Similar grazing mechanisms explain contrasting intake and sward-height dynamics under different grazing management

Abstract

Context Average sward height has traditionally been used as a grazing management variable. However, such approach ignores the spatio-temporal heterogeneity of swards. Because the intake rate responds non-linearly to average height at plant level, we expected that sheep exposed to contrasting grazing methods with similar average heights would behave differently. Aims We hypothesised that low-intensity, high-frequency grazing results in a higher daily intake rate than does high-intensity, low-frequency grazing, despite both methods having similar average sward heights over space and time. Also, we asked whether sheep exhibit the same foraging mechanisms for a given instantaneous sward state regardless of long-term grazing conditions imposed. Methods We exposed sheep to two contrasting grazing methods with similar average height; low-intensity, high-frequency (LIHF), and high-intensity, low-frequency (HILF). We then evaluated their foraging behaviour using continuous bite-monitoring paired with detailed sward measurements during grazing down. Key results Sward height decreased markedly during the grazing period in both treatments, but the sward height depletion was faster under HILF (7.5 cm and 20.3 cm for LIHF and HILF), which means that sward structure was severely disrupted. The animals exhibited markedly different herbage intake patterns between the grazing methods, with sheep under LIHF presenting greater total daily herbage intake (850 vs 630 g DM per individual). Also, sheep demonstrated a more stable and higher instantaneous intake rate in the LIHF. Despite the difference in total daily herbage intake, the grazing treatments did not affect the behavioural mechanisms beyond their indirect effects on the rate of change in the instantaneous sward conditions. Grazing treatments created different patterns of sward change and intake rate over time, but the local and instantaneous sward conditions determined intake rate regardless of the grazing method. Conclusions Intake responses of sheep differed between grazing methods with similar average height due to sward heterogeneity over time and space. Grazing mechanisms were not directly affected by the treatments but were influenced by the sward heterogeneity imposed by the grazing management. Implications Grazing management requires not only consideration of the average sward height, but also an understanding of how the resource is distributed in time and space.