Changes in growth rate and morphology of perennial ryegrass swards at high and low nitrogen levels

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Abstract
The nitrogen requirements for maximum production of perennial ryegrass swards in August/September were shown to be of the order of 4 kg N/ha/day. Further increases above this level had no appreciable effect on dry-matter production, leaf area or light intercepted, but maximum tiller numbers were considerably enhanced. Shortage of nitrogenous fertilizer had comparatively little effect on crop growth rate in the early stages of regrowth, but thereafter caused the rate to fall increasingly short of potential. At high fertilizer levels crop growth rate based on total above-ground parts was linearly related to percentage light intercepted in the first month after defoliation, but values subsequently became erratic and at times negative. This change in crop growth rate and the resulting halt in effective net dry-matter production could be associated with the overall pattern of leaf and tiller formation and death, maximum net yield being achieved at the point in time when three new leaves had been produced on each tiller since cutting. It is concluded that in August and September worth-while increases in harvestable net dry matter are unlikely to occur after this stage has been reached, and that managements based on the maintenance of a complete crop cover are not likely to be successful at this time of year.
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