Productivity of grazing Angus steers of different structural size

With the objective of studying the response patterns of two biotypes of dif ferent body (structural) size to stocking rate (SR) in terms of individual growth rate (ADG), meat production per ha (PROD) and backfat thickness (BFT ) under grazing conditions, 64 steers of two biotypes of the Angus breed of different body size were used: small-sized (SB: frame score of 1.3 (s.d. 0 .7)) and medium-sized (MB: 2.9 (s.d. 0.6)). Four levels of SR were applied (1.8, 2.3, 2.8 and 3.3 steers per ha). The experiment lasted 241 days (Apri l to December). Animals were weighed every 14 days, forage availability (FA ) was estimated on four occasions and BFT was recorded at the end of the ex periment (between 12th and 13th ribs). There were no differences between bi otypes in ADG. SE tended to be earlier maturing showing higher BFT (4.39 v. 3.97 mm; P = 0.11). ADG was affected by SX and was higher at lower SR (0.6 12, 0.529, 0.414 and 0.375 kg/day, for 1.8, 2.3, 2.8 and 3.3 steers per ha, respectively). Individual average daily gain of the MB was move variable a cross the seasons of the year. None of the variables showed effects of the biotype x SR interaction but differences in ADG between biotypes as a funct ion of the SR were observed as a trend (P = 0.29). Despite the difference i n frame score between biotypes it was not possible to associate it with con sistent differences in the response variables. Individual productivity (pot ential) of MB was not reflected in a greater productivity per surface unit. The data from this experiment do not allow the choice of a particular biot ype in order to optimize the productivity of the pastoral production system s.