Levy, D., Agricultural Research Organization, Newe Ya'ar Regional Experiment Station, Haifa Post, Israel
Holzer, Z., Agricultural Research Organization, Newe Ya'ar Regional Experiment Station, Haifa Post, Israel
Samuel, V., Agricultural Research Organization, Newe Ya'ar Regional Experiment Station, Haifa Post, Israel
Bruckental, I., Agricultural Research Organization, Institute of Animal Science, Bet Dagan, Israel

Two feeding trails were made and in both all the diets given contained about 11 MJ metabolizable energy (ME) per kg dry matter (DM). In trial 1, diets containing three concentrations of crude protein (CP) were given to a total of 84 Friesian bulls in each of three stages of growth. They were, in stage 1 (150 to 250 kg live weight) 130, 145 and 160 g CP per kg dietary DM; in stage 2 (250 to 350 kg live weight) 110, 130 and 145 g CP per kg dietary DM; in stage 3 (350 kg to slaughter) 95, 110 and 130 g CP per kg dietary DM. In stages 2 and 3, the source of supplementary nitrogen at each CP concentration was either soya-bean meal or non-protein nitrogen (NPN) giving six combinations of concentration and source of dietary CP. Live-weight gains in stage 1 were 1·52, 1·60 and 1·58 kg/day respectively (P > 0·05). In the stages involving both concentrations and source of nitrogen (soya-bean meal in parentheses), the mean daily live-weight gains were (1·40), 1·38, (1·32), 1·34 (1·34) and 1·31 kg for stage 2 (F > 0·05) and (1·14), 1·14, (1.24), 1·16, (1·06) and 1·08 kg for stage 3 (P < 0·05), for the diets of 130, 145 and 160 g CP per kg diet respectively. Trial 2 consisted of six stages of growth, the first five of 45 days duration and the sixth of varying length up to a predicted slaughter weight, and seven dietary treatments. Treatment 1 was a negative control (NC) with a dietary CP concentration of 90 g/kg DM. For the other six treatments two concentrations of dietary CP were given; 120 g/kg and 140 g/kg in stages 1 and 2 and 100 g/kg or 120 g/kg in stages 3 to 6. Dietary CP concentration was increased by the addition of NPN, fish meal or soya-bean meal. A total of 98 Friesian bulls were used with an initial average live weight of 185 kg. Daily gain of the NC group in the six stages of the trial was 0·86, 0·97, 101, 1·13, 1·07 and 1·00 kg for stages 1 to 6 respectively. From the results it was concluded that feeding NPN is not efficient at live weights lower than 250 kg and that there is no advantage to feeding fish meal over soya-bean meal in diets of medium energy concentration at live weights heavier than 250 kg. © 1986, British Society of Animal Science. All rights reserved.