DEFOLIATION EFFECTS ON SEASONAL PRODUCTION AND GROWTH-RATE OF COOL-SEASON GRASSES

Knowledge of seasonal distribution of herbage growth is essential for design of pasture management schemes that optimize season-long herbage productivity and nutrients for grazing livestock. Current knowledge of cool-season grass growth is derived largely from either hay production or high-input systems. A 3-yr field study was conducted to determine the productivity and growth rates of cool-season grasses defoliated according to sward height criteria in a marginal-soil environment. Defoliation regimes included hay harvest, lenient (50%) removal of short (10 cm) canopies, and both lenient and intensive (75%) removal of tall (20 cm) canopies. Species included orchard-grass (Dactylis glomerata L.), tall fescue x perennial ryegrass hybrid (Festuca arundinacea Schreb. x Lolium perenne L.), and prairie grass (Bromus willdenowii Kunth). Cumulative yield curves were fit to the Gompertz growth equation and growth rates were derived from the fitted data. All species had low yields relative to other defoliations and growth rates when managed for removal of 50% of canopy height regardless of height. Managing pastures on the premise of take half, leave half may not always contribute to plant persistence and sustained productivity. Frequent defoliation beginning in spring resulted in a greater portion of annual yield occurring in late than in early season, resulting in more stable distribution but less herbage within a season, compared with canopies managed as hay. High growth rates of prairie grass early in the study were followed by stand degradation, regardless of defoliation treatment, and may indicate unsuitability for use in low-input or marginal environments. Autumn recovery of herbage production did not occur under any defoliation regime.