Abstract
Chlorophyll meters have been used to estimate leaf chlorophyll content, and therefore nitrogen (N) status and fertilization requirements for several crops. However, until now C4 forage grasses have not been investigated in this regard. A field study was conducted in 1995 and 1996 to evaluate the potential of chlorophyll (SPAD) meter readings to determine switchgrass N concentration and herbage yield. Meter readings were taken on the top most fully expanded leaves of switchgrass grown on a free draining sandy clay loam soil (St Bernard, Typic Hapludalf) fertilized at 0, 75, or 150 kg N ha‐1. The switchgrass was either cut at the end of the season or every four or six weeks. Herbage N concentration in uncut stands declined with time while SPAD readings increased to a mid‐season maximum, beyond which they declined rapidly. The relationship between N concentration and SPAD readings was linear (r2=0.62–0.93; p<0.01) for the cut systems. Exceptforthe 1st cut under the 4‐week harvest regime, there were positive correlations between dry matter yield and SPAD meter readings (r2=0.58–0.96; p<0.01). These results indicate that SPAD meter readings can be used to measure N concentration and/or monitor N availability for seed production, and when N is the limiting factor, to estimate yield of switchgrass (Panicum virgatum L.)
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