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Eight Hereford steers with an initial weight of 344 kg and a final weight of 476 kg were allotted randomly to two groups that were alternated between low (120 x maintenance) and high (2.27 x maintenance) intakes during five consecutive seasons in order to measure effects of season (cold stress) and intake on thermoneutral energy requirements. The steers were housed outdoors with neither shelter nor bedding and adapted to the specified intake levels for 21 to 31 d before each season's measurement of maintenance. Maintenance energy requirements were estimated as fasting heat production (FHP) and maintenance metabolized energy (MEm) by respiration calorimetry at thermoneutral temperatures. The high plane of nutrition caused a consistent increase of about 7% in FHP and 14% in MEm averaged across seasons (P less than .05). Within plane of nutrition, season had no effect on FHP or MEm, with the exception that values recorded in fall were lower than those recorded during the first summer (P less than .05). Nevertheless, daily gain during the winter acclimatization period was lower (P less than .05). We conclude that season had no effect on thermoneutral maintenance energy requirements (FHP, MEm) even though cold increased requirements for weight maintenance or gain. No interactions were found between season and plane of nutrition for the parameters measured. Acute cold stress, rather than chronically elevated metabolic rate, likely was responsible for poorer feedlot performance during the cold months of the year.
Birkelo et al. (Tue,) studied this question.