Cool-Season Pasture Growth Rate and Root Density Responses to Defoliation at Differing Severities and Leaf Area Indices.

Pasture growth rate and botanical composition vary in relation to season and 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 sward maturity following accumulation of ungrazed 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. Our objective was to compare 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 defoliation to 10-12 cm; B) periodic defoliation from 25-30 cm to 5-6 cm; C) periodic defoliation from 25-30 cm to 10-12 cm; and D) periodic defoliation from mature hay stage to 10-12 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 half 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 bar containing 80 photosynthetically-active radiation (PAR) sensors immediately prior to each defoliation. Results will be presented.