Abstract
Six steers of each breed type, Hereford ($H{\times}H$), Brahman ($B{\times}B$) and Brahman $\times$ Hereford ($B{\times}H$) were ranked on liveweight and allocated to three treatments, basal hay diet (Basal), Basal plus 1 kg/head/day of cottonseed meal (Basal + CSM) and Basal plus 1 kg/head/day of formaldehyde-treated sunflower meal, $Norpro^{(R)}$ (Basal + NPO). The hay was made from a pasture based predominantly on carpet grass (Axonopus affinus) growing in subtropical New South Wales, and had an estimated organic matter digestibility of 52% and a nitrogen (N) content of 7.8 g/kg dry matter (DM). The steers were accustomed to the Basal diet over 15 days and supplements offered over 42 days. Intake of the basal hay diet by steers was not increased by supplementation. When intakes were adjusted for differences between breed types in liveweight the $B{\times}H$ steers ate 25% (P<0.01) more hay than $H{\times}H$ steers (6.3 V 5.0 kg/head/d) and $B{\times}B$ steers ate 8% less hay than $H{\times}H$ steers. Supplementation significantly (P<0.01) increased liveweight gain during the experiment, being ($g/head/d\;{\pm}\;s.e.d.$) 290, 770 and $795{\pm}118$ respectively for Basal, Basal + NPO and Basal + CSM. There was no difference between supplements in the liveweight gain of steers nor between steers of different genotypes. However, there was a significant interaction (P<0.01) between breeds and treatments such that $B{\times}B$ steers gained most on the basal diet but least of the breed types when supplemented. The estimated non-degradeable fraction of N in the protein meals was 58.5 and 44.5%, respectively for NPO and CSM. Both meals increased (P<0.01) plasma urea N and rumen ammonia N concentrations.