基于断奶羔羊生理和生长性能的热应激发生阈值预测

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

热应激对绵羊的生长和健康产生了较大影响,但目前对羊热应激的发生程度尚无评价标准。为有效预防绵羊热应激,该研究基于不同温湿度条件下断奶羔羊生理指标和生长性能的变化,对热应激发生的温湿度阈值进行了预测。试验采用环控舱将12只健康状况、体质量相近的断奶羔羊(小尾寒羊×杜泊羊,2月龄,(18±2.1)kg)分为两组,分别饲养于相对湿度60%和80%的舱内,舱内温度采用梯度爬坡法逐渐升高(21.5~35.5℃),试验期24 d。通过非线性回归模型,根据不同温湿度条件下(60%和80%,21.5~35.5℃)断奶羔羊生理指标(体表温度(body surface temperature,BT)、呼吸频率(respiratory rate,RR)和直肠温度(rectal temperature,RT))及生长性能指标(采食量(dry matter intake,DMI)和饮水量(average daily water intake,ADW))的变化,预测各项指标发生拐点(inflection point,I)的温度和温湿指数(temperature-humidity index,T_(HI))。结果表明,1)在该试验条件下羔羊RR变化范围为28~211次/min,BT和RT则分别为29.90~35.65℃和38.38~41.42℃。2)3项生理指标(BT、RR和RT)在2种湿度条件下I出现的时间不同。60%时,各项生理指标出现I的环境温度分别为:I_(BT)=22.99℃(R2=0.98)、I_(RR)=23.25℃(R^(2)=0.99)和I_(RT)=26.69℃(R^(2)=0.99)。80%时,各项生理指标出现I的环境温度分别为:I_(BT)=22.33℃(R^(2)=0.97)、I_(RR)=23.02℃(R^(2)=0.99)和I_(RT)=25.41℃(R^(2)=0.98)。3)随着环境温度的升高,2项生长性能指标中ADW先出现I,I_(ADW)=23.02℃(60%)和22.43℃(80%),当环境温度超过I温度时,ADW开始线性增加;而DMI出现I的环境温度为:I_(DMI)=24.95℃(60%)和23.21℃(80%),当超过I温度时,DMI开始线性下降。4)随着T_(HI)升高,各项指标出现I的早晚顺序依次为:I_(BT)(69.84)<I_(ADW)(70.12)<I_(RR)(71.46)<I_(DMI)(72.11)<I_(RT)(74.83)。综上所述,T_(HI)≤71.46认定为无热应激区,71.46<T_(HI)≤72.11为临界热应激区,72.11<T_(HI)≤74.83为热应激区,74.83<T_(HI)为警惕区。该结果可为断奶羔羊热应激的评判提供依据。

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℃(R^(2)=0.98),23.25℃(R^(2)=0.99),and 26.69℃(R^(2)=0.99),respectively.At the humidity of 80%,the three physiological parameters inflection point ambient temperature were as follows:22.33(R^(2)=0.97),23.02℃(R^(2)=0.99),and 25.41℃(R^(2)=0.98).3)The threshold values of two growth parameters(DMI and ADW)were:the inflection point ambient temperature of DMI was 24.95℃(R^(2)=0.94)at humidity of 60%and 23.21℃(R^(2)=0.89)at humidity of 80%;the inflection point ambient temperature of ADW was 23.02℃(R^(2)=0.92)at 60%and 22.43℃(R^(2)=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.