Opportunities to improve the irrigation efficiency of dairy pasture systems through management of water resources and an understanding of plant water relations
Creators: name: Holloway-Phillips, MM
Publisher: name: University Of Tasmania
Year: 2023

description: This thesis investigates the capacity to improve irrigation efficiency in dairy pastures through the practice of deficit irrigation. In this practice, less water is applied than is needed to meet full losses from evapotranspiration (ET), thus creating a soil water deficit and exposing plants to mild water stress. Deficit irrigation has been successfully used in various crops to reduce irrigation demand with minimal penalty to yield. However constraining production of pasture growth under reduced water availability is the linear relationship between biomass and ET. Two options investigated in this thesis to reduce the risks to pasture production were the potential to improve, instantaneous water use efficiency through stomatal regulation of water use (A/gs; WUEl) (leaf-level response), and irrigation efficiency (yield/irrigation applied) through increased rainfall capture (field-level response). Management options to mitigate the increased risks of practicing deficit irrigation associated with spatial and temporal variability of water requirements and climate uncertainty were also considered, including soil moisture monitoring to improve irrigation scheduling precision, climate forecasting to reduce uncertainty around irrigation requirements, and breeding opportunities for improved drought resistance. Knowledge of how stomata regulate water use and the importance of maintaining hydraulic connection between leaves and the soil for growth and survival (leaf area maintenance) under soil water deficits, were used as the basis to investigate both the limitations and opportunities to augment WUEl. Lolium perenne L. (perennial ryegrass) is the dominant pasture species grown in temperate dairy systems, and was the focus of this study. Perennial ryegrass was identified as being intrinsically drought-sensitive according to the leaf xylem, which was highly susceptible to water stress-induced declines in hydraulic conductivity. Furthermore, stomata provided no protection against hydraulic dysfunction, closing well after the point where hydraulic conductivity had declined by 50 %. Despite the lack of safety conferred by stomata, hydraulic dysfunction was neither detrimental to assimilation up to a point, nor restrictive to the recovery of hydraulic conductivity on rewatering. As a result diurnal soil water availability could be manipulated in order to achieve increases in WUEl without affecting dry matter (DM) yield (g/plant). A range of deficit irrigation strategies were tested to improve irrigation efficiency under field conditions. Increasing the cumulated potential evapotranspiration (PET) deficit from 20 mm to 60 mm before irrigation was triggered, resulted in a 50 % saving in irrigation inputs over the experimental period with no significant effect on herbage DM yield (t DM/ha) or nutritive value. When the scheduling practice was further tested at the paddock scale with grazing cows, there was however a linear decline in DM production with irrigation inputs. However, under conditions where the distribution uniformity of irrigation application was low, the variability in DM yield was similar between well-watered and deficit irrigation treatments suggesting that there was no additional risk to yield from increasing the soil water deficit to a maximum threshold of 60 mm. Furthermore, gas exchange and leaf water potential measurements sampled across a range of soil water potentials in the field indicated that on average the increase in WUEl between well-watered and deficit irrigated plants was 18.7 %, compared with a 53.5 % increase which had been achieved under glasshouse conditions. High variability in leaf water potential and hence WUEl with soil water potential, demonstrated the difficulty in the field to maintain tight control on WUEl. A comparison between irrigation scheduling methods showed that the use of granular matrix sensors improved irrigation scheduling precision resulting in a 0.24 t DM/ML increase in the response of pasture to irrigation inputs, with a water-saving of 20-33 % compared with where irrigation was scheduled according to a PET-based rainfall deficit. Seasonal climate forecasts based on the Southern Oscillation Index 5-Phase system were used to optimise the choice of the irrigation scheduling practice for Elliott, on the north-west coast of Tasmania. Probability distributions of modelled irrigation requirements were constructed using historical climate data from 1901 to 2008, as well as yield distributions under 4 different deficit irrigation strategies using the biophysical model, DairyMod. The reliability of the forecast system to predict differences in irrigation requirements (mm/year) between the SOI Phases appeared poor with statistical differences between the cumulative distribution functions of irrigation requirement non-significant when the P-values were adjusted for multiplicity. This was consistent with the considerable overlap observed between 95 % confidence intervals. However in terms of user value, the forecast system maximised DM production over a fixed scheduling strategy when water was non-limiting, whilst reducing the water requirements by 8.5%. When the irrigation allocation was capped at 250 mm (50% reduction of maximum requirements), there was less differentiation of irrigation requirements between SOI Phase distributions, negating the need for pre-season information and limiting the opportunity to improve irrigation efficiency.Breeding presents another potential avenue for improving the DM response to water inputs and was approached from the point of view of understanding differences in water transport characteristics between closely related forage grasses to assess the potential DM trade-offs associated with increased drought resistance. Cultivars of two common forage grass species were assessed with different drought performance ratings, including Lolium multiflorum Lam. and Festuca arundinacea Schreb. Species of the Festuca-Lolium complex were specifically chosen as their chromosomes share sufficient homology to pair and recombine providing much opportunity for the breeding of model genotypes. It was found that whilst vulnerability of xylem to hydraulic dysfunction (a measure of dehydration tolerance) was comparable, for F. arundinacea cultivars there was greater hydraulic risk associated with reduced stomatal sensitivity to leaf hydration. In contrast, L. multiflorum cultivars expressed a higher capacity for water transport, but more conservative stomatal regulation. Under acute soil drying, F. arundinacea was as a result more susceptible to leaf damage. However, under the sustained moderate drought conditions in this experiment, the two strategies were balanced in terms of water conservation and hydraulic utilisation, resulting in similar DM production. This thesis uses an integrated approach to investigate the opportunities for improving the irrigation efficiency of dairy pasture systems. Leaf level assessment indicated that the forage grass species tested had the capacity to lose more water than required to maintain optimal DM production. However at the field level, regulating soil water availability through irrigation scheduling to restrict stomatal regulation of water use was difficult to achieve due to the large amplitude in soil water experienced in each of the irrigation treatments. Options for mitigating the risks associated with deficit irrigation, including the use of soil moisture monitoring and climate forecasting, were demonstrated as well as implications for the breeding of drought resistant grass species based on how the coordination between hydraulic capacity and water use regulation influences biomass production and water use efficiency. The results of this thesis provide important information to the management of pastures in water-limited environments, as well as illustrate the general need for management practices to be designed in context of the crop's physiology.