Adaptation of feedlot cattle to repeated sinusoidal heat challenge

摘要

An important component of cattle response to heat challenge is the ability to adapt. A study was conducted to assess adaptation to repeated sinusoidal heat waves. Twelve Angus x Simmental steers (419+-9.8 kg) were provided a typical finishing diet and water ad libitum, and first exposed to either 21 days thermoneutral (TN; 19+/-7 deg C, 30-80 percent RH) or 7 days TN followed by 14 days heat challenge (HS; 33+/-7 deg C, 15-55 percent RH). After a 14-day recovery (TN), all cattle were exposed to 7 days TN followed by 14 days HS. Thermal conditions were recorded continuously, and temperature humidity index was calculated. Intraperitoneal telemetric transmitters monitored body core temperature (T_(core)). The predicted point at which T_(core) increased was best determined by using a T_(core) lag of 3 hours relative to air temperature (T_a). The breakpoints between prior heat-challenged animals and 1st exposure animals were similar (24.8-25.1 deg C). Temperatures above these break points were used toplot the slope of T_(core) to T_a relationships for beginning and ending periods of HS for both groups (days 11-13 and 19-21). The slope of the line did not change with repeated HS exposure, but the line shifted downward approximately 0.3 deg C for both1st exposure groups during the 14 day HS period. There was overlap of T_(core) to T_a relationship at the start of HS for steers exposed for the first or second time to HS indicating little effect of previous exposure. However, steers exposed a secondtime to HS experienced a 0.5 deg C reduction in T_(core) response to T_a with repeated exposure. The results show that adaptation does occur with continuous heat challenge, but there is tio clear indication that adaptation carries over following a 14-day recovery period at thermoneutrality.