Mitigating the detrimental effects of hot and dry summers on the south-eastern Australian dairy feedbase

This thesis investigated strategies for mitigating detrimental effects of hot and dry summers (i.e. supraoptimal temperatures and soil water deficit stress) on home-grown feed production in south-eastern Australian dairying regions. Strategies included incorporating alternative species into the feed-base and modifying aspects of feed-base management. Field research was conducted in north-west Tasmania, where summer conditions are forecast to become hotter during the 21st century, challenging the existing perennial ryegrass (Lolium perenne L.) feed-base. A glasshouse study (Chapter 3) identified summer-active temperate perennial forages more tolerant than perennial ryegrass of the combined stress of supraoptimal temperatures and water deficit. These species were examined as they are known to possess key attributes underpinning perennial ryegrass' popularity (i.e. they are perennial, grazeable, and of high nutritive value). Species exhibiting greater tolerance than perennial ryegrass included chicory (Cichorium intybus L.), and to a lesser extent lucerne (Medicago sativa L.), plantain (Plantago lanceolata L.), and tall fescue (Festuca arundinacea Schreb.). Tolerance did not pertain to growth occurring when exposed to the combination of stresses, but was a result of these plants capacity to survive and resume growth when more favourable conditions returned. An accompanying field experiment (Chapter 6) tested the potential of chicory and tall fescue to increase summer-autumn growth beyond that achievable by perennial ryegrass in north-west Tasmania. Under both irrigated and rain-fed conditions, chicory did not increase summer-autumn growth beyond that achieved by perennial ryegrass. In contrast, irrigated tall fescue achieved a similar total yield to comparably managed perennial ryegrass over a 15-month period, and in the second of two growing seasons, 10% greater summer-autumn yields. The potential role of tall fescue in north-west Tasmania was further demonstrated by the species high marginal irrigation water-use index (1.6-2.7 t DM/ML). Field research showed rain-fed chicory and second year tall fescue swards experienced less extreme crown (plant-soil interface) temperatures than perennial ryegrass (Chapter 4). This feature may contribute to their superior growth in hotter summer environments than north-west Tasmania. Field research confirmed the crown temperatures of these species can be manipulated by both stubble height (height of stubble above the soil surface post-defoliation) and irrigation management. Defoliating to shorter stubble heights (35 mm or 55 mm cf. 115 mm) elevated the upper distribution of crown temperature (75th and 90th percentiles) in subsequent regrowth cycles (Chapter 4). In the second of two summers, defoliating to shorter stubble heights increased the 90th percentile of crown temperature by an average of 4.2°C for perennial ryegrass, 3.6°C for tall fescue, and 1.8°C for chicory. Differences in crown temperatures may be partly attributed to the canopy base being less shaded when defoliated to shorter stubble heights (i.e. swards having lower leaf area indices). Despite experiencing these higher summer crown temperatures, each species achieved greater summer-autumn yields when defoliated to shorter stubble heights (Chapter 6). This is explained by north-west Tasmania's cool temperate climate, limiting the extent and/or duration of supraoptimal crown temperatures. Irrigation mitigated detrimental effects of supraoptimal temperature stress on growth and survival of all species examined in the glasshouse study (Chapter 3). Field research tested the role of irrigation frequency in mitigating the detrimental effects of supraoptimal temperature stress on perennial ryegrass pastures (Chapter 5). Addressing this research question necessitated the development of a novel outdoor warming system to reliably simulate heat wave conditions. The developed outdoor warming system was successfully deployed, with results showing irrigating at higher frequencies suppressed elevation of perennial ryegrass crown temperature under heaters. The median crown temperature differential between heated and unheated plots equalled 5.5°C, 6.5°C, and 7.0°C for swards irrigated every 2, 5, and 10 days respectively. Because soil water deficits never developed to an extent where transpiration was unduly constrained, irrigating more frequently did not mitigate the detrimental effects of applied heating on growth of perennial ryegrass. These experiments have: (i) identified summer-active temperate perennial forages more tolerant than perennial ryegrass of the combined stress of supraoptimal temperatures and water deficit; (ii) found, in north-west Tasmania, irrigated tall fescue summer-autumn growth can exceed that achieved by comparably managed perennial ryegrass; (iii) shown, defoliating to higher stubble heights and irrigating more frequently maintains cooler crown temperatures; and (iv) developed a novel outdoor warming system for use in evaluating heat wave mitigation strategies. Identification of species more tolerant of hot and dry conditions, and management strategies that limit crown temperature elevation, are important steps in future-proofing north-west Tasmanian dairy farming systems against a warming climate. When interpreting results from field experiments, the influence of environment as a component of the genotype by environment by management interaction should not be ignored. In south-eastern Australian dairying regions warmer than north-west Tasmania, the application of management practices that maintain cooler crown temperatures may benefit temperate pasture production. This requires confirmation, with future research also required to evaluate the potential role of these management practices at a whole farm system level.