Abstract Mississippi is characterized by hot and humid weather during summer, coinciding with the most productive season of forage. To analyze the effect of weather on physiological variables associated with heat stress in grazing goats, a trial was conducted at the H.H. Leveck Research Center, Mississippi State University. Six Kiko (K) and six Spanish (S) goats were randomly selected from a group of 30 weaned males (average initial weight 26 ± 1.2 kg and age 5 to 6 months old), grazing warm-season forages in a rotational system, with water and minerals provided ad libitum and 1% body weight supplementation with a commercial concentrate (16% CP). Animals were restrained weekly for 10 weeks during summer (June to August), at the hottest time of the day (2 pm), to measure four variables: Respiration Rate (RR): through visual inspection of flank movements for 15 seconds x 4 to obtain breaths/min; Panting Score (PS): using a goat-specific scale from 0 to 4 (0 = no panting, normal breathing and difficult to see chest movement; 1 = slight panting, easy to see chest movement, mouth closed, no drool; 2 = moderate panting, easy to see chest movement, occasionally mouth open enough to see teeth; 3 = fast panting, easy to see chest movement, occasionally mouth open enough to see teeth and tongue inside, possibly small drool from the nose; 4 = fast panting, teeth and tongue often visible, occasional open mouth, possibly small drool from the nose); Heartbeat (HB): measured with a stethoscope for 15 seconds and multiplied by 4 to obtain beats per minute; and Rectal Temperature (RT): measured with a clinical thermometer inserted into the rectum (°C). Environmental temperature and humidity were recorded every 15 minutes using a weather station (Watch Dog 3000 Series) to calculate the THI. Data were analyzed in SAS 9.4 using a mixed model with THI as a covariate and Animal as a random effect. RR was significantly affected by breed (P 0.001), being lower in K (113.63 ± 3.99) than in S (128.15 ± 4.39). RT was higher in K (39.94 ± 0.07) than in S (39.63 ± 0.06). HB and PS were not significant, with averages of 125 ± 3.3 and 0.72 ± 0.06, respectively. The average THI during the experiment was 86.06, indicating that the animals were experiencing heat stress. THI proved to be a good predictor, especially for RR, as increases in THI corresponded to increases in RR = −157.78 + 3.23(THI) R2=0.32 (P 0.001), meaning that for every unit of increment in THI, the respirations increase by 3.23. Further studies measuring cortisol and heat shock proteins could help elucidate how these variables interact under grazing conditions, helping these breeds tolerate harsh conditions to maintain productivity.
Jackson et al. (Wed,) studied this question.