BREED EFFECTS, DIETARY ENERGY DENSITY EFFECTS, AND RETAINED HETEROSISON DIFFERENT MEASURES OF GAIN EFFICIENCY IN BEEF-CATTLE

Retained heterosis for different measures of gain efficiency was estim ated in F-3 generation castrate male progeny in three composite popula tions finished on two levels of dietary energy density (2.82 and 3.07 Meal of ME and 11.50% CP) and serially slaughtered at four end points at intervals of 20 to 22 d. Breed effects were evaluated in nine paren tal breeds (Red Poll [R], Hereford [H], Angus [A], Limousin [L], Braun vieh [B], Pinzgauer [P], Gelbvieh [G], Simmental [S], and Charolais [C ]) that contributed to the three composite populations (MARC I = 1/4 B , 1/4 C, 1/4 L, 1/8 H, 1/8 A; MARC II = 1/4 G, 1/4 S, 1/4 H, 1/4 A; an d MARC III = 1/4 R, 1/4 P, 1/4 H, and 1/4 A). Gain efficiency was eval uated in time constant (0 to 207 d), gain constant (310 to 540 kg), ca rcass weight constant (333 kg), and retail product weight constant (22 5 and 210 kg) end points and to different marbling score and longissim us muscle fat end points. Expressions of gain efficiency included live weight gain/megacalories of ME and retail product weight/megacalories of ME. Significant differences were observed among breeds in all meas ures of gain efficiency. Breeds that had the smallest weight to mainta in tended to be more efficient in live weight gain in the time constan t period, whereas breeds with the highest rate of gain tended to be mo re efficient in the gain constant period. To marbling score or longiss imus muscle fat end points, breeds with the lowest marbling scores and smallest percentage fat in the longissimus muscle on an age constant basis (e.g., Limousin and Gelbvieh) tended to be less efficient, where as breeds with the highest marbling score and highest percentage of fa t in the longissimus muscle on an age constant basis (e.g., Hereford a nd Angus) tended to be more efficient. Breeds with the highest percent age of retail product (Limousin and Gelbvieh) were more efficient to r etail product weight end points, or when retail product weight was the measure of output. Steers fed the higher energy density diet were mor e efficient in live weight gain to time constant, live weight gain con stant, marbling score constant, and longissimus fat constant end point s and were more efficient in the production of retail product weight t o retail product weight end points because of the shorter period of ma intenance. Retained heterosis was not consistent among composites in d ifferent measures of gain efficiency. Higher initial weights resulting in greater requirements for maintenance had a negative effect in comp osites for most measures of gain efficiency.