VARIATION AMONG TWIN BEEF-CATTLE IN MAINTENANCE ENERGY-REQUIREMENTS

The genetic variation in energy expenditures of cattle at fasting (FHP) and maintenance (ME(m)) was determined by using 12 pairs of monozygous twins at 20 mo of age. The pairs were of two breed types, eight Angus x Hereford (three steers, five heifers) and four Barzona x Hereford (three steers, one heifer). The heifers were 132 +/- 13 d pregnant at the time of measurement. The pairs were fed at 1.15 x maintenance energy requirements for a minimum of 30 d prior to heat production (HP) measurements in dual indirect respiration calorimetry chambers. The diet fed was cracked corn: alfalfa hay (45:55) with a determined ME of 2.47 +/- .02 Mcal/kg DM. This diet was fed individually for 7 d prior to and during two consecutive 22-h HP measurements. The animals then were fasted for 2 d and fasting heat production measurements (FHP) were made on d 3 and 4 of the fast. Metabolizable energy required for ME(m) was calculated iteratively by assuming a semi-log relationship between HP and metabolizable energy intake. There were no differences (P < .10) in measured energy expenditures due to different breed type. The FHP and efficiency of ME use for ME(m) (K(m)) were similar between sexes, although heifers had lower (P < .025) ME(m) than steers. Twin pair effects were detected for FHP (P < .005) and ME(m) (P < .05) but not for K(m). Broad sense heritability estimates were calculated as the intraclass correlation between members of monozygous twin pairs. Heritability estimates for ME(m), FHP, and K(m) were .52 +/- .22, .75 +/- .13, and .34 +/- .27, respectively. These results demonstrate the presence of a genetic component to maintenance energy metabolism measured as FHP and ME(m); selection to change these traits should prove effective.