Abstract
Simple SummaryHeat stress in sheep initiates physiological methods to dissipate heat that result in decreased production. This study investigated the use of a dietary supplement, the osmolyte betaine fed at two doses (2 or 4 g/day), on the physiological responses to heat in sheep. Heat exposure initiated physiological responses such as an increased rectal temperature and respiration rate as expected, while betaine supplemented at 2 g/day ameliorated these responses. Thus, dietary betaine supplementation may have beneficial effects for sheep exposed to heat.Heat exposure (HE) results in decreased production in ruminant species and betaine is proposed as a dietary mitigation method. Merino ewes (n = 36, 40 kg, n = 6 per group) were maintained at thermoneutral (TN, n = 18, 21 °C) or cyclical HE (n = 18, 18–43 °C) conditions for 21 days, and supplemented with either 0 (control), 2 or 4 g betaine/day. Sheep had ad libitum access to water and were pair fed such that intake of sheep on the TN treatment matched that of HE animals. Heart rate (HR), respiration rate (RR), rectal (TR) and skin temperatures (TS) were measured 3 times daily (0900 h, 1300 h, 1700 h). Plasma samples were obtained on 8 days for glucose and NEFA analysis. The HE treatment increased TR by 0.7 °C (40.1 vs. 39.4 °C for HE and TN respectively p < 0.001), TS by +1.8 °C (39.3 vs. 37.5 °C, p < 0.001) and RR by +46 breaths/min (133 vs. 87 breaths/min, p < 0.001) compared to TN. The 2 g betaine/day treatment decreased TR (39.8, 39.6 and 39.8 °C, p < 0.001), TS (38.7, 38.0 and 38.5 °C, p < 0.001) and RR (114, 102 and 116 breaths/min for control, 2 and 4 g betaine/day, p < 0.001) compared to control. Betaine supplementation decreased plasma NEFA concentrations by ~25 μM (80, 55 and 54 μmol/L for 0, 2 and 4 g/day respectively, p = 0.05). These data indicate that dietary betaine supplementation at 2 g betaine/day provides improvements in physiological responses typical of ewes exposed to heat stress and may be a beneficial supplement for the management of sheep during summer.
Highlights
Temperature is one of the most important physiological stressors [1] and heat stress is a global problem for agriculture resulting in decreased reproduction, growth and production, increased health issues and increased mortality [2,3]
It is generally expected that production animals in Australia will be exposed to more frequent chronic heat exposure (HE) in the future resulting in an increased need for effective mitigation strategies
The data presented in this experiment demonstrated the novel finding that there are dose dependent responses to dietary betaine supplementation in sheep
Summary
Temperature is one of the most important physiological stressors [1] and heat stress is a global problem for agriculture resulting in decreased reproduction, growth and production, increased health issues and increased mortality [2,3]. It is generally expected that production animals in Australia will be exposed to more frequent chronic heat exposure (HE) in the future resulting in an increased need for effective mitigation strategies. The basal RR and magnitude of increase in response to high ambient temperature is influenced by sheep breed whereby temperate breeds tend to have a greater basal RR compared to Middle Eastern breeds [8]. This can amount to an increased RR from the normal range of 20 to 40 breaths/min to 100 to 200 breaths/min when environmental temperatures exceed 35 ◦ C [9]
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