ABSTRACT
Summary To improve our understanding of grassland utilization under grazing, the physiology of a grazed sward was investigated. An enclosure method, infrared gas analysis, and radiocarbon were used to measure sward and single-leaf photosynthesis and assimilate utilization. Intake by sheep was also measured. Expanding and recently expanded leaves had high rates of net photosynthesis but leaf sheaths, whose projected area in a “hard” grazed sward equaled that of the lamina, had low efficiency and contributed little to canopy photosynthesis. But because high leaf efficiency did not compensate for the small leaf area, canopy photosynthesis decreased markedly as grazing intensity increased. Twice as much of the photosynthetic production was harvested as intake in hard grazing as in lenient grazing, but actual intake /ha was only one-third greater because the initial photosynthetic production was less. Conversely, much more carbon was lost to death and decay in the leniently grazed sward. Maximum animal production requires the consumption of as much as possible of the young and most nutritious foliage, whereas maximum sward production requires a fully light-intercepting canopy of young, photosynthetically efficient leaves. The resolution of this conflict poses formidable problems in the continuously grazed sward, and rotational grazing appears to have provided only a partial solution.
