The present study was conducted to examine the behaviour, blood constituents, growth rate and feed utilization of beef calves when the tryptophan was orally given over 14 days following weaning at 3 months of age. Ten naturally suckled Japanese Black calves were allocated to one of two treatments : tryptophan and control (water), 5 animals each after balancing the two groups for sex and weaning weight. Each calf in the tryptophan-fed group received the granules of rumen-bypassed tryptophan (80 mg kg^<-1> of BW, including 30% L-tryptophan) on pre-weaning day (Day -1), weaning day (Day 0) and alternate days after weaning. Maintenance and social behaviours were recorded between 1300 and 1800 h using a video camera on Days 0,1,2,4,6 and 9,and blood samples were taken by jugular venipuncture on Days 1,4 and 9 in order to determine the plasma glucose concentration, the percentage of eosinophils to total white blood cell counts and the neutrophils/lymphocytes ratio (N/L). Additionally average daily weight gain and feed conversion were calculated during the post-weaning period (14days). There was a tendency for tryptophan-fed calves to spend more time in lying and less time in exploratory behaviour than controls (P<0.10). Agonistic behaviour, social investigative behaviour and play (mainly mockfighting) were markedly reduced by tryptophan administration (P<0.05). Though the N/L of tryptophan-fed calves tended to be higher than that of controls on Day 1 (P<0.10), there were no marked differences in plasma glucose, the percentage of eosinophils and N/L between treatments on other post-weaning days. Tryptophan-fed calves showed higher growth rate than controls (1.5kg vs. 0.9kg, P<0.01). These results indicated that the oral administration of tryptophan tended to increase lying behaviour, and suppressed agonistic and social investigative behaviours, which might induce the improvement of growth even though the current tryptophan dose was reduced by half as much as our previous study. Jpn. J. Livest. Management, 33(3) : 65-72,1998 Received August 20,1997 Accepted February 5,1998

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