The use of a rising plate meter to evaluate lucerne (Medicago sativa L.) height as an important agronomic trait enabling yield estimation

A rising plate meter (RPM) is an instrument used for the measurement of compressed sward height and estimation of yield on grasslands. The aim of this study was to investigate the use of an RPM to evaluate lucerne height as an agronomic trait enabling yield estimation. Compressed height (CH), stem length, plant and stem density and yield measurements were recorded from the late‐vegetative stage to flowering in the first cut and from the late‐bud stage to flowering in the second and third cuts, during three successive years. For lucerne maximal stem‐length prediction, CH could be used successfully (with R ² over 0·90) in developmental stages before flowering and for stem lengths up to 80 cm, irrespective of stand density or order of cut. The dry‐matter yield of lucerne was better predicted using CH (R ² = 0·72) than maximal stem length (MSL) (R ² = 0·53). The recommended number of observations was 7–9 per treatment in the first cut and twice this in subsequent cuts because of higher variability in stand structure. The use of RPM to measure CH of lucerne is not recommended when MSL exceeds 80 cm, or in the case of lodging. RPM may also be suitable for similar forage crops with erect or semi‐erect stems, subject to consideration of disc weight and diameter. Heavy discs are unsuitable for lucerne height prediction but could be more suitable for yield prediction.