Quantifying the interactions between defoliation interval, defoliation intensity and nitrogen fertiliser application on the nutritive value of rainfed and irrigated perennial ryegrass

A key goal of temperate pasture management is to optimise the nutritive value and production of pastures. Extensive research has examined the influence of single components such as irrigation, nitrogen (N) fertiliser, and grazing interval and grazing intensity, yet conjecture remains regarding practices that optimise pasture nutritive value. Much of this conjecture relates to interactions between inputs and grazing management. A two-year split-split plot experiment was undertaken to investigate these interactions sing a perennial ryegrass dominant pasture at Elliott, Tasmania. Irrigation treatments (rainfed or irrigated) were main plots and defoliation intervals (leaf regrowth stages; 1-leaf, 2-leaf or 3-leaf) were subplots. Defoliation intensity (30, 55 or 80 mm defoliation height) and N fertiliser (0.0, 1.5 or 3.0 kg N/ha/day) were crossed within sub-subplots. Herbage samples were collected from each plot four times over the experimental period and were analysed for neutral detergent fibre (NDF), acid detergent fibre (ADF) and crude protein (CP) concentrations (% dry matter (DM)). Metabolisable energy (ME) concentration (MJ/kg DM) was estimated from these values. The ME concentration decreased as defoliation height and defoliation interval increased for all time points except during winter. The CP concentration increased with increasing N fertiliser applications in the plots defoliated at the 1-leaf stage, but this increase only occurred as N applications increased from 1.5 to 3.0 kg N/ha/day for the plots defoliated at the 2-leaf and 3-leaf stages of regrowth. As N application rates increased from 0 to 1.5 kg N/ha/day, plots defoliated at the 3-leaf stage had greater increases in NDF concentration compared to plots defoliated at the 1-leaf stage of regrowth, except during spring. As defoliation height and interval increased ADF concentration increased in both spring and summer. While defoliating at frequent intervals (1-leaf stage) and lower heights (30 mm) produced pasture of a marginally higher nutritional value, these benefits are mitigated by the previously established negative consequences of lower pasture yield and poor pasture persistence. Consequently, grazing management that maximises pasture productivity and persistence (defoliated between the 2- and 3-leaf regrowth stages to a height of 55 mm) should be applied to perennial ryegrass pastures irrespective of input management