Relationship between sward height and herbage mass for integrated crop-livestock systems with trees

Sward height (SH) is an important management variable for both continuous and intermittent stocking, since it is easily measured and highly correlated to herbage mass (HM). However, in systems with trees, tree shading can alter the relationship between SH and HM by changes in the sward structure. Understanding these relationships under shading could help identify and design management practices for swards in association with trees. The aim of this study was to investigate the SH and HM relationship for a continuously stocked mixed black oat (Avena strigosa Schreb.) + annual ryegrass (Lolium multiflorum Lam.) pasture in two systems, i.e. with and without trees, with two levels of nitrogen (N) (90 vs. 180kg N ha(-1)), three replicates and over 2 years (2014/2015). Comparison of regression lines was performed between SH (x-variable) and HM (y-variable), showing mainly a system effect. Shading altered the relationship between SH and HM in order of magnitude (i.e. different intercepts), but not in the rate of change (i.e. equal slopes). Regardless the SH, oat + ryegrass mixture under trees produced 698kg of dry matter (DM) ha(-1) less than those without trees. Therefore, a target SH used at full sun systems as an index for management (e.g. 20cm) might not apply under trees with >50% shade, since HM may be limiting to ensure intake of grazing animals (e.g. <1200kg DM ha(-1)). The shade level must be controlled to optimize the yield of these forage species and to maintain the necessary HM to ensure the sustainability of no-till silvopastoral systems.