Developing Heat Stress Thresholds for Sheep

Considerable improvements in management of livestock have come about via the development of species specific climate indices. The objective of the study was to define thermal thresholds for sheep. One hundred and forty-four Merino wethers (44.02±0.32kg) were used in a replicated climate control chamber study. Each replicate (n=2) was conducted over 29 d and consisted of 2 treatments: hot (HOT) and thermoneutral (TN). There were 6 sheep/pen and 3 pens/room. Individual respiration rate (RR) and panting score data were obtained for each animal at 3 h intervals between 0800 h and 1700 h daily for the duration of the study. Ten minute rumen temperature (T_RUM) data were obtained using rumen boluses. The development of temperature humidity index (THI) categories were undertaken by determining the THI values at which there were marked increases in RR and T_RUM. A backwards regression model (PROC REG, SAS Institute, Cary, NC) was used to determine the relationship between RR, T_RUM, ambient temperature and relative humidity. The validity of the thresholds were tested by comparing the RR and T_RUM means within each THI threshold category. If the differences between the mean values were different (P<0.05), the thresholds were confirmed. Adjustments to the THI thresholds were then made until best fit was obtained. The statistical validity of the THI categories were then tested. The model included treatment, pen, observation time, day, THI category and the interaction of day×observation time as fixed effects, with animal included as a random effect.

 

Mean rumen temperature (T_RUM, ^oC) ± standard error (SE) and mean respiration rate (RR, breaths/min) ± standard error (SE) for sheep exposed to thermoneutral (TN) and hot conditions (HOT) under four temperature humidity (THI) heat load categories

Treatment	THI Category	THI Values	Descriptor	TRUM1	RR1
TNC	1	≤ 24.3	No HL	39.9a ± 0.02	50.5a ± 0.46
HOT	1	≤ 24.3	No HL	39.7a ± 0.08	93.3b ± 0.71
HOT	2	>24.3 ≤ 28.6	Moderate	40.3c ± 0.02	146.1c ± 0.63
HOT	3	>28.6 ≤32.3	High	40.4d ± 0.06	181.1d± 0.79
HOT	4	>32.3	Extreme	40.7e ± 0.02	222.4e± 5.90

¹Means within columns with different superscripts are significantly different (P<0.0001)

These thermal thresholds are an initial categorization of heat stress thresholds, further testing is required to validate their accuracy. Determining the thresholds based on climatic variables will provide a management tool to assess the impact of hot conditions on sheep.