Calibration of a rising plate meter for herbage mass estimation in ryegrass-white clover mixed swards containing different proportions of plantain (Plantago lanceolata L.)

Abstract

The objective of this study was to develop calibration equations for a rising plate meter (RPM) to estimate herbage mass in irrigated ryegrass/white clover (RGWC) swards with varying proportions of plantain. A 32-ha area, divided into 0.3-ha paddocks, was randomly allocated to three treatments sown with increasing plantain seeding rates: (i) RGWC with no plantain, (ii) RGWC + 3 kg/ha, and (iii) RGWC + 6 kg kg/ha. Over two production years (2021/22 and 2022/23), 1138 quadrat cuts and 228 botanical samples were collected. Data were grouped by season (late-winter, spring, summer, and autumn) across treatments and years. A multivariate linear regression model incorporating compressed sward height, plantain % in sward DM and season explained 73.5% of the variation in herbage mass (R² = 0.735; P<0.001). Compressed sward height was positively associated with herbage mass (estimate ± SE; 145 ± 6.2 kg DM/ha per RPM unit; P < 0.001), while plantain % was negatively associated (–4.5 ± 0.12 kg DM/ha per 1% increase; P < 0.001). Season significantly affected the intercept (P < 0.001), being -128 ± 135.0 (late-winter), 82 ± 104.0 (spring), 251 ± 106.0 (summer) and -81 ± 111.0 (autumn) kg DM/ha. Validation using an independent dataset from the 2023/24 production year showed the model predicted herbage mass with a root mean square error (RMSE) of 350 kg DM/ha. These equations can be integrated into RPM devices to estimate herbage mass in rotationally grazed, irrigated RGWC swards containing plantain, accounting for both plantain content and seasonal variation.