David Roget

On-farm assessment of environmental and management constraints to wheat yield and efficiency in the use of rainfall in the Mallee

The responses of wheat grain yield to soil properties, weather, root diseases, and management practices were investigated in 75 grower-managed crops in the Mallee region of South Australia, Victoria, and New South Wales during 3 growing seasons. Fourteen cultivars were represented in the sampled crops, with Frame being the most common (56%). The most widespread crop sequence was wheat after pasture (43% of wheat crops), followed by wheat after fallow or cereal (both about 20%); 12% of the wheat was sown after legumes. Wheat after cereal was more common in drier sites, and wheat after fallow in wetter sites. Wheat yield was proportional to Fischer’s photothermal coefficient around flowering, and ranged from nil to 4.7 t/ha. On average, wheat crops sown after cereals yielded 0.4 t/ha less than their counterparts sown after fallow, and 0.7 t/ha less than those after legumes. Sowing date ranged from 24 April to 21 July; yield declined with delayed sowing at an average rate of 17 kg/ha.day. Growing season rainfall (April–October) ranged from 111 to 266 mm, and accounted for 27% of the variation in grain yield. Soil water content at sowing (0–1 m) ranged from 32 to 330 mm; yield increased with initial soil water at an average rate of 6 kg/ha.mm. Grain yield per unit growing season rainfall was generally low, with 75% of crops producing <12 kg grain/ha.mm; the maximum ratio was 21 kg/ha.mm. Soil constraints, including sodicity, alkalinity, salinity, and boron toxicity, reduced yield in part by reducing availability of stored soil water. Owing to severity of chemical constraints increasing with soil depth, grain yield and yield per unit growing season rainfall were both inversely related to the proportion of water stored deeper in the soil (0.5–1 m). Yield was unrelated to nitrogen, both initial and applied. Larger amounts of nitrogen accumulated in soils with more severe constraints partially accounted for the lack of association between yield and nitrogen.

Dynamic cropping strategies for risk management in dry-land farming systems

Abstract Low rainfall and soils with low water holding capacity are major features of the Mallee region of south-eastern Australia. To reduce risk related to likely water deficits, many growers adopt a conservative cropping strategy based on low-input cereal crops. This approach has, however, a substantial opportunity cost by missing the potential profit from more intense cereal cropping and high yielding oilseed crops in more favourable seasons. We tested the hypothesis that whole-farm profitability could be enhanced by the adoption of a dynamic cropping strategy shifting from a cereal-only, conservative strategy in dry years, to a more risky strategy involving both cereals and canola in wet years. To test this hypothesis, we used 40-years rainfall series to (1) investigate rainfall features in 11 locations in the Mallee, (2) test the skill of simple rules to predict seasonal rainfall, as developed by local farmers, and (3) calculate whole-farm profit for conservative, risky and dynamic cropping strategies. Rainfall and profit were linked with a whole-farm model that estimates crop yield as a function of seasonal rainfall (i.e. rainfall from April to October) and water-use efficiency. Among locations, annual rainfall ranged from 259 to 358 mm. For each location, two types of seasons were defined: likely wet, when April rain was above the median, and likely dry otherwise. The strength of the association between April and seasonal rain varied widely among sites; it was stronger in locations with more marked rainfall seasonality. Contrasting whole-farm profit responses to cropping strategies were found in locations with annual rainfall below or above a threshold around 300 mm. For wetter locations (annual rain above the threshold), the more risky cropping strategy including canola was generally more profitable than the more conservative strategy. For farms in drier areas, the cereal-based conservative strategy outperformed the more risky strategy in seasons predicted to be dry, but was less profitable in wet seasons. The dynamic cropping strategy had a substantial effect on extreme years, alleviating economic losses associated with the risky strategy in dry seasons, while being able to capture the benefits of more favourable seasons. Analysis of rainfall patterns, development of a rainfall forecasting procedure, and quantification of whole-farm profit in response to cropping strategies, all highlighted the need for decision support tools that account for small-scale variation in rainfall characteristics.

Fungal community structure in disease suppressive soils assessed by 28S LSU gene sequencing.

Natural biological suppression of soil-borne diseases is a function of the activity and composition of soil microbial communities. Soil microbe and phytopathogen interactions can occur prior to crop sowing and/or in the rhizosphere, subsequently influencing both plant growth and productivity. Research on suppressive microbial communities has concentrated on bacteria although fungi can also influence soil-borne disease. Fungi were analyzed in co-located soils ‘suppressive’ or ‘non-suppressive’ for disease caused by Rhizoctonia solani AG 8 at two sites in South Australia using 454 pyrosequencing targeting the fungal 28S LSU rRNA gene. DNA was extracted from a minimum of 125 g of soil per replicate to reduce the micro-scale community variability, and from soil samples taken at sowing and from the rhizosphere at 7 weeks to cover the peak Rhizoctonia infection period. A total of ∼994,000 reads were classified into 917 genera covering 54% of the RDP Fungal Classifier database, a high diversity for an alkaline, low organic matter soil. Statistical analyses and community ordinations revealed significant differences in fungal community composition between suppressive and non-suppressive soil and between soil type/location. The majority of differences associated with suppressive soils were attributed to less than 40 genera including a number of endophytic species with plant pathogen suppression potentials and mycoparasites such as Xylaria spp. Non-suppressive soils were dominated by Alternaria, Gibberella and Penicillum. Pyrosequencing generated a detailed description of fungal community structure and identified candidate taxa that may influence pathogen-plant interactions in stable disease suppression.

Principles and Management of Soil Biological Factors for Sustainable Rainfed Farming Systems

Soil microflora and fauna are important for organic matter decomposition and hence nutrient cycling, organic matter turnover, disease incidence and suppression, agrochemical degradation and soil structure. Soil moisture, temperature and availability of energy source (carbon) determine the activity of these organisms. Biological activity and plant growth must be synchronised for optimum production. Control of soil-borne root pathogens is important in maximising water use efficiency. The beneficial influences of soil biota include nitrogen fixation, nutrient cycling and supply, improved soil structure, promotion of plant and root growth, and disease control or suppression. Detrimental influences include those of root pathogens and deleterious rhizobacteria.

Potential for non-symbiotic N2-fixation in different agroecological zones of southern Australia

Nitrogen fixation by symbiotic and non-symbiotic bacteria can be a significant source of nitrogen in cropping systems. However, contributions from non-symbiotic nitrogen fixation (NSNF) are dependent on available carbon in the soil and environmental conditions (soil moisture and temperature). In Australia, measurements of NSNF have been made in the field by quantifying nitrogenase activity. These studies have included determinations of the moisture and temperature requirements for NSNF and for crop residue decomposition that supplies carbon to NSNF bacteria. Other studies have determined the N input by NSNF using N budget calculations. These data together with information about carbon supply and environmental conditions were used to estimate potential NSNF in the cropping zones of southern Australia. Using the ArcviewGIS Spatial Analyst (v3.1), maps of Australia showing estimates of NSNF in different cropping zones as determined by rainfall and temperature or carbon availability were generated. In Western Australia (represented by Wongan Hills) and South Australia (represented by Avon), where summers are dry, estimates of NSNF were generally low (10–15 kg N/ha from January to June) due to limitations of soil moisture. In New South Wales, particularly in the north where summer rainfall patterns develop (represented by Gunnedah), the warm, moist conditions produced higher estimates of NSNF (totaling 32–38 kg N/ha from January to June). In this region, the majority of estimated NSNF occurred in January and February leading to the depletion of carbon supplies and reduced NSNF in autumn (March–June). Information about potential supplies of N from NSNF across the cropping zones should be useful for researchers to select and study areas that are most likely to benefit from NSNF. It should also help agronomists and extension officers explain changes in N status within paddocks or within specific farming systems and to provide more accurate advice on N fertiliser requirements, particularly in low-input farming systems.

Suppression of Rhizoctonia solani AG-8 induced disease on wheat by the interaction between Pantoea, Exiguobacterium, and Microbacteria

Rhizoctonia solani AG-8 is a major wheat root pathogen; however, soils can become suppressive to the expression of disease under intensive cropping with retention of crop residues. This is in part due to the action of soil microorganisms. A step-wise approach was used to determine which microorganisms contributed to suppression of R. solani induced disease in a disease-suppressive soil. Using wheat-soil-pathogen bioassays it was determined that the interaction between 3 phylogenetically diverse groups of bacteria, Pantoea agglomerans, Exiguobacterium acetylicum, and Microbacteria (family Microbacteriaceae), was a major contributor to disease suppression. Inoculation of a sterilised soil with the combination of these groups resulted in greatly increased seedling shoot dry weight and reduced infection compared with diseased control plants with no bacterial inoculation, or inoculated with individual types of bacteria. These groups, however, did not reduce levels of pathogen DNA, although inoculation with suppressive soil (at 10% w/w) did reduce pathogen DNA. Root associated P. agglomerans and E. acetylicum promoted the growth of infected wheat plants and soil associated Microbacteria reduced root infection by R. solani.

On-farm assessment of environmental and management factors influencing wheat grain quality in the Mallee

We used data from 63 grower-managed wheat crops during 3 growing seasons in the Mallee to explore grain protein responses to environmental and management factors. Allometric coefficients were calculated as the slope of the regression between the mass of log-transformed protein and non-protein grain components to account for the effect of ontogenetic drift on grain protein concentration. Test weight and screenings were also investigated. Grain protein concentration ranged from 8.7 to 16.2%; 90% of crops had less than 5% screenings, and 95% had test weight above 74 kg/hL. Screenings increased and test weight declined with increasing concentration of protein, particularly for protein concentration above 13%. Fourteen cultivars were represented in the sampled crops. In comparison with crops of varieties eligible as Australian Premium White, crops of hard wheats had greater protein content, more screenings, lower test weight, and a greater protein : non-protein allometric coefficient, indicating differences in the pattern of protein allocation between these groups of cultivars. Protein concentration declined with increasing yield at a rate a1%/t.ha. It decreased with increasing seasonal rainfall at a rate of 0.014%/mm, and increased with the proportion of water stored below 0.5 m at a rate of 0.121%/%. Delayed sowing between mid April and mid July generated a size-dependent increase in grain protein concentration of 0.027%/day. Increasing protein content could attenuate the profit lost due to delayed sowing by up to AU

Evaluating the contribution of take-all control to the break-crop effect in wheat

39/ha in hard wheats. Wheat grown after legumes accumulated 64% more protein and 47% more non-protein material in the grain than their counterparts grown after cereal, and grain protein concentrations averaged 13.3 and 12.2% respectively. Protein concentration was unrelated to the amount of nitrogen in the whole soil profile (0-1 m), and weakly associated with the amount of initial nitrogen in the 0-0.1 m soil layer; it increased at a rate of 0.038%/kg N.ha. Chemical constraints in the subsoil probably affected the ability of the crop to use, and contributed to the accumulation of nitrogen in deep soil layers.

Are Legumes Doing Their Job? The Effect of Herbicides on N2 Fixation in Southern Australian Agricultural Systems

Abstract. Break-crops such as legumes and oilseeds increase the yield of subsequent cereal crops by reducing the level of diseases and weeds that build in continuous cereal crops, and can also improve water and nitrogen supply. Although the term ‘break-crop’ originates from their role in breaking disease cycles of soil-borne diseases such as take-all (caused by Gaeumannomyces graminis var. tritici), the contribution of take-all control to the overall break-crop effect has not been separated in most studies. We re-analysed a range of medium- and short-term crop-sequencing experiments comprising 18 year × site combinations in diverse environments in southern Australia. An analysis using linear mixed effects models was conducted to: (i) define the agro-environments that lead to increases in take-all incidence in continuous wheat crop sequences, (ii) quantify the effect of take-all on wheat yield, and (iii) ascertain the contribution of the reduction in take-all following break-crops to the size of the total break-crop effect on wheat crop yield. Break-crop effects on wheat yield averaged 0.7 t/ha and ranged from 0 to 2.1 t/ha. On 14 of 18 occasions, take-all contributed to reduced wheat yield in continuous wheat rotations, although the estimated effect exceeded 0.1 t/ha on just six of those occasions. As a result, reduced take-all by break-crops contributed to <20% of the total break-crop effect in all but one instance, where the suppression accounted for 80% of the break-crop effect. In summary, although the break-crops improved wheat yield by 0.7 t/ha, the contribution from take-all control in the 14 locations where it could be quantified was just 0.1 t/ha. Correlation analysis revealed that take-all incidence in wheat was most likely to proliferate in colder, wetter environments. Take-all can severely damage crop yield, and the reduction contributes to the break-crop effect, but the average impact on wheat yield is small and poorly correlated with the potential yield of the wheat crop. The analytical approach helped to quantify the effect of take-all damage on crop yield, to provide further insight into the agro-environment that contributes to high levels of take-all incidence, and to demonstrate that take-all, like many other processes, operates in an episodic manner that is rare but, on occasions, severe.

Soil Microbiota: a Gold Mine and a Minefield for Biotechnology

Introduction Grain and pasture legumes are an integral part of cropping systems in southern Australia. They can (i) act as a break crop for cereal root diseases, (ii) offer an opportunity to control grass weeds through the use of herbicides and (iii) increase soil nitrogen (N). The culmination of these benefits generally results in improved cereal yields following a legume crop (Khan et al. 2003). However, in the dryland farming regions of southern Australia (the Mallee), legumes have failed to provide the ‘expected’ N benefits, due primarily to low N2 fixation (about 25% that of expected) (Gupta, V.V.S.R., CSIRO, personal communication). The aim of this research was to determine if recommended post-emergent herbicides affect the legume-rhizobia symbiosis, and thereby the soil N balance.