The objective of this study was to determine physiological parameters (rectal temperature = RT, respiratory rate = RR, heart rate = HR) and production parameters (dry matter intake = DMI, water intake = WI, average daily weight gain = ADG, and feed conversion = FC) of crossbred and Katahdin lambs raised in rotational grazing of native pastures (NP) and in confinement under heat stress conditions. Nine crossbred lambs (average initial weight = 19.4 kg) were used and fed in a rotational grazing system for 70 days with occupation and rest periods of seven and 21 days, respectively. The lambs had daily access to the grazing area for four to six hours, grass hay and water ad libitum, a daily supplement of 100 g of commercial concentrate and a nutritional block. In the confined system, six lambs of the Katahdin breed (average initial weight = 27.3 kg) were used, and for 28 days these were fed total mixed rations (TMR) containing 30% grass hay, 15.4% soybean meal, 54.6% corn grain and a mineral block. The diet contained 14% Crude Protein (CP) and 66% Total Digestible Nutrients (TDN) and was formulated for an ADG of 200 g. The offered and rejected feed was quantified every day [4% body weight (BW)/dry matter basis (DMB)] to determine the DMI, and the volume of water offered and rejected to determine WI. To validate the stress conditions, the maximum environmental temperature and relative humidity were monitored to calculate the temperature and humidity index (THI), and the RT, HR and RR of the 15 lambs were determined. The lambs were weighed every seven days. In both experiments, the THI oscillated between 78 and 80. The lambs were under environmental heat stress conditions (THI x̅ = 79) during both studies. The averages of HR and RT in the grazing system were 118.0 beats per minute and 38.6° C, while the averages under the confinement system were 108.6 beats per minute and 39.4° C, respectively. The average RR (53.6 and 62.6 breaths per minute, in grazing and in confinement, respectively) was greater than the normal value considered for thermal comfort. Respiratory rate is the main mechanism used by lambs to dissipate heat. The heart rate also showed slightly higher values than those reported for animals in their comfort zone. However, the rectal temperature values were constant. As expected, crossbred lambs fed under grazing conditions with naturalized pastures (NP) had a low ADG (55.6 g). Nevertheless, Katahdin lambs fed in confinement with TMR had a DMI of 935.4 g/day (3.14% BW/DMB), a WI of 3.5 L/day, a FC of 4.74 and an ADG of 226.2 g. In conclusion, raising crossbred sheep in rotational grazing of NP without any type of agronomic management results in ADG that could not be considered viable in traditional production systems. The introduction of pure breeds and feeding them in confined systems with a TMR results in a pre-determined ADG (>200 g/day) and is an alternative that can be evaluated economically for its practical implementation.
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