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This paper quantifies the effects of N-stress on development and growth of sorghum by identifying critical values for stover N% (SNC) and specific leaf nitrogen (SLN) for a range of physiological processes (rates of leaf appearance, leaf expansion, leaf area increase, and biomass accumulation). It also compares the merits of the SNC and SLN approach for implementation in the N-routines of crop growth simulation models. Data from experiments covering a range of nitrogen treatments, grown at three contrasting locations in Australia, were used in the analyses. For the rate of biomass accumulation, the critical and minimum SLN were constant for most of the growing season, except for early stages prior to approximately panicle initiation. For SNC, however, the critical and minimum values continued to decline throughout the growing season, although the decline was limited at later development stages. During the stage of leaf growth, the critical N content was similar for all processes considered. Although critical SLN was less sensitive to development stage than SNC, identification of a critical SLN was hampered if environmental effects on specific leaf area were present. The merit of each approach in simulation models will depend upon their ability to capture genotypic differences in N-dynamics. (C) 2001 Elsevier Science B.V. All rights reserved.