Species Population Dynamics in a Mixed Pasture under Two Rotational Sward Grazing Height Regimes

Abstract

Describing tiller/leaf density and weight dynamics of pasture plants can improve our understanding of the seasonal productivity and persistence of different species under grazing. We hypothesized that within current rotational-stocking recommendations, shorter grazing heights would reduce orchardgrass (Dactylis glomerata L.) population and productivity, and favor more rhizomatous species, such as Kentucky bluegrass (Poa pratensis L.) and quackgrass (Elytrigia repens L.). We compared tall and short sward grazing height regimes in a mixed species pasture rotationally stocked by cattle. ‘Tall’ pastures were grazed based on orchardgrass height, from 27 cm down to 7 cm, and ‘short’ pastures were grazed from 20 to 5 cm. Before each grazing event, herbage was sampled at ground level, tillers/leaves were counted by species, dried, weighed; and species' tiller/leaf density, weight, and (total) herbage mass were calculated. Population dynamics of all species were influenced by climate and date of grazing events. Kentucky bluegrass was most sensitive to dry, warm periods, and quackgrass production was least affected. Quackgrass produced fewer reproductive tillers and more stable tiller density than Kentucky bluegrass and orchardgrass. Orchardgrass and bluegrass tiller density and weight were similar between grazing regimes. Quackgrass and dandelion (Taraxacum officinale Weber) tiller/leaf density were higher in short than tall pastures, and quackgrass herbage mass increased. Grazing treatments shifted botanical composition and influenced seasonal herbage mass distribution, but total pasture mass was similar under both grazing regimes. Limited precipitation and warm temperatures most consistently explained reductions of orchardgrass, bluegrass, and total pasture productivity, and were significant causes of variability.