Evidence of increasing thermal requirement in young, unadapted calves during 6 to 11 days of age.

Changes in thermal demand in restrictively fed, unadapted, young calves were studied during the first days after transportation. Twenty-three 6-d-old Holstein-Friesian male calves were assigned to one of four ambient temperature treatments: 5, 9, 13, or 18 degrees C. Calves were fed at a constant level below the maintenance requirement (300 kJ of ME.kg-.75.d-1). After arrival, heat production (HP) and energy and nitrogen balance were measured for each calf for 5.5 d. At ambient temperatures of 5 and 9 degrees C, HP was increased compared with temperatures of 13 and 18 degrees C (P < .001). As a consequence, energy retention was decreased at low ambient temperatures (P < .05). At temperatures of 5 and 9 degrees C, body fat mobilization was increased compared with that at temperatures of 13 and 18 degrees C (P < .001). Energy retained as protein was not affected by ambient temperature. During the experimental period, HP decreased with time. Ambient temperature affected this decrease (P < .001), indicating that the relationship between HP and ambient temperature changed with time. The rate of increase in HP below the lower critical temperature was not affected by time, averaging 9.5 kJ.kg-.75.d-1.C degrees-1. Lower critical temperature, however, increased with time by .89 C degrees/d (P < .05). The changing relation between HP and ambient temperature with time in young, unadapted calves after transportation is thus reflected in an alteration in thermal requirement with time.