Extreme or rapid changes in environmental conditions can often be detrimental to cattle performance and well-being (Webster, 1973; Hahn, 1995). However, if climatic changes are not too abrupt, cattle can buffer effects of, and adapt to, changing environmental conditions through physiological and metabolic processes. Kamal and Ibrahim (1969) reported that thyroid gland activity in summer was 16% less than in winter allowing for a decrease in metabolic rate and muscle activity to occur and overall heat production to be reduced. In addition, when cattle were exposed to heat stress, blood urea nitrogen levels were found to decrease by 16% in lactating cows and by 28 to 30% in calves (Habeeb et al., 1992). The depression in blood urea nitrogen appears to be a result of re-absorption from the blood to the rumen to compensate for the decrease in ruminal ammonia-N due to reduced feed intake (Habeeb et al., 1992). When cattle are exposed to cold stress, gastrointestinal tract motility increases due to an elevated metabolic rate, resulting from an increase in thyroid hormone activity (Westra and Christopherson, 1976; Kennedy et al., 1977). Thus, feed intake is often enhanced in cattle exposed to cold environments (NRC, 1996). There is evidence that when fed oestrogenic anabolic agents, IGFI concentrations are increased (Breier et al., 1988; Johnson et al., 1998). Change in body temperature, particularly when cattle are under environmental stress, is another physiological characteristic that could be influenced by anabolic agents. However, no data are available regarding effects of anabolic agents on body temperature. The aim of the present experiment was to determine the effect of season on feed intake, blood metabolites and body daily weight gain of Holstein steers fed diets providing different concentrate:lucerne hay ratios (C60: L40, C80: L20) in a semi-arid climate.

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