilis reduced wheat spikes per unit area and late competition reduced the 1,000-grain weight.
Figure 1. (A) Wheat yield reduction (%) and (B) weed seed production as influenced by weed infestation level. AF, Avena fatua; AL, Avena sterilis. Lines represent a three-parameter logarithm regression model.
Based on the logarithmic model, A. fatua and A. sterilis biomass values corresponding to the 50% yield reduction were 221 and 237 g m−2, respectively (Figure 2). The logarithmic model also indicated that A. fatua and A. sterilis produced 3,671 and 3,021 seeds m−2 at the weed infestation level of 24 plants m−2, and seed production of A. fatua and A. sterilis further increased to 5,065 and 3,938 seeds m−2, respectively, at the 48 plants m−2 density (Figure 1). The high level of seed production of A. fatua and A. sterilis enhances their adaptive potential to become dominant weeds. This study revealed that seed retention was higher in A. sterilis (64% to 80%) than A. fatua (17% to 38%), suggesting a better opportunity for harvest weed seed control for managing A. sterilis. Further, lower seed retention of A. fatua and A. sterilis at 48 and 24 plants m−2, respectively, suggests that a very high infestation level of A. fatua and A. sterilis in the wheat field could change the impact of seed destruction, as the high density of weeds increased their seed-shattering tendency.
Figure 2. Relationship of wheat yield reduction (%) and weed biomass. (A) Avena fatua (AF) and (B) Avena sterilis (AL). Lines represent a three-parameter logarithm regression model.
Weeds compete with crop plants for growth resources; therefore, the reduction in grain yield of wheat due to infestation of A. fatua and A. sterilis is logical. Overall, our results are in close conformity with previous weed interference studies, which revealed that with the increasing density of A. fatua and A. sterilis, the grain yield of wheat was decreased (Balyan et al. Reference Balyan, Malik, Panwar and Singh1991; Martin et al. Reference Martin, Cullis and McNamara1987; Walia and Brar Reference Walia and Brar2001).
Previous studies suggested that biological attributes of A. fatua and A. sterilis enabled these weeds to survive harsh conditions, which aided in the successful completion of life cycles in a wide range of environments and water-stress conditions (Mahajan and Chauhan Reference Mahajan and Chauhan2021a; Sahil et al. Reference Sahil, Loura, Raymont and Chauhan2020). These studies also suggested that A. fatua and A. sterilis tend to produce enough seeds to replenish the soil seedbank, leading to their persistence in the agroecosystems. In another study in Australia, it was found that A. fatua and A. sterilis tend to produce multiple cohorts under a wide range of climatic conditions (Mahajan and Chauhan Reference Mahajan and Chauhan2021b). These studies suggest A. fatua and A. sterilis can infest a wheat crop at varied planting times and cause substantial yield losses in wheat if not managed in a timely way. Sole reliance on chemical weed control may not provide effective control of A. fatua and A. sterilis due to their evolution of herbicide resistance against a large number of herbicides in Australia (Storrie Reference Storrie2007, Reference Storrie2019). Therefore, IWM approaches involving cultural weed control methods, such as improved crop competition, harvest weed seed control tactics, and judicious herbicide use could provide better control of A. fatua and A. sterilis.
The present study revealed that there is a great opportunity for harvest weed seed control for A. sterilis, as its seed retention at crop maturity is very high. Our results also demonstrated that high infestation levels of A. fatua and A. sterilis in a wheat field could change the impact of seed destruction, as the shattering tendency of weeds increased at high weed pressure. In another study, seed longevity of A. fatua and A. sterilis was found to be less than 12 mo for the surface seeds (Mahajan and Chauhan Reference Mahajan and Chauhan2021b). These results suggest that in no-till production systems in Australia, effective control of A. sterilis can be achieved by adopting harvest weed seed control tactics in an IWM program. As these tactics (harvest weed seed control in IWM) could restrict seed replenishment in the soil, the remaining seedbank on the surface could decay within a year as the seed persistence of A. sterilis on the surface is short. However, in paddocks where A. fatua is dominant, or where mixed populations of A. fatua and A. sterilis occur, an attempt at early control of these species should be made with suitable PRE and POST herbicides to restrict yield loss in wheat. Delayed crop sowing, pre- and post-sowing tillage, summer fallowing, closer row spacing, and exploring weed-competitive cultivars that help in early canopy closure are valuable cultural strategies that can be combined with PRE and POST herbicides for early control of A. fatua and A. sterilis in the field (Brown Reference Brown1953; Harker et al. Reference Harker, O’Donovan, Turkington, Blackshaw, Lupwayi, Smith, Johnson, Pageau, Shirtliffe, Gulden, Rowsell, Hall and Willenborg2016).
Various studies reported that the use of integrated approaches reduced biomass of A. fatua and A. sterilis by up to 90%, even when the populations were herbicide resistant (Anderson Reference Anderson2003; Beckie Reference Beckie2006; Blackshaw et al. Reference Blackshaw, Harker, O’Donovan, Beckie and Smith2008; Harker et al. Reference Harker, O’Donovan, Irvine, Turkington and Clayton2009). Similarly, our results suggest that the adoption of a suitable integrated management program, including harvest weed seed control, could be the key to the successful management of A. fatua and A. sterilis. Our results demonstrated that the lowest weed density (i.e., 3 plants m−2) did not cause a significant yield loss in wheat, but this infestation level produced sufficient seeds for reinfestation if not controlled. Therefore, an attempt should be made to completely control A. fatua and A. sterilis in the field.
In conclusion, this study revealed that A. fatua and A. sterilis infestation levels corresponding to 50% wheat yield loss were 15 and 16 plants m−2, respectively. Further, our study suggests that more data are needed to relate yield loss parameters to genetic or environmental variables. Carlson and Hill (Reference Carlson and Hill1985) suggested that the competitive ability of Avena spp. may vary with different wheat cultivars, fertilizer management, moisture regimes, and variation in Avena species and biotypes. Tillering capacity of wheat cultivars and relative time of emergence of Avena spp. and wheat may also influence the competitive ability of Avena spp. and wheat. In the eastern grain region of Australia, the major factors limiting weed-free wheat yields, like available soil water, nitrates, and delayed sowing, may affect the extent of competition between the crop and weed. Therefore, further research is needed to explore the potential of these practices under a wide range of environmental conditions and variables.
Supplementary material
To view supplementary material for this article, please visit https://doi.org/10.1017/wsc.2021.25
Acknowledgments
This research received funding from Grains Research and Development Corporation (GRDC). The authors declare no conflicts of interest.
