Quantifying the response to heat stress in Sarda dairy sheep housed indoor with indicators measurable on field.

Sheep are considered one of the most favoured species to adapt to climate change. Nevertheless, they suffer heat stress more intensively than cattle due to their high metabolic effort per kg of BW and the wool coat. Biological markers of heat stress need to be defined at field level. This PhD dissertation was developed to quantify the response to heat stress of Sarda sheep housed indoor in several experiments. The Thesis was developed within 7 Chapters. Chapter 1 presents a literature review on the heat stress in sheep and, at its end, the main objectives of the work. In Chapter 2, was presented a trial in which fifteen Sarda dairy ewes were divided in three groups different for milk yield (MY): High MY, Low MY and Dry with 1900, 1200, 0 g/d of milk per ewe (n=5), respectively. Air temperature (AT) and relative humidity (RH) were recorded and nine physiological variables were measured on the individual animals, five times per day, for 60 days. With increasing of THI classes, a significant increase was observed for all animal measures (P< .001). Lactating animals showed high stress in comparison to dry groups. In conclusion, respiration rate (RR) was the most valuable marker for heat stress in housed sheep. In Chapter 3, records from dry ewes raised in the same experimental barn and in two different seasons (winter 2022 and summer 2021), were studied to quantify the ranges of RR and rectal temperatures. RR and rectal temperature (RT) were not significantly affected by AT in winter, showing low values and small standard deviations, equal to 27.8±7.2 breath/min and 38.86±0.55 °C, respectively. In summer, RR and RT were equal to 71.1±34.8 breath/min and 38.95±0.30, respectively. Daily variation of indicators showed that ewes have strong summer accumulation of heat from morning to evening even with medium THI values. In Chapter 4, twenty-one Sarda dairy ewes, separated in 3 groups fed different diets for the NDF quality, were monitored for 21 days in summer 2022. RR, RT, DMI and MY were recorded. DMI was not affected by THI but MY showed a strong decrease with THI>76 (-11.2% compared to THI=71-72; p<0.05) without differences among groups. RR ranged from 30-50 breaths/min (at THI of 65) to a max of 90-170 breaths/min (at THI of 80) as predictable by the equation: RR = 0.2555^ 0.0739*THI . In Chapter 5, was presented a trial carried out on 16 sheep housed in ventilated hoods for direct measurements of methane emissions. The data showed that respiration rate is positively correlated with methane emissions during measuring trials. In Chapter 6, a descriptive statistical analysis was carried out using biometrics measurements of ewes to develop a simple geometric model to estimate volume and surface of the animals. Furthermore, a conceptual model of heat production and dissipation in sheep was developed as preliminary basis to develop a dynamic model of heat stress response in sheep. In Chapter 7, the general conclusions and practical implications of the work were presented for scientific and applicative uses.