The threshold of occurrence of heat stress in weaned lambs based on physiological and growth performance

The object of this study was to evaluate the effects of ambient temperature and humidity on physiological indexes and growth performance in weaned lambs. The threshold for heat stress (HS) occurrence was also predicted to prevent the HS in weaned lambs. Environmental control cabins were used to control the indoor temperature and humidity. 12 healthy weaned lambs Small-tailed Han sheep × Dupo sheep, 2 months, (18±2.1) kg were assigned into two treated groups, and fed in the individual cabin with relative humidity of 60% and 80%, respectively. The ambient temperature in the cabin gradually increased from 21.5 to 35.5 ℃ with a gradient of 2 ℃. The temperature in each gradient was kept for 3 days, and the entire experiment lasted for 24 days. The physiological indexes included the body surface temperature (BT), respiratory rate (RR), and rectal temperature (RT). The growth performance was represented by the dry matter intake (DMI) and average daily water intake (ADW) of weaned lambs under temperatures from 21.5 to 35.5 ℃ at two humidity conditions of 60% and 80%. The threshold of inflection point ambient temperature and temperature-humidity index (THI) were predicted for each parameter using nonlinear regression. The results showed that: 1) RR ranged from 28 to 211 times/min, BT from 29.90 to 35.65 ℃, and RT from 38.38 to 41.42 ℃ under present experimental conditions. 2) Three physiological indexes (BT, RR, and RT) were relatively stable in the early period of the trial, and then increased linearly with increasing temperature under two humidity conditions. At the humidity of 60%, the threshold value of BT, RR, and RT were 22.99 ℃ (R²=0.98), 23.25 ℃ (R²=0.99), and 26.69 ℃ (R²=0.99), respectively. At the humidity of 80%, the three physiological parameters inflection point ambient temperature were as follows: 22.33 (R²=0.97), 23.02 ℃ (R²=0.99), and 25.41 ℃ (R²=0.98). 3) The threshold values of two growth parameters (DMI and ADW) were: the inflection point ambient temperature of DMI was 24.95 ℃ (R²=0.94) at humidity of 60% and 23.21 ℃ (R²=0.89) at humidity of 80%; the inflection point ambient temperature of ADW was 23.02 ℃ (R²=0.92) at 60% and 22.43 ℃ (R²=0.92) at 80%. Furthermore, the DMI decreased and ADW increased linearly when the ambient temperature exceeded inflection point ambient temperature. 4) With the increase in THI, each index of inflection was ranked in the order of: BT (69.84) <ADW (70.12) <RR (71.46) <DMI (72.11) <RT (74.83). According to the nonlinear regression between various parameters and THI, it was found that: with every 1 increase in THI, BB, RR, and RT increased by 0.30 ℃, 9.30 times/min, and 0.19 ℃, respectively. DMI decreased by 18.04 g, while ADW increased by 203.80 mL, as the THI increased by 1. The optimal threshold values were achieved by temperature, where the 21.5-22.99 ℃ at 60% or 21.5-22.33 ℃ at 80% was as a comfortable zone; 22.99-23.25 ℃ at 60% or 22.33-23.02 ℃ at 80% was as a non-HS zone; 23.25-24.95 ℃ at 60% or 23.02-23.21 ℃ at 80% was as an early-warning zone, 24.95-26.69 ℃ at 60% or 23.21-25.41 ℃ at 80% was as a HS zone, and above 26.69 ℃ at 60% or 25.41 ℃ at 80% was as alert zone. Besides, below 71.46 was defined as non-HS, 71.46-72.11 was as a critical HS zone, 72.11-74.83 was as an HS zone, and above 74.83 was as an alert zone. The findings can provide a strong reference for the HS evaluation in weaned lambs.