Effects of extreme events on the productivity of dairy farms

The Intergovernmental Panel on Climate Change (IPCC) has estimated that by 2050, Australia will be one of the most negatively impacted regions by climate change. More frequent and severe extreme events such as heatwaves, droughts or floods present significant challenges to the productivity and profitability of climate-dependant agricultural sectors, with economic losses of up to 19%. Australian dairy regions are expected to experience greater frequency, duration and severity of extreme climatic events in the next 20 years. Thus, the primary objective of this research was to analyse the impacts of extreme heat events and other extreme climatic events on the plant and animal productivity of pasture-based dairy farms in Australia. In the first part of this thesis, we reviewed the impact of climate change on pasture-based dairy systems with a focus on extreme climatic events. The review provides insight into current methods for assessing and quantifying the risk of heat stress impacts on pastures and animals. We found the temperature humidity index (THI) to be one of the most appropriate indicators of heat stress in dairy cattle. Adapting milking routines, calving times and introducing heat tolerant animals are some proposed strategies to reduce the incidence of animal heat stress while alternative pasture species, better adapted to heat waves and prolonged periods of water deficit, are proposed for improved pasture production in future climates. We next investigated regional milk production losses in 99 commercial dairy farms during a real, pervasive period of excessive heat in Australia. Using an auto-regressive time-series approach, we demonstrated that present heat events have significant detrimental effects on the milk production of dairy farms. Cumulative regional milk production during a spring heat event in November 2009 were reduced by 3.4% in milk yield, 3.9% in energy-corrected milk and by 4.1% in terms of milk solids. Aggregated losses for the nine-day heat event were estimated at 13,000 kg milk solids, with a farm gate value of AUD$65,000. This study found significant spatial variability in production losses at the regional level, providing insight into the impact of extreme heat events on the dairy industry. Such economic losses demonstrated the impact of heat stress on dairy cows as well as the need for the dairy industry to proactively implement adaptations, particularly in spring and summer, when the risk of heat events is higher. Given the importance of heat stress in dairy systems, we conducted a systematic literature review and meta-analysis to quantify the effects of heat stress on dairy cows through a relationship between dry matter intake (DMI) and THI. The review accounted for differences between countries, breeds, stage in lactation and parity. There was a significant negative correlation (r =-0.82) between THI and DMI, wherein DMI was reduced by 0.45 kg/d for every unit increase in THI. This result facilitates standardisation of heat stress and feed intake comparison across studies, including differences between countries, breeds, stage in lactation and parity. Primiparous and multiparous cows did not show significant differences in the reduction of DMI at increasing THIs. While differences in the DMITHI relationship between lactating and non-lactating cows were not significant, effects of THI on DMI were significantly different across lactation stages. We also investigated the efficacy of cooling strategies and showed that cooling adaptations became important above THI 68, indicating that this THI could be viewed as a threshold at which cooling should be provided. Passive cooling (e.g. shading) was more effective than active strategies (e.g. fans and sprinklers) at alleviating the reduction in DMI. The meta-analysis and development of the THI-DMI relationship allows users to predict effects of heat stress across environments and animal genotypes and will be useful both from disciplinary (e.g. animal nutrition) and systems modelling perspectives. Having examined historical impacts of extreme climatic events on dairy farms, we next evaluated the relative influence of pasture and animal heat stress on the productivity of whole dairy farming systems and under two future climate scenarios, accounting for variation in extreme climatic events. Future climate projections were developed using monthly regional climate scaling factors based on Representative Concentration Pathway 8.5 for 2050 from 40 global circulation models. Climate projections accounted for increased frequencies of extreme events with more heatwaves, longer droughts and more extreme rainfall events than historical climates. A whole farm systems model was used to quantify the relative effects of heat stress on pasture growth rates, animal DMI and milk production in three major dairy locations in Australia. Although contemporary systems models are well equipped to account for pasture responses to temperature, often they do not account for heat stress effects on livestock. Hence, we used the THI-DMI relationship developed in the previous chapter to account for the relative importance of animal heat stress and contrasted this to the effects of heat stress on plants that were implicit to the model outputs. We found that effects of animal heat stress on DMI and lactation were greater than the effects on the same variables caused by plant heat stress. Across sites and climate scenarios, the relative impact of animal heat stress ranged between 10-30% versus 2-15% in plants, suggesting that past modelling of the impacts of heat stress on pasture-based dairy systems may have underestimated the extent to which heat stress influenced DMI and lactation. Under future climates, effects of heat stress on pasture and animal DMI were more pronounced than under historical climates, as future climates were both hotter and drier. Relative to historical climates, lactation was reduced by 11-35% by 2050 across sites when both plant and animal heat stress were considered. This thesis provides insight into the effect of future climates on dairy systems as extreme events increase in frequency, severity and duration. Extreme heat events will challenge the productivity of temperate perennial ryegrass grazing systems by compromising pasture quality and quantity, creating a seasonal feed-supply gap on pasture-based dairy farms. As heat events become more frequent in many regions, pasture-based dairy farmers will be forced to adapt to remain profitable and maintain good animal welfare standards. Beneficial adaptations may include the nutritional management of dairy cows to account for the energetic and nutritional costs associated with heat stress while maintaining low metabolic heat production, planting trees for outdoor shading, and incorporating heat- and drought tolerant pasture species. Future systems modelling work should account for the impacts of heat stress on dairy cows, as this aspect can be even larger than impacts of heat stress on pastures. In addition, future experimental and modelling work of pasture-based dairy should consider the implementation of ‘animal-focused’ heat mitigation adaptations. While shading and sprinklers are beneficial in countries such as the US, where animals are confined, such strategies are not always practical in Australian pasture-based systems. Successful adaptation of future grazing systems can only be truly achieved through systematic analyses of future challenges and opportunities arising from changing climates, highlighting the need for studies such as those conducted here