Improving pooled calibration of a rising‐plate meter for estimating herbage mass over a season in cool‐season grass pasture

Abstract

To construct a new calibration method that combines usability and accuracy for estimating herbage mass from rising‐plate meter readings, we derived four models differing in the way their parameters are related to sampling date and compared their estimation accuracies using cross‐validation. The parameters of the linear regression for each sampling date showed seasonal variations, which had a steep decrease from early April to early June and a gradual increase thereafter. The pooled models were less accurate for estimating herbage mass than a separate model, which had specific parameters for each sampling date (S model). Among the pooled models, however, those in which the parameters were assumed to be linear functions (PL model) or combined functions (PC model) of the sampling date showed substantively improved estimation accuracy compared with the traditional pooled model, in which the parameters were assumed to be fixed throughout the year (PF model). Moreover, at the beginning of the season, the models derived from previous years' data were suggested to be applicable as a practical method. Thus, it can be concluded that these types of pooled calibration could be used as ‘compromise methods’ that combine both accuracy and usability.

Effects of sites and years on the coefficients of rising plate meter calibration under varying coefficient models

“…The former would be attributed to the accumulation of dead residue caused by the management in the previous season (Braga et al 2009;Ferraro et al 2012;Nakagami and Itano 2014) and the latter to the management practice and the meteorological condition affecting the dead-material content after the reproductive stage.…”

“…The following two models proposed in Nakagami and Itano (2014) were derived: 1. PL model: The coefficients were assumed to have simple linear type relationships with the sampling date:…”

“…However, seasonal patterns in the individual calibrations of RPM reading and herbage mass (e.g. Braga et al 2009;Ferraro et al 2012;Nakagami and Itano 2014) would require increased effort to maintain estimation accuracy. Our previous study (Nakagami and Itano 2014) proposed a calibration method by assuming "varying coefficients" (Hastie and Tibshirani 1993), where the coefficients are functional upon individual sampling dates.…”

Evaluation of a Rising Plate Meter for Use in Multispecies Swards

“…In pasture-based dairy systems, accurate budgeting of available forage is crucial to maintaining milk production while maximizing limited pasture resources. However, accurate calibration of the RPM is difficult due to variability in sward composition and forage growth (Nakagami and Itano, 2013).…”

A method for approximate on‐farm estimation of herbage mass by using two assessments per pasture

“…The RPM was used to take readings at 9–10 points at approximately 5‐m intervals (three points × three rows or five points × two rows) in each area. Herbage height was converted to herbage DM at 5 cm above ground level using pooled calibration formulae in which the parameters were assumed to be linear functions of the day of year (Nakagami and Itano, ). Finally, the average mass from the two selected areas in each pasture was corrected by multiplying by the factor derived in the simulation study after setting negative mass values to zero.…”