Cool-Season Mixed Pasture Growth and Root Density Under Differing Defoliation Regimes and Leaf Area Indices.

Pasture growth rate varies with timing and severity of defoliation of cool-season mixed-species swards. Relationships of pasture regrowth rate and root density to canopy leaf area index (LAI) at defoliation stages including maturity following accumulation of surplus spring growth are less well-understood. We hypothesized that root density, but not pasture growth rate and total seasonal forage dry matter (DM) production, may be greater for relatively few harvests of late-maturity pasture at LAI>8 than for more frequent harvests at LAI<5-6. We compared LAI, herbage mass, growth rate, and root density of mixed cool-season perennial pastures containing grasses, legumes, and non-legume forbs under the following defoliation frequencies and residual heights: a) weekly to 10 cm; b) periodically from 25-30 cm to 6 cm; c) periodically from 25-30 cm to 10 cm; and d) periodically from mature hay stage to 10 cm. Half of the experimental units had been clipped twice to 3-4 cm residual height during April to simulate severe continuous stocking during early spring and the remainder were protected from defoliation during April. Herbage mass was estimated with a calibrated rising plate meter and LAI was estimated from canopy light interception measurements with a line quantum sensor immediately prior to each defoliation. Results will be presented.