This study was conducted to determine lysine requirement in laying hens, using plasma free lysine levels as a response criterion. The basal diet used was based on corn- corn gluten meal- soybean meal, and contained 0.58% lysine, Nine graded levels of lysine (0.04, 0.08, 0.12, 0.16, 0.2, 0.24, 0.28, 0.32, 0.36%) were added to the basal diet. Each experimental diet was fed to 48 laying hens in a randomized complete design with 4 replicates in each treatment. Broken-line method was employed for estimation of lysine requirement. Dietary lysine levels exerted a significant effect on egg weight, egg mass output, feed intake (P< 0.01), and feed converstion ratio (P< 0.05), but did not signifcantly affect egg production. Based on egg mass output, feed conversion ratio and plasma free lysine responses, lysine requirements were estimated at 675, 790 and 785 mg per hen - day; respectively. The results suggest that plasma free lysine could be a precise indicator (R2=0.96) for determining lysine requirement in laying hens. In conclusion, based on the results of this experiment, daily lysine requirement of laying hens during peak egg production was estimated at 785- 790 mg per hen- day.
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