Stephen P. Milroy

Large root systems: are they useful in adapting wheat to dry environments?

There is little consensus on whether having a large root system is the best strategy in adapting wheat (Triticum aestivum L.) to water-limited environments. We explore the reasons for the lack of consensus and aim to answer the question of whether a large root system is useful in adapting wheat to dry environments. We used unpublished data from glasshouse and field experiments examining the relationship between root system size and their functional implication for water capture. Individual root traits for water uptake do not describe a root system as being large or small. However, the recent invigoration of the root system in wheat by indirect selection for increased leaf vigour has enlarged the root system through increases in root biomass and length and root length density. This large root system contributes to increasing the capture of water and nitrogen early in the season, and facilitates the capture of additional water for grain filling. The usefulness of a vigorous root system in increasing wheat yields under water-limited conditions maybe greater in environments where crops rely largely on seasonal rainfall, such as the Mediterranean-type environments. In environments where crops are reliant on stored soil water, a vigorous root system increases the risk of depleting soil water before completion of grain filling.

Improving water productivity in the Australian Grains industry—a nationally coordinated approach

Abstract. Improving the water-limited yield of dryland crops and farming systems has been an underpinning objective of research within the Australian grains industry since the concept was defined in the 1970s. Recent slowing in productivity growth has stimulated a search for new sources of improvement, but few previous research investments have been targeted on a national scale. In 2008, the Australian grains industry established the 5-year, AU

Wheat genotypes with high early vigour accumulate more nitrogen and have higher photosynthetic nitrogen use efficiency during early growth

17.6 million, Water Use Efficiency (WUE) Initiative, which challenged growers and researchers to lift WUE of grain-based production systems by 10%. Sixteen regional grower research teams distributed across southern Australia (300–700 mm annual rainfall) proposed a range of agronomic management strategies to improve water-limited productivity. A coordinating project involving a team of agronomists, plant physiologists, soil scientists and system modellers was funded to provide consistent understanding and benchmarking of water-limited yield, experimental advice and assistance, integrating system science and modelling, and to play an integration and communication role. The 16 diverse regional project activities were organised into four themes related to the type of innovation pursued (integrating break-crops, managing summer fallows, managing in-season water-use, managing variable and constraining soils), and the important interactions between these at the farm-scale were explored and emphasised. At annual meetings, the teams compared the impacts of various management strategies across different regions, and the interactions from management combinations. Simulation studies provided predictions of both a priori outcomes that were tested experimentally and extrapolation of results across sites, seasons and up to the whole-farm scale. We demonstrated experimentally that potential exists to improve water productivity at paddock scale by levels well above the 10% target by better summer weed control (37–140%), inclusion of break crops (16–83%), earlier sowing of appropriate varieties (21–33%) and matching N supply to soil type (91% on deep sands). Capturing synergies from combinations of pre- and in-crop management could increase wheat yield at farm scale by 11–47%, and significant on-farm validation and adoption of some innovations has occurred during the Initiative. An ex post economic analysis of the Initiative estimated a benefit : cost ratio of 3.7 : 1, and an internal return on investment of 18.5%. We briefly review the structure and operation of the initiative and summarise some of the key strategies that emerged to improve WUE at paddock and farm-scale.

The impact of wheat stubble on evaporation from a sandy soil

Genotypic differences in early growth and nitrogen (N) uptake among 24 wheat (Triticum aestivum L.) genotypes were assessed in a field trial. At late tillering, large genetic variation was observed for shoot biomass (23-56gm-2 ground area) and N uptake (1.1-1.8gm-2 ground area). A strong correlation between aboveground biomass and N uptake was observed. Variation around this relationship was also found, with some genotypes having similar N uptake but large differences in aboveground biomass. A controlled environment experiment was conducted to investigate the underlying mechanisms for this variation in aboveground biomass using three vigorous genotypes (38-19, 92-11 and CV97) and a non-vigorous commercial cultivar (Janz). Vigorous genotypes had lower specific leaf N in the youngest fully expanded leaf than Janz. However, there was no difference in chlorophyll content, maximum Rubisco activity or the rate of electron transport per unit area. This suggests that Janz invested more N in non-photosynthetic components than the vigorous lines, which could explain the higher photosynthetic N use efficiency of the vigorous genotypes. The results suggest that the utilisation of wheat genotypes with high early vigour could improve the efficiency of N use for biomass production in addition to improving N uptake during early growth.

Impact of short-term exposure to cold night temperatures on early development of cotton (Gossypium hirsutum L.)

In Mediterranean-type climates, dryland soil water storage and evaporation during the hot and dry summer are poorly understood, particularly for sandy-textured soils. Continued evaporation during summer, and any effects of crop stubble management, could have a significant impact on annual components of the water balance and crop yield. In this research, the effect of wheat stubble management on summer evaporation and soil water storage was investigated for a sandy soil in south-western Australia, during the summers of 2005–06 and 2006–07. Treatments comprised: retained standing stubble; retained flattened stubble; removed stubble; and removed stubble followed by burying the crowns with topsoil from an adjacent area. Under ‘dry’ conditions, evaporation continued at ~0.2 mm/day. In contrast to previous results for finer textured soil types, stubble retention did not decrease the rate of evaporation, but marginally (10–30%) increased evaporation on 7 out of 14 days when measurements were taken. Significant differences due to stubble management were observed in two successive summers, but only for relatively dry soil conditions. There were no significant differences observed for several days after irrigation or rainfall. Under dry conditions in the absence of rainfall, total decrease in water storage during a 90-day summer period could be ~20 mm, but differences attributable to stubble management are likely to be a few mm.

Development and use of a barley crop simulation model to evaluate production management strategies in north-eastern Australia

Regression analysis of field data has indicated that minimum daily temperatures below 11°C delay the development of cotton (Gossypium hirsutum L.) seedlings beyond what would be expected based on the accumulated degree-day sum. In Australian cotton production systems, events where the minimum daily temperature falls below this value are referred to as ‘cold shocks’. The number of cold shocks is used by growers and advisors in assessing retardation of crops in their areas. However, this effect has not been tested explicitly. The aim of this work was to empirically assess effects of cold shock on pre-flower development of cotton plants. Cotton seedlings were grown in controlled-temperature glasshouses. Plants were transferred to cold chambers ranging from 5 to 22°C during the night period for durations from 3 to 10 days. Negative effects were not seen until plants had been exposed to at least 10 nights at 10°C, or for at least 5 nights at 5°C. When differences were generated it did not delay development to first square any more than 4 days, nor was the effect consistent. These differences translated into delays to first flower, but had little effect on plant morphology, or on dry weight measured soon after flowering. In one experiment, a significant reduction in leaf photosynthesis was measured at two times of day on the day after cold shock at 5°C. Improving understanding of the effects of temperature extremes on cotton growth and development will help in developing more functional decision-support tools and field management strategies.

Managing yields of high fruit retention in transgenic cotton (Gossypium hirsutum L.) using sowing date

A study was undertaken to identify improved management strategies for barley (Hordeum vulgare L.), particularly in relation to time of planting, location, and frost risk in the variable climate of north-eastern Australia. To achieve this objective, a crop growth simulation model (QBAR) was constructed to integrate the understanding, gained from field experiments, of the dynamics of crop growth as influenced by soil moisture and environmental variables. QBAR simulates the growth and yield potential of barley grown under optimal nutrient supply, in the absence of pests, diseases, and weeds. Genotypic variables have been determined for 4 cultivars commonly grown in the northern cereal production areas. Simulations were conducted using long- term weather data to generate the probabilistic yield outcome of cv. Grimmer for a range of times of planting at 10 locations in the north-eastern Australian grain belt. The study indicated that the common planting times used by growers could be too late under certain circumstances to gain full yield potential. Further applications of QBAR to generating information suitable for crop management decision support packages and crop yield forecasting are discussed.

The influence of shoot and root size on nitrogen uptake in wheat is affected by nitrate affinity in the roots during early growth

Recently, genetically engineered (transgenic) cottons expressing genes from Bacillis thuringiensis (Bt) have been made available to cotton growers throughout the world. In Australia, cotton growers have access to Bt cotton that contains genes expressing the insecticidal proteins Cry1Ac and Cry2Ab (Bollgard II®). Bollgard II offers significant potential to reduce pesticide use for the control of major Lepidopteran pests (particularly Helicoverpa spp. in Australia). As a consequence of the improved insect control, retention of squares (flower buds) and young bolls is higher in Bollgard II varieties than in non-Bollgard varieties. A concern raised by Australian cotton growers is that in some regions, yield potential for Bollgard II may be limited because the demands of earlier high fruit retention reduce resources for continued growth and fruiting, thus leading to earlier maturity and reduced yield. Non-Bollgard crops with high early retention are known to mature earlier sometimes reducing yield. Three field experiments over three seasons, which varied sowing date and compared non-Bollgard II and Bollgard II cotton cultivars, were conducted to test the hypothesis that delaying sowing date in Bollgard II will increase canopy size (without delaying crop development) and alleviate the potential concerns for the effect of higher fruit retention reducing canopy size and the time to maturity, limiting the yield of Bollgard II. In non-Bollgard II crops, larger canopies resulting from early loss of fruit or apical meristem damage can support more fruit growth for longer, provided season length allows fruit to mature. Results showed that delayed sowing did not increase the yield of the Bollgard II cultivar through increased leaf area index at flowering compared with normal sowing dates. However, in comparison with the conventional cultivar, which had yields that became lower with later sowings, Bollgard II maintained its yield presumably through the shorter fruiting cycle (because of its consistently higher earlier fruit retention), allowing time to support growth of the same number of bolls as earlier sowings. Improvements in fibre quality were also recorded with later sowings for both cultivars. Varying sowing dates for Bollgard II in different production regions may offer opportunities for Australian growers to help optimise yield, fibre quality, and reduce risks associated with poor crop establishment when crops are sown too early.

Green and Brown Manures in Dryland Wheat Production Systems in Mediterranean-Type Environments

Shoot and root system size influences N uptake in wheat (Triticum aestivum L.). Previously, we showed that four wheat genotypes with different biomass had similar N uptake at tillering. In the present study, we determined whether the similarity in N uptake in these genotypes was associated with genotypic differences in the affinity of the root system for NO3- uptake. Kinetic parameters of NO3- uptake were measured in hydroponic seedlings of vigorous and nonvigorous early growth wheat genotypes by exposing them to solutions with differing concentrations of K15NO3 for 15min. In the low concentration range, the high-affinity transport system of the nonvigorous cultivar Janz showed a higher maximum influx rate than the three vigorous lines and a higher affinity than two of the three vigorous lines. At high NO3- concentrations, where the low-affinity transport system was functional, the responsiveness of NO3- uptake to external concentrations was greater in Janz than in the vigorous lines. Both the high- and low-affinity transport systems were inducible. The genotypic variation in the kinetic parameters of NO3- uptake was large enough to offset differences in morphological traits and should be considered in efforts to improve N uptake. In a field trial, the growth and N uptake performance of the four wheat genotypes was investigated over the winter-spring growing season (June-November of 2010). The field trial showed that although early N uptake was disproportionately large relative to biomass accumulation, the differences in uptake at tillering can be changed by subsequent patterns of uptake.

Towards biological control of Spongospora subterranea f. sp. subterranea, the causal agent of powdery scab in potato

Abstract In this review, we draw together research on the use of green and brown manures in wheat cropping systems in Mediterranean-type environments in the light of contemporary pressures on cropping systems including changing climates, increasing costs and declining profit margins. Green and brown manuring have been demonstrated to have benefits in terms of weed control, delaying the development of resistance to herbicides, reducing populations of disease organisms, altering soil water, soil quality and biology, erosion control, and contributing to the nutrition of subsequent crops. However, few researchers have attempted to measure more than one of these variables, which presents difficulties in both interpreting the causes behind results of field trials and in estimating the total benefit of manuring, and hence its consequences for profitability. Well-designed experiments have been reported on component mechanisms (such as weed numbers or N2 fixation). However, these experiments are often not taken through to maturity of the crop following the manuring treatment. As a result, there is limited yield and grain quality data on which to base sound analyses of profitability. A few reports are available which present the impact of manuring on wheat yield and profitability in specific areas and systems but the results vary widely. For such reports to be of value, further research is needed into the factors inducing the changes in response (climate, soil type, or the specifics of the farming system at the time the treatments are imposed) and the mechanisms by which these act. Thus, research is needed into both the mechanisms and yield benefits that flow from the individual responses to manuring. Two further limitations to determining the economic benefit of manuring emerge. Firstly, impacts are primarily reported for a single year after a single manuring treatment. However, if measurements are made over a number of years, effects can often still be detected. More studies aimed to assess the longer-term impacts of manuring on soil health, disease prevalence, and weed populations are required. Secondly, there has been very little effort to explore the whole-farm impact of using manures. These impacts could include effects on other farm enterprises as well as business-level impacts such as potential changes in labor requirements, cash flow, and risk. The incorporation of manuring into wheat production systems may have multiple on-farm and off-farm benefits. However, there is a substantial research requirement before these approaches could be recommended. The highest priority is a sound demonstration of short- to medium-term economic benefits to growers. Without this, adoption can be expected to be poor.