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Resistance to herbicides in weeds is widespread in the northern grains region of Australia. To reduce continued reliance on herbicides and improve the control of herbicide resistant weeds, alternative non-chemical and agronomic approaches for weed management are being explored. One such option is growing a competitive crop by manipulating crop row spacing, density and cultivar. Over a period of five years (2017 – 2022), researchers in the northern grains region have been investigating the effect of growing competitive summer (sorghum and mungbean) and winter (faba bean, chickpea and wheat) crops on key weeds Echinochloa colona, Chloris virgata (summer crops) and Sonchus oleraceus and Conyza bonariensis (winter crops). Over the six experimental years, a total of 22 summer and 49 winter field trials were conducted in Wagga Wagga (winter only), Narrabri and Southeast Queensland at either Hermitage, Wellcamp or Kingaroy. Competition effects were assessed through the collection of weed biomass, weed seed production and crop yield data. The large data set is being analysed to identify consistent trends across sites and seasons with each crop by weed combination being analysed independently. While results are still pending, preliminary summation of the data suggests narrow row spacing is a reliable agronomic approach for increasing crop competitiveness and reducing weed growth and seed production. Increasing crop density has also been shown to reduce weed growth and reproduction but results have been more variable. The impact of cultivar is also variable, being greatly influenced by location and seasonal conditions. The impact of narrow row spacing and increased crop density on crop yield has differed between crops, sites and seasons. However, yield responses were generally found to be positive in favourable growing seasons. Results of the meta analysis will help inform decision making on the fit and application of growing a competitive crop.

In the northern grain region of Australia, it is increasingly common for ‘summer’ weed species to emerge in early spring within winter grown crops. This emergence typically coincides with the loss of residual herbicide activity, at a stage when advanced crop growth and canopy closure make seedlings difficult to target. However, these weeds can potentially be suppressed by enhancing winter crop competition effects on late-season emerging weed seedlings. The aim of this study then was to determine if reduced row spacing and increased wheat plant density prevented the establishment of summer weeds late in the winter growing season. Field trials at Narrabri, NSW and Hermitage, Qld measured the impact of wheat row spacing (25 cm and 50 cm) and plant density (75, 100 and 125 plants m-2) on the growth and seed production of common sowthistle, fleabane, awnless barnyard grass and feathertop Rhodes grass plants emerging at wheat pre-booting stage. Weeds biomass was significantly reduced to 83% when row spacing was reduced from 50 to 25cm. Increasing wheat plant density from 75 to 125 plants m-2 reduced the biomass of weeds by 49%. These results indicate that early emerging summer weeds can be controlled in wheat by maximizing crop competition through manipulating agronomic practices.

Take home message • Feathertop Rhodes grass (FTR) and awnless barnyard grass (ABG) are both difficult to control summer grass weeds with both species prone to herbicide resistance evolution • Growing a competitive sorghum or mungbean crop can reduce growth and seed production of FTR and ABG • ABG is more susceptible to the impacts of crop competition than FTR • Sorghum competitiveness can be increased by growing the crop at a narrow row spacing (50 cm) and increased density (10 to 15 plants/m2) • Mungbean competitiveness is most effectively increased through the use of narrow row spacing (25 and 50 cm) • Consider growing a competitive summer crop to take pressure off relying solely on in-crop herbicides for summer grass control.

Trials in the Condamine-Balonne basin, Australia, compared 11 promising perennial pasture grass accessions (4 Bothriochloa, 2 Cenchrus, 2 Urochloa and 1 each of Digitaria, Eragrostis and Panicum species) against the best similar commercial cultivars on the basis of ease of establishment from seed, persistence once established, forage yield and ease of seed production. Accessions sown at a site were determined by prior experience with them on a range of soils. High quality seed was relatively easy to produce for both Urochloa species and for Eragrostis curvula CPI 30374 but problematic for the Bothriochloa spp. Once established, all accessions persisted for 3–5 years and most were well grazed, but adequate establishment was sometimes a problem with Panicum stapfianum and Bothriochloa ewartiana. The dry matter yield ratings of the non-commercial lines were similar to those of the commercial equivalents of the same species. While agronomically valuable, none of the promising new grasses was considered worthy of commercialization at this point because their strengths did not warrant the setting up of a seed-production business in competition with current commercial enterprises. Long-standing cultivars such as Gayndah buffel and Nixon sabi grass continued to exhibit their superior pasture qualities.

Stay-green plants retain green leaves longer after anthesis and can have improved yield, particularly under water limitation. As senescence is a dynamic process, genotypes with different senescence patterns may exhibit similar final normalised difference vegetative index (NDVI). By monitoring NDVI from as early as awn emergence to maturity, we demonstrate that analysing senescence dynamics improves insight into genotypic stay-green variation. A senescence evaluation tool was developed to fit a logistic function to NDVI data and used to analyse data from three environments for a wheat (Triticum aestivum L.) population whose lines contrast for stay-green. Key stay-green traits were estimated including, maximum NDVI, senescence rate and a trait integrating NDVI variation after anthesis, as well as the timing from anthesis to onset, midpoint and conclusion of senescence. The integrative trait and the timing to onset and mid-senescence exhibited high positive correlations with yield and a high heritability in the three studied environments. Senescence rate was correlated with yield in some environments, whereas maximum NDVI was associated with yield in a drought-stressed environment. Where resources preclude frequent measurements, we found that NDVI measurements may be restricted to the period of rapid senescence, but caution is required when dealing with lines of different phenology. In contrast, regular monitoring during the whole period after flowering allows the estimation of senescence dynamics traits that may be reliably compared across genotypes and environments. We anticipate that selection for stay-green traits will enhance genetic progress towards high-yielding, stay-green germplasm.

Cereal crops can suffer substantial damage if frosts occur at heading. Identification of post-head-emergence frost (PHEF) resistance in cereals poses a number of unique and difficult challenges. Many decades of research have failed to identify genotypes with PHEF resistance that could offer economically significant benefit to growers. Research and breeding gains have been limited by the available screening systems. Using traditional frost screening systems, genotypes that escape frost injury in trials due to spatial temperature differences and/or small differences in phenology can be misidentified as resistant. We believe that by improving techniques to minimize frost escapes, such ofalse-positive' results can be confidently identified and eliminated. Artificial freezing chambers or manipulated natural frost treatments offer many potential advantages but are not yet at the stage where they can be reliably used for frost screening in breeding programmes. Here we describe the development of a novel photoperiod gradient method (PGM) that facilitates screening of genotypes of different phenology under natural field frosts at matched developmental stages. By identifying frost escapes and increasing the efficiency of field screening, the PGM ensures that research effort can be focused on finding genotypes with improved PHEF resistance. To maximize the likelihood of identifying PHEF resistance, we propose that the PGM form part of an integrated strategy to (i) source germplasm;(ii) facilitate high throughput screening; and (iii) permit detailed validation. PGM may also be useful in other studies where either a range of developmental stages and/or synchronized development are desired.

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