A plate meter inadequately estimated herbage mass in a semi‐arid grassland

A resting plate meter was tested in the laboratory and on a field site to determine its effectiveness for estimating peak herbage mass on semi‐arid grasslands. In laboratory tests, data from the plate meter closely predicted the herbage mass of four perennial bunch grasses (thirteen of sixteen tests with r 2 ≥ 0·90) but the closeness of the prediction varied with the pressure of the plate. Field tests took place on a southern Arizona, USA semi‐arid grassland in 2005, 2006, 2007 and 2008. Twenty teams of two or three people sampled the site; five teams in each year measured eight to thirteen plate heights and clipped the vegetation beneath. Consistent with a need for yearly calibration, a significantly different (P < 0·05) linear relationship was detected between 3 of the 4 years which was associated with differences in average herbage mass of dry matter (DM) (1525 kg ha−1 for 2005, 2093 kg ha−1 for 2006, 1338 kg ha−1 for 2007 and 1370 kg ha−1 for 2008). Plate height poorly predicted herbage mass within years (r 2 = 0·21, 0·51, 0·49 and 0·41 respectively) with plate heights explaining half or less of the variability in field herbage mass and having a mean prediction error of 466 kg DM ha−1. The plate meter technique had limited potential for estimating peak herbage mass in semi‐arid grasslands.