A standardized plate meter for estimating pasture mass in on-farm research trials

A plate meter for estimating pasture mass (PM) was constructed of 5.6-mm-thick acrylic plastic cut into a 46-cm square and fitted with a 3.8-cm center hole. (The acrylic is stable under varying humidity, readily available, inexpensive, and easy to work.) A meter stick through the center hole measures the plate height (PH) resting on the pasture canopy. The meter was tested on six rotationally grazed pastures (0.2-2.4 ha), measured before eight grazing events. Each pasture was double-sampled for PH and clipped PM at 10 to 30 locations. The calibration regression slope was 452 kg (ha cm)-1, with a residual standard deviation (RSD) of 753 kg ha-1 and an r2 of 0.52. The high RSD is typical of forage stands, which vary widely in forage bulk density. The relatively low r2 was probably due to the uniform height of these rotationally grazed (as opposed to continuously grazed) pastures resulting in a small range in PH. The validation regression had a slope of 0.72 and a RSD of 808 for individual samples and a slope of 1.11 and RSD of 155 for means of 20 samples. The reduction in RSD with increasing sample size points to the need to take 15 to 30 measurements when calibrating the plate meter or sampling rotationally grazed pastures, to have an estimate within 10% of the mean PM. This simple plate meter provides as accurate a measure of PH and PM as the more sophisticated metal plate meters, is practical for carrying in the field, and in 1996 cost $12