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Production output characteristics for cows of breeds or breed crosses varying in genetic potential for growth rate, mature size, and lactation have been previously reported. These differences among breeds were reported for characteristics associated with output potential (e.g., gestation length, calving difficulty, calf crop weaned, and for wt at birth, weaning, and at puberty). We have reported differences among nonpregnant, nonlactating mature cows of breed crosses varying in genetic potential for mature size and lactation yield for dry matter required for weight stasis and metabolizable energy (ME) required for zero body energy change. We have reported significant differences among various breed crosses for energetic efficiency (wt gain of the calf/total ME intake of the cow and calf) during a 138-daylactation period, thus documenting the need for consideration of variation in energy requirements among germ plasm resources that vary in mature size or lactation yield potential.
Variation exists among breeds of beef cattle in energy requirements for maintenance or fasting heat production during the postweaning interval. The trend observed in reviewing this literature was that a positive relationship appears to exist between genetic potential for rate of postweaning avg daily gain and fasting heat production. Researchers have reported that rate of gain as mediated through dietary energy intake affected fasting heat production (maintenance) and mass of metabolically active organs such as the liver, empty gastrointestinal tract, and the kidneys. The relationships between visceral organs and postweaning avg daily gain was documented with rams of similar genetic potential for growth receiving feed ad libitum.Results from this study indicated that the rate of protein accretion for an aggregate of the visceral components such as the gastrointestinal tract, kidneys, and liver was most closely related to the rate of gain.