Brosh A, Aharoni Y, Degen A A, Wright D, Young B A
Department of Beef Cattle, Agricultural Research Organization, Institute of Animal Science, Newe Ya'ar Research Center, Ramat Yishay, Israel.
J Anim Sci. 1998 Oct;76(10):2671-7. doi: 10.2527/1998.76102671x.
Ten growing heifers were either exposed to or protected from solar radiation, offered a diet of either high (H) or low (L) ME, and fed either in the morning or afternoon during a hot summer. Heifers that consumed the H diet had a greater water intake, DMI, metabolizable energy intake, energy expenditure, and retained energy than heifers that consumed the L diet. Solar radiation did not have an effect on any of these variables. Furthermore, dietary energy and time of measurement had an effect on rectal temperature (Tr), respiration rate (RR), heart rate (HR), and rate of oxygen uptake (VO2); solar radiation had an effect on Tr and RR but not on HR and VO2; and time of feeding had an effect only on VO2. Heifers coped with greater heat loads by increasing RR and the difference in Tr between morning and afternoon. It seems that a lowered body temperature in the morning is a physiological mechanism used by animals to prepare for the heat load that develops during the day. Heat production (HP) and HR throughout the day were affected mainly by the time of feeding and not by the environmental heat load. Feeding in the afternoon increased HP in the cooler hours of the day when heat losses from the animal through conduction and radiation were more efficient. With a pending high heat load situation, reducing feed quality and(or) changing the time of feeding to the late afternoon could be beneficial to the animals in reducing their heat loads.
十头生长中的小母牛在炎热的夏季要么暴露于太阳辐射下，要么受到太阳辐射的保护，分别给予高（H）或低（L）代谢能的日粮，并在上午或下午喂食。与摄入低代谢能（L）日粮的小母牛相比，摄入高代谢能（H）日粮的小母牛饮水量、干物质采食量、可代谢能量摄入量、能量消耗和能量留存更高。太阳辐射对这些变量均无影响。此外，日粮能量和测量时间对直肠温度（Tr）、呼吸频率（RR）、心率（HR）和摄氧率（VO2）有影响；太阳辐射对Tr和RR有影响，但对HR和VO2无影响；而喂食时间仅对VO2有影响。小母牛通过增加RR以及上午和下午Tr的差值来应对更大的热负荷。似乎早晨体温降低是动物为应对白天出现的热负荷而采用的一种生理机制。全天的产热（HP）和HR主要受喂食时间影响，而非环境热负荷影响。在一天中较凉爽的时候喂食，此时动物通过传导和辐射散热更有效，下午喂食会增加HP。在即将面临高热负荷情况时，降低饲料质量和（或）将喂食时间改为下午晚些时候可能有利于动物减轻热负荷。
