PSI-A-3 Validation of Rising Plate Meter for Predicting Herbage Mass in Tall-Fescue Pastures

Abstract

Abstract The objective was to compare herbage mass (HM) estimates using a rising plate meter (RPM) and hand-sampled total collections using a forage ring. Accurate measurement of HM can be used to budget available forage in grazing systems, particularly as grass is a dynamic resource. While accurate estimation of HM can be achieved through total forage collection obtained from clipped quadrants, this is labor-intensive and time-consuming. For optimal and informed management, grass needs to be measured quickly and reliably in relation to both accuracy and precision. An RPM can be used to estimate the HM of grasslands based on the compressed sward height (CSH). However, device reliability can be affected by the naturally large variation of dry matter (DM) within CSH, which is governed by numerous factors, such as plant growth state, season species composition, and grassland management regime. Forage samples and heights were obtained weekly from grazed paddocks, which were rotated daily, over a 10-week period from June 6 to August 2, 2022. Paddocks were predominately endophyte-infected tall fescue. Each week two paddocks were randomly selected for sampling. Across both paddocks a total of 9 hand-clipped samples were randomly collected using a 0.25 m2 forage ring. All the forage within the forage ring was collected and dried for 48 h at 55 °C to determine forage DM. A Jenquip Manual Platemeter was used to measure the CSH within the forage ring before clipping. Herbage mass was estimated using the rising plate meter equation (kg DM/hectare = rising plate meter height × 158 + 200). The data were analyzed using CORR procedure of SAS 9.4. Herbage mass measured by total ring collection and RPM were not correlated (r = 0.15; P = 0.17). When RPM and total ring collection forage mass were averaged by pasture, they tended to be correlated (r = 0.44; P = 0.07). When the outlier weeks (samples 1 ,5, 6, and 7) were removed, HM measured by the total ring collection was correlated (r = 0.41; P = 0.01) to RPM. The variability in correlations across pastures and time suggest that further work is needed to understand how variables like season, weather, and clipping procedures affect reliability of RPM to accurately predict forage mass.