Estimación de los umbrales de estrés por calor en vacas Cinisara sicilianas en lactación en condiciones naturales

The efficiency of thermoregulation varies among breeds of cattle and can be part of the adaptive ability of local cows, such as Cinisara. This study aimed to assess the resilience to heat stress (HS) of Cinisara cows in a semi-extensive system during summer, evaluating rectal temperature (RT), milk yield (MY) and composition. The study was carried out from June to September 2024 on a commercial dairy farm located in Cinisi (Sicily, Italy) involving 15 primiparous and 20 multiparous lactating Cinisara cows. Temperature-humidity index (THI) was calculated based on temperature and humidity data registered by 3 probes every 15 min. The RT was recorded monthly (once a month) after the morning and evening milking. The MY was recorded monthly and milk samples were collected. Data were analyzed with PROC GLIMMIX and NLIN of SAS. Parity did not affect RT or milk performance response to HS. Overall, the highest RT was registered in August in both morning and afternoon, when THI was above 77 (38.51°C and 39.21°C, respectively). The nonlinear regression (NLIN) analysis revealed a strong and statistically significant relationship between the THI and RT (adjusted R² = 0.89). A THI breakpoint of 77.65 was identified for RT, beyond which RT increased significantly and abruptly by approximately +0.12°C for each unit increase in THI above the threshold. The nonlinear regression analysis applied to MY data revealed a strong relationship between THI and milk production (adjusted R² = 0.90). A THI breakpoint of 76.9 was identified, beyond which MY declined sharply by approximately -8.77 kg/d for each unit increase in THI above the threshold, compared with a much milder decline (-0.69 kg/d for each unit) observed below the breakpoint. This highlights the breed's notable heat tolerance and its suitability for semi-extensive dairy systems in hot climates. Further studies exploring the genetic basis of this adaptation may support conservation and enhancement strategies for native breeds in Mediterranean regions.