GROWTH RESPONSE OF YOUNG AND MATURE STEERS WITH RESPECT TO FEED TYPE (GRAIN OR FORAGE) AND SUPPLEMENT TYPE (ENERGY OR PROTEIN-BASED SUPPLEMENT)

Two experiments were conducted to compare the growth response of young (200 ± 1.5 kg live-weight) and mature (451 ± 5.8 kg live-weight) steers with respect to levels and type of supplement [Experiment 1, barley grain (BG) versus cottonseed meal (CSM) supplements] or feed [Experiment 2, ryegrass haylage (RH) versus concentrate-based diet (CBD)]. The young and mature steers were ca. 0.33 and 0.73 as a proportion of standard reference weight (SRW), respectively. It was hypothesized that young steers would grow faster than mature steers because of the composition of the gain and that diet and supplement type would have most effect on live-weight gain (LWG) of mature fattening steers because of their requirement for glucogenic precursors for fat deposition. Both experiments used a randomised block design with two age groups (young and mature steers); two supplement types (BG versus CSM) for Experiment 1 or two feed types (RH and CBD) for Experiment 2. Live-weight gain, body condition score (BCS) or fat depth (at 13th rib and p8 sites), feed intake and digestibility were measured along with the concentration of plasma glucose and plasma urea nitrogen, rumen pH, and the concentration of rumen ammonia-N (NH3-N) and volatile fatty acids (VFAs). In Experiment 1, 21 young and 21 mature steers were offered a basal diet of pangola hay [42 g crude protein (CP)/kg dry matter (DM)] ad libitum, either alone (control) or supplemented with either CSM (482 g CP/kg DM) or BG (110 g CP/kg DM), in one of four amounts. Cottonseed meal was offered at 2.5, 5.0, 7.5 or 10.0 g/kg live-weight (W)/day (d), while BG was offered at 5, 10, 15 or 20 g/kg W/d. The results showed that there were significant quadratic relationships (