Effects of pasture and management variables on forage intake by grazing sheep

Graduation date: 1990Grazing experiments were conducted in summer, autumn and winter\ud to assess the effects of stock density, grazing duration, pre-grazing\ud pasture mass and stock type on forage intake by sheep grazing at high\ud stock densities for short durations. In summer and winter, sheep were\ud stocked at 200 sheep/ha on 0.1 ha plots for 9 and 4 days,\ud respectively. Total dry matter (TDM) and green dry matter (GDM) were\ud estimated each day by clipping 0.2 m² plots to ground level. In\ud autumn, ewes and lambs separately grazed 0.333 ha pastures at stock\ud densities of 270, 540 or 810 sheep/ha for 1, 2, or 3 days. Pre- and\ud post-grazing TDM were estimated with a single probe capacitance meter.\ud The percent GDM of TDM was estimated using a spectophotometer. In each\ud study TDM and GDM pasture mass were analyzed using a non-linear\ud (negative exponential) multi-variate regression model. Intake was\ud estimated from results of pasture mass analysis. In autumn, ewes and\ud lambs consumed similar amounts of TDM, but ewes consumed 43% more GDM\ud (P<.05) than lambs; differences between treatments at the same grazing\ud pressure (stock density X grazing duration) in intake of both TDM and\ud GDM were not significantly different. In all analyses regression\ud coefficients for grazing pressure, i.e. instantaneous rates of forage\ud decline per stock day, were significant (P<.01). Instantaneous rates\ud per stock day in TDM analyses were 0.0007, 0.0001 and 0.0002, and in\ud GDM analyses were 0.0019, 0.0003 and 0.0010 for summer, autumn and\ud winter, respectively. A simulation model was also developed which\ud predicted intake of GDM and DDM by sheep grazing at high stock\ud densities for short durations. Model predictions of intake agreed\ud favorably with summer and winter observations, but the model\ud overestimated intake during autumn. In agreement with previous\ud studies, the model predicted intake would increase at a decreasing\ud rate as forage allowance (kg/sheep/day) increased