Periodic changes in body composition and in priorities for tissue storage and retrieval in mature beef cows

Sixty mature (4 to 6 yr), nonpregnant, nonlactating beef cows of two breed types, Angus (n = 30) and Simmental (n = 30), were used in a four-period, split-plot study (371 d) to determine the seasonal differences in body composition. Periods were chosen to represent each of the four seasons in one year that ran consecutively from June 14, 1986 to June 20, 1987. Initially, all cows were assigned randomly within breed type to one of four feeding levels based on the cows' estimated maintenance requirement (75, 87.5, 112.5, and 125% of weight maintenance) and were rotated such that all animals were fed at all four feeding levels during the course of the study. Body composition was estimated via isotope dilution initially and at the end of Periods 1 through 3 and via derived regression relationships after Period 4. Body composition differed by breed; Simmental cows had greater (P < 0.05) amounts of empty body protein (79.4 vs 55.8 kg, respectively) and less (P < 0.05) empty body fat (85.3 vs 93.9 kg, respectively) than the Angus cows. These differences also were apparent when expressed as a percentage of live weight. Season altered components of the empty body. On the average, both breeds mobilized (P < 0.05) empty body protein (-3.6 and -5.4 kg for Angus and Simmental cows, respectively) in the summer while simultaneously gaining (P < 0.05) empty body fat (15.3 and 30.1 kg for Angus and Simmental cows, respectively). Both breeds subsequently tended to gain empty body protein in the winter and spring. Body composition differed with season, perhaps due to differences in priorities for deposition or mobilization of protein vs fat of mature beef cows. Therefore, nutrient requirements may differ with season due to changes in body composition and tissue exchange.