Glyphosate Efficacy in Chloris virgata Sw. in Response to Temperature and Tank Mixing.

Glyphosate alone or a tank mixture of glyphosate and 2,4-D is commonly used for broad-spectrum weed control under fallow conditions in Australia. Air temperature or mixing glyphosate with 2,4-D, may influence the efficacy of glyphosate on feather fingergrass (Chloris virgata Sw.), a problematic summer-season weed of Australia. Dose-response studies were conducted with four populations of feather fingergrass under temperature-controlled glasshouse conditions (35/25 °C and 25/15 °C at 12 h/12 h) to assess the level of glyphosate resistance in relation to temperature regimes. Four parameter log-logistic models were used to develop dose-response curves. Based on plant mortality percentage, LD50 (lethal dose for 50% mortality) values of glyphosate at 25/15 °C for populations Ch, SGM2, SGW2, and CP2 were 137, 60, 650, and 1067 g ae ha-1, respectively. However, at 35/25 °C, the corresponding LD50 values were 209, 557, 2108, and 2554 g ae ha-1, respectively. A similar response was observed for the parameter GR50 (dose for 50% growth reduction) values of glyphosate. These results indicate that populations SGW2 and CP2 are highly glyphosate-resistant and in the summer season, it may be very difficult to control these populations due to poor glyphosate efficacy. These results further suggest that the efficacy of glyphosate for feather fingergrass control can be improved if applied during cooler temperatures (25/15 °C) or the spring season compared with warmer temperatures (35/25 °C) or the summer season. In another study, 2,4-D antagonized glyphosate remarkably in the CP2 (glyphosate-resistant) population but only marginally in the Ch (glyphosate-susceptible) population. Thus, it is not advisable to mix 2,4-D with glyphosate for the control of glyphosate-resistant feather fingergrass populations. The results further suggest that the use of this mixture is useful if the feather fingergrass is not glyphosate-resistant; however, the use of the mixture is to be avoided if the population is glyphosate-resistant in order to not exacerbate the potential resistance problem.

Seed germination ecology of hood canarygrass (Phalaris paradoxa L.) and herbicide options for its control.

Hood canarygrass (Phalaris paradoxa L.) is a problematic weed in winter crops of Australia. Experiments were conducted to determine the effects of environmental factors on seed germination of P. paradoxa and wheat (Triticum aestivum L.) and herbicide options for P. paradoxa control. Results revealed that P. paradoxa had higher germination (> 89%) at a temperature range from 15/5 ℃ to 25/15 ℃ [day/night (12 h/12 h] compared with 30/20 ℃ and 35/25 ℃. At a temperature regime of 30/20 ℃, P. paradoxa had 1% germination; however, wheat at this temperature range resulted in 79% germination. Exposure of seeds of P. paradoxa to > 150 ℃ pretreatment (radiant heat for 5 min) resulted in no germination. These results suggest that infestation of P. paradoxa could be reduced by residue burning or by planting wheat crops early in the season when the temperature is relatively high. At a water potential of -0.8 MPa, seed germination of P. paradoxa and wheat was 75 and 96%, respectively. Similarly, at the highest salt concentration (200 mM sodium chloride), seed germination of P. paradoxa and wheat was 73 and 79%, respectively. These observations suggest that like wheat, P. paradoxa is also highly tolerant to water and salt stress conditions, therefore, it could invade the agro-ecosystem under water and salt stress situations. Germination of P. paradoxa was found to be low (10%) on the soil surface, suggesting that no-till systems could inhibit the germination of P. paradoxa. Pre-emergent (PRE) herbicides, namely cinmethylin, pyroxasulfone, and trifluralin, provided 100% control of P. paradoxa; however, in the presence of 2 t ha-1 of residue cover, pyroxasulfone provided better control of P. paradoxa compared with other herbicides. Post-emergent (POST) herbicides clethodim, haloxyfop, and paraquat provided excellent control of P. paradoxa, even if the plant size was large (10-leaf stage). Knowledge generated from this study will help in strengthening the integrated management of P. paradoxa.

Management options for large plants of glyphosate-resistant feather fingergrass (Chloris virgata) in Australian fallow conditions.

Chloris virgata has become one of the most difficult glyphosate-resistant (GR) grass weeds in summer fallows in the eastern region of Australia. It germinates in several cohorts following rainfall events; therefore, growers are often tempted to wait for most of the weeds to emerge before herbicide application. However, by that time, some seedlings have reached an advanced stage and there is limited information on the efficacy and reliability of alternate herbicides when targeting large plants of GR C. virgata. A series of experiments were conducted to determine the efficacy of alternate herbicides for the control of GR C. virgata. Haloxyfop (80 g a.i. ha-1) on its own, in mixtures, or sequential applications of haloxyfop and paraquat or glufosinate provided 97 to 100% mortality of the 8-10 leaf stage plants. Glufosinate (1500 g a.i. ha-1) also provided complete control of plants at this growth stage. For larger plants at the 24-28 leaf stage, glufosinate, with or without additional tank-mixed adjuvants, generally did not provide full control, however did show very high levels of biomass reduction and panicle suppression at application rates of 750 or 1500 g a.i. ha-1. Haloxyfop (40 to 160 g a.i. ha-1) and clethodim (180 g a.i. ha-1) on their own achieved 96 to 100% mortality at this growth stage. When applied to large plants (40-50 leaf stage), a tank-mix of isoxaflutole plus paraquat demonstrated significantly higher levels of plant mortality and biomass reduction than either herbicide used alone, and this mixture appears to be synergistic when tested via the Colby equation for synergy or antagonism. Plant mortality was greater (83%) when isoxaflutole (75 g a.i. ha-1) plus paraquat (300 g a.i. ha-1) was taken up through the foliage and soil, compared with the foliage alone. This study identified alternative herbicide options for large plants of GR C. virgata.

Evaluation of Preemergent Herbicides for Chloris virgata Control in Mungbean.

Chloris virgata is a problematic weed in mungbean crops due to its high seed production, resistance to glyphosate and high dispersal ability. Pot and field experiments were conducted in 2020 and 2021 to evaluate a range of preemergent (PRE) herbicides for C. virgata control in mungbean. In the field and pot studies, isoxaflutole 75 g ai ha-1 caused crop injury, and in the field experiment, it reduced mungbean yield by 61% compared with the best treatment (pyroxasulfone 100 g ai ha-1). In the field and pot experiments, dimethenamid-P 720 g ai ha-1, pyroxasulfone 100 g ai ha-1 and S-metolachlor 1400 g ai ha-1 provided >88% control of C. virgata (for reduced biomass) and in the field experiment, these herbicides resulted in improved yield by 230%, 270% and 170%, respectively, compared with nontreated control (250 kg ha-1). Similarly, pendimethalin 1000 g ai ha-1 and trifluralin 600 g ai ha-1 provided >89% control (biomass) of C. virgata, and in the field experiment, these resulted in improved yields of 230% and 160%, respectively, compared with the nontreated control. PRE herbicides such as diuron 750 g ai ha-1, linuron 1100 g ai ha-1, metribuzin 360 g ha-1, terbuthylazine 750 g ai ha-1, imazapic 48 g ai ha-1 and imazethapyr 70 g ha-1 although did not cause crop injury; however, these herbicides did not control C. virgata. Flumioxazin 90 g ai ha-1 caused reduced biomass of C. virgata by 80% compared with the nontreated control, and in the field experiment, it resulted in improved yield by 140% compared with the nontreated control. This study suggests the potential use of herbicides, such as dimethenamid-P, pyroxasulfone and S-metolachlor in addition to pendimethalin and trifluralin, for C. virgata control in mungbean. Further studies are needed to determine the efficacy of dimethenamid-P, S-metolachlor and pyroxasulfone for controlling other troublesome weeds in mungbean.

Influence of soil moisture levels on the growth and reproductive behaviour of Avena fatua and Avena ludoviciana.

Adaptation of weeds to water stress could result in the broader distribution, and make weed control task increasingly difficult. Therefore, a clear understanding of the biology of weeds under water stress could assist in the development of sustainable weed management strategies. Avena fatua (wild oat) and A. ludoviciana (sterile oat) are problematic weeds in Australian winter crops. The objectives of this study were to determine the growth and reproductive behaviour of A. fatua and A. ludoviciana at different soil moisture levels [20, 40, 60, 80, and 100% water holding capacity (WHC)]. Results revealed that A. fatua did not survive and failed to produce seeds at 20 and 40% WHC. However, A. ludoviciana survived at 40% WHC and produced 54 seeds plant-1. A. fatua produced a higher number of seeds per plant than A. ludoviciana at 80 (474 vs 406 seeds plant-1) and 100% WHC (480 vs 417 seeds plant-1). Seed production for both species remained similar at 80 and 100% WHC; however, higher than 60% WHC. Seed production of A. fatua and A. ludoviciana was 235 and 282 seeds plant-1, respectively, at 60% WHC. The 60% WHC reduced seed production of A. fatua and A. ludoviciana by 51 and 32% respectively, compared to 100% WHC. The plant height, leaf weight, stem weight, and root weight per plant of A. fatua at 60% WHC reduced by 45, 27, 32, and 59%, respectively, as compared with 100% WHC. Similarly, the plant height, leaf weight, stem weight, and root weight per plant of A. ludoviciana at 60% WHC reduced by 45, 35, 47 and 76%, respectively, as compared with 100% WHC. Results indicate that A. ludoviciana can survive and produce seeds at 40% of WHC, indicating the adaptation of the species to dryland conditions. The results also suggest that A. ludoviciana is likely to be robust under water stress conditions, potentially reducing crop yield. The ability of A. fatua and A. ludoviciana to produce seeds under water-stressed conditions (60% WHC) necessitates integrated weed management strategies that suppress these weeds whilst taking into account the efficient utilization of stored moisture for winter crops.

Performance of Dry-Seeded Rice Genotypes under Varied Soil Moisture Regimes and Foliar-Applied Hormones.

Plant hormones influence various physiological processes during the growth and development of plants, but their critical roles in influencing yield and antioxidant activities in dry-seeded rice (DSR) have not been adequately explored. This study aims to analyze the performance and antioxidant activity of contrasting genotypes of DSR in response to soil moisture regimes and foliar-applied hormones. The study comprised sixteen treatments that were evaluated under field conditions as per split-plot design in three replications. Treatments comprised combinations of two soil moisture tension regimes (10 kPa and 20 kPa) and two genotypes (PR-111, non-stay-green type and PR-123, stay-green type) applied to the main plots and foliar application of three hormones (gibberellic acid (GA3) 40 mg kg-1, abscisic acid (ABA) 20 mg kg-1, and cytokinin (CK) 40 mg kg-1)) and a control (unsprayed) to subplots. The non-stay-green genotype (PR-111) resulted in 34.6% more grain yield (6.48 t ha-1) than the stay-green genotype (PR-123) at the lower soil moisture tension regime (SMTR) (10 kPa) due to the increased number of filled grains per panicle and improvement in harvest index (HI). At the higher SMTR (20 kPa), the stay-green genotype (PR-123) produced 26.4% more grain yield (5.21 t ha-1) than non-stay green genotype (4.12 t ha-1) and showed enhanced superoxide dismutase (SOD) and peroxide dismutase (POD) activity that may have contributed in maintaining sink size through improved chlorophyll content. Grain yield (6.35 t ha-1) with foliar-applied GA3 (40 mg kg-1) at SMTR of 10 kPa was higher by 12.2% and 24.0% than with foliar-applied ABA (20 mg kg-1) and unsprayed treatments, respectively. Irrigation application at SMTR of 20 kPa and foliar application of ABA gave 24.1% higher grain yield (5.15 t ha-1) than the unsprayed treatment, but it was similar to foliar-applied GA3 and CK. This study implied that the stay-green genotype (PR-123) was more suitable under moisture stress conditions (20 kPa) in DSR, as it maintained sink size even under moisture stress conditions by improving dry matter translocation and enhancing SOD and POD activity. The study suggests the need to find out the endogenous level of these plant hormones in rice genotypes under a range of water regimes to develop high yielding and water use efficient genotypes of DSR.

Growth behavior and glyphosate resistance level in 10 populations of Echinochloa colona in Australia.

Recently, poor control of Echinochloa colona with glyphosate has been reported in no-till agriculture systems of the northern grain region (NGR) of Australia. Two experiments were conducted using 10 populations of E. colona selected from the NGR of Australia to understand differences in their growth behavior and resistance pattern. Growth studies revealed that these populations differed in plant height (53-70 cm plant-1), tiller production (30-52 tillers plant-1), leaf production (124-186 leaves plant-1) and seed head production (37-65 seed heads plant-1). Days taken to seed heads and shoot biomass in these populations ranged between 40-48 d and 21-27 g plant-1, respectively. Seed production in these populations ranged between 5380 and 10244 seeds plant-1; lowest for population B17/25 and highest for population B17/13. Correlation studies revealed that seed number plant-1 had a positive correlation with tiller number plant-1 (r = 0.73) and negative relation with days taken to seed head initiation (r = - 0.65). The glyphosate dose-response study showed a wide range of responses in these populations and the glyphosate dose required to kill 50% plants (LD50 values) was estimated between 161 to 2339 g a.e. glyphosate ha-1. LD50 values of populations B17/16, B 17/34 and B17/35 were 1086, 2339 and 1153 g ha-1, respectively, making them 6.7, 15.1 and 7.2-fold resistant to glyphosate compared with the susceptible population B17/37. Growth behavior and seed production potential in these populations had no correlation with the resistance index. These results suggest that some populations of E. colona are highly problematic; for example, population B17/34 was not only highly glyphosate-resistant, but also produced a high seed number (9300 seeds plant-1). This study demonstrated that there is a possibility of great risk with the increased use of glyphosate for managing E. colona in the NGR of Australia. The results warrant integrated weed management strategies and improved stewardship guidelines are required for managing glyphosate-resistant populations of E. colona and to restrict further movement of resistant populations to other regions of Australia.

Evaluation of dormancy breaking methods for enhanced germination in four biotypes of Brassica tournefortii.

Brassica tournefortii is an important broadleaf weed of the winter season in the northern grain region of Australia. Knowledge of germination ecology of this weed would help in implementing effective weed control programs. A series of experiments were conducted to study the germination and dormancy behavior of four biotypes of B. tournefortii seeds, biotypes A (collected from barley crop), B (barley fence lines), C (chickpea crop), and D (chickpea fence lines), collected from the St George region of Queensland. The aim of this research was to determine the effectiveness of various methods on the seed dormancy release of B. tournefortii. Water, potassium nitrate and a soil extract did not release dormancy in B. tournefortii seeds (biotype A) at 20/10 °C in the light/dark regime. Cold stratification (5 °C) also did not improve germination. However, gibberellic acid (GA3; 100-300 mg kg-1) stimulated germination (>88%). Germination also improved when seeds were immersed in sodium hypochlorite (NaOCl; 42 g L-1) for 10 minutes and the effect was more pronounced under the complete dark environment (89% germination at a day/night temperature of 20/10 °C). The NaOCl treatment makes seeds more porous and decreases sensitivity to light. Another experiment in light/dark conducted at 25/15 °C with two biotypes (A and D) showed that, without NaOCl treatment, biotype A was more sensitive to light (29% germination) as compared to biotype D (92% germination). Our results suggest that dormancy in B. tournefortii seeds can be broken by the combination of NaOCl (10 min) and a dark environment. A day/night temperature of 25/15 °C was found best for optimum germination (>87%) for all the biotypes (A-D) when incubated in dark after treating with NaOCl. This research indicated a high degree of variability in germination responses for various biotypes of B. tournefortii seeds to various sets of conditions, which may be due to metabolic changes in response to maternal environments or genetically controlled mechanisms. Information gained from this study will be important in developing a better understanding of the dormancy behavior of B. tournefortii seeds in response to tillage systems or maternal environments that could influence the weed seed bank in the soil and therefore help in designing suitable weed management programs.

Effect of Soil Moisture Regimes on Growth and Seed Production of Two Australian Biotypes of Sisymbrium thellungii O. E. Schulz.

Sisymbrium thellungii O. E. Schulz is an emerging problematic weed in the northern grain region of Australia. Several different biotypes exist in this region but not all biotypes exhibit the same growth and reproduction behavior. This might be due to local adaptation to the different agro-ecosystems, however, information on this aspect is limited. To determine whether adaptation to water stress was a factor in biotype demographic growth and reproduction behavior, we evaluated the physiological and biochemical responses of two Australian S. thellungii biotypes, selected from high (Dalby) and medium (St. George) rainfall areas, to different pot soil moisture levels corresponding to 100, 75, 50, and 25% of soil water holding capacity (WHC). Averaged across moisture levels, the St. George biotype (medium rainfall area) had 89% greater biomass and produced 321% more seeds than the Dalby biotype. The St. George biotype was less affected by increased levels of water stress than the Dalby biotype. The Dalby biotype produced 4,787 seeds plant-1 at 100% WHC and only 28 seeds plant-1 at 25% WHC. On the other hand, the St. George biotype produced 4,061 seeds plant-1 at 25% WHC and its seed production at 100% WHC was 9,834 seeds plant-1. On a per leaf area basis and averaged across all moisture levels, the St. George had significantly lower net carbon assimilation compared with the Dalby biotype, accompanied by a trend for lower stomatal conductance, which might indicate an adaptation to water stress. Across the moisture levels, the St. George biotype had higher phenolics and total soluble sugar, but free proline content was higher in the Dalby biotype compared with the St. George biotype. Like total soluble sugar, proline content increased with water stress in both biotypes, but it increased to a greater extent in the Dalby biotype, particularly at the 25% of WHC. Branching, flowering and maturity occurred earlier in the St. George biotype compared with the Dalby biotype, indicating relatively faster growth of the St. George biotype, which again seems to be an adaptation to water-limited environments. In conclusion, the St. George biotype of S. thellungii had higher reproductive capacity than the Dalby biotype across all the moisture regimes, which suggests greater invasiveness. Overall, the large size and rapid growth of the S. thellungii population from the medium rainfall area, together with its physiological response to water stress and its ability to maintain seed production in dry conditions, may enable this biotype to become widespread in Australia.

Emerging Challenges and Opportunities for Education and Research in Weed Science.

In modern agriculture, with more emphasis on high input systems, weed problems are likely to increase and become more complex. With heightened awareness of adverse effects of herbicide residues on human health and environment and the evolution of herbicide-resistant weed biotypes, a significant focus within weed science has now shifted to the development of eco-friendly technologies with reduced reliance on herbicides. Further, with the large-scale adoption of herbicide-resistant crops, and uncertain climatic optima under climate change, the problems for weed science have become multi-faceted. To handle these complex weed problems, a holistic line of action with multi-disciplinary approaches is required, including adjustments to technology, management practices, and legislation. Improved knowledge of weed ecology, biology, genetics, and molecular biology is essential for developing sustainable weed control practices. Additionally, judicious use of advanced technologies, such as site-specific weed management systems and decision support modeling, will play a significant role in reducing costs associated with weed control. Further, effective linkages between farmers and weed researchers will be necessary to facilitate the adoption of technological developments. To meet these challenges, priorities in research need to be determined and the education system for weed science needs to be reoriented. In respect of the latter imperative, closer collaboration between weed scientists and other disciplines can help in defining and solving the complex weed management challenges of the 21st century. This consensus will provide more versatile and diverse approaches to innovative teaching and training practices, which will be needed to prepare future weed science graduates who are capable of handling the anticipated challenges of weed science facing in contemporary agriculture. To build this capacity, mobilizing additional funding for both weed research and weed management education is essential.

Protein and starch characteristics of milled rice from different cultivars affected by transplantation date.

The effects of transplantation date on milled rice (physicochemical, amino acids composition and protein profiling) of different cultivars and their starch characteristics (granules size distribution, pasting and thermal) were investigated. Head rice yield increased (2.0-4.1 %) and chalky grains (5-10 %) decreased with delaying the paddy transplantation of different cultivars by 20 days. Delayed transplantation of paddy significantly increased asparagine, glutamine, threonine, cysteine, methionine, tryptophan, lysine and proline content in milled rice. Early transplantation of paddy showed higher accumulation of glutelin and prolamines than that in milled rice from delayed transplantation. The change in amino acid composition of milled rice with delay in transplantation was related to variation in accumulation of glutelin and prolamines. Starch from delayed transplanted paddy showed higher peak viscosity and lower breakdown viscosity than those from early transplanted paddy. These differences were due to higher accumulation of amylose in starch from delayed transplanted paddy than that from early transplanted paddy due to exposure of former to lower night air temperature during starch synthesis.

Effect of different doses of nitrogen on protein profiling, pasting and quality attributes of rice from different cultivars.

Effect of different level (60, 120 and 180 kg N/ha) of nitrogen (N) application on protein profiling, pasting and cooking quality characteristics of milled rice from different paddy cultivars was evaluated. N dose showed positive correlation with protein content and negative correlation with L*, whiteness and amylose content. N application significantly affected the protein profile, textural and pasting properties of different cultivars. All the cultivars expect PR120 and PAU201, showed an increase in the amount of accumulation of 60 kDa polypeptide with increase in N application. Accumulation of prolamines (16 and 14 kDa) and polypeptides of 38 and 35 kDa increased in all the cultivars. Size exclusion chromatography revealed decrease in low molecular weight subunits and increase in medium molecular weight subunits in all the cultivars upon N application. However, high molecular weight subunits increased in IET21214 and decreased in PR120 and PAU20 upon N application. N application resulted in increase in glutelins and decrease in peak and breakdown viscosity. PAU201 and PR120 showed lower AAC due to low accumulation of 60 kDa granule-bound starch synthase (GBSS), in response to N application. Gumminess and hardness of cooked rice increased with the increase in N dose and the increase was significant at 60 kg N/ha.

Response of rice genotypes to weed competition in dry direct-seeded rice in India.

The differential weed-competitive abilities of eight rice genotypes and the traits that may confer such attributes were investigated under partial weedy and weed-free conditions in naturally occurring weed flora in dry direct-seeded rice during the rainy seasons of 2011 and 2012 at Ludhiana, Punjab, India. The results showed genotypic differences in competitiveness against weeds. In weed-free plots, grain yield varied from 6.6 to 8.9 t ha(-1) across different genotypes; it was lowest for PR-115 and highest for the hybrid H-97158. In partial weedy plots, grain yield and weed biomass at flowering varied from 3.6 to 6.7 t ha(-1) and from 174 to 419 g m(-2), respectively. In partial weedy plots, grain yield was lowest for PR-115 and highest for PR-120. Average yield loss due to weed competition ranged from 21 to 46% in different rice genotypes. The study showed that early canopy closure, high leaf area index at early stage, and high root biomass and volume correlated positively with competitiveness. This study suggests that some traits (root biomass, leaf area index, and shoot biomass at the early stage) could play an important role in conferring weed competitiveness and these traits can be explored for dry-seeded rice.