In temperate regions of Australasia, feed-base management is the key determinant of the economic viability of dairy enterprises. However, conjecture exists regarding agreed grazing management practices for pasture-based dairy systems, because of the combined effects of variable seasonal conditions and input management (irrigation and nitrogen (N) usage). To address this conjecture a 2-year defoliation study was undertaken in the high-rainfall zone of north-western Tasmania, to examine the effect of these interactions on the yield of perennial ryegrass (Lolium perenne L.) dominated pasture. Treatments were imposed in a split-split-plot design with moisture availability the main plot treatment (irrigated or dryland), defoliation intervals (full emergence of 1, 2, or 3 new leaves/tiller) assigned to subplots, and both defoliation intensity (30, 55 and 80 mm) and N application rate (0.0, 1.5 and 3.0 kg N/ha.day) treatments crossed within sub-subplots. Although the independent effects of each treatment on total yield were significant (P < 0.05), the effect of N application was found to diminish with time (P < 0.05). Furthermore, under periods of high pasture growth resulting from the absence of moisture stress (irrigation), shortening the grazing rotation via defoliating at the second leaf stage had no detrimental impact on growth rates. However, to optimise growth rates during periods of either soil moisture deficits or low temperatures, longer rotation lengths (to the 3-leaf stage) were required. High response rates to N fertiliser were found during the initial (first 6 month) period of this 2-year study; however, these responses diminished with time, with plots receiving zero N fertiliser achieving growth rates comparable to those plots that received rates as high as 3 kg N/ha.day.