Live Weight Change as a Dynamic Indicator of Equation-Derived Herbage Intake in Part-Time Grazing Dairy Ewes

Accurate estimation of individual herbage intake in grazing ruminants is essential for improving feeding efficiency and animal performance, yet it remains challenging under field conditions. This study evaluated the association between live weight dynamics recorded by an automated walk-over-weighing (WoW) system and equation-derived herbage dry matter intake (HDMI) in dairy ewes and whether this association changed throughout lactation. The study was conducted over two years (2023–2024) using 92 Sarda dairy ewes managed under part-time grazing with two daily pasture access times (2 or 6 h day−1). Live weight was recorded daily before and after pasture access, and daily live weight change was averaged weekly. HDMI was estimated using a published prediction equation based on milk yield, herbage neutral detergent fibre concentration, and pasture access time. Live weight change was partitioned into between-animal (DW_mean) and within-animal (DW_dev) components, allowing persistent differences among animals to be distinguished from short-term within-animal fluctuations, and analysed using linear mixed-effects models. In the pooled model, both components were positively associated with equation-derived HDMI (p < 0.001), while the significant DW_dev × days-in-milk interaction (p = 0.004) indicated that the association between within-animal live weight change and equation-derived HDMI decreased as lactation advanced. However, pasture access-time-specific sensitivity analyses showed that the associations with DW_mean and DW_dev persisted within AT-6h but were not detected within AT-2h, while adjustment for milk yield attenuated the within-animal association, which was no longer statistically significant, and also removed statistical support for the DW_dev × days-in-milk interaction. The mixed-effects model explained a substantial proportion of the variability in equation-derived HDMI (marginal R2 = 0.64; conditional R2 = 0.94), and equation-derived and model-fitted HDMI were positively correlated (r = 0.77). These findings indicate that WoW-derived live weight dynamics provide non-invasive information associated with equation-derived HDMI under the conditions of this study, but independent validation against directly measured individual herbage intake is required before their use for individual intake estimation.