Species contribution to seasonal productivity of a mixed pasture under two sward grazing height regimes

Mixed pastures often include tall and short plant species that respond differently to grazing intensity. We evaluated the effect of two rotational grazing height regimes on species productivity and availability to animals. We compared tall and short grazing height regimes within current recommendations in a Pennsylvania pasture dominated by orchardgrass (Dactylis glomerata L.) and Kentucky bluegrass (Poa pratensis L.) and grazed by cow-calf (Bos taurus) pairs. Orchardgrass height defined grazing regimes. Tall pastures were grazed from 27 cm down to 7 cm and short pastures from 20 to 5 cm from April 1998 to May 2000. Before each grazing period, herbage was sampled at grazing regime residual heights of 7 or 5 cm (herbage harvested) and at ground level (herbage mass). Species harvest index (HI) was calculated as the ratio of herbage harvested over live herbage mass to compare herbage availability among species and grazing regimes. In tall vs. short pastures, there was 50% more herbage harvested at each grazing event and 23% more total herbage harvested over the 2.3-yr experiment. Higher production was mainly due to orchardgrass (51% more herbage harvested in tall vs. short pastures) and tall legumes. Herbage harvested decreased during dry, warm periods independent of season, with bluegrass decreasing most. Orchardgrass HI was highest, quackgrass (Elytrigia repens L.) and bluegrass HIs were intermediate, and white clover (Trifolium repens L.) HI was lowest. Small differences in grazing height recommendations had significant effects. Tall grazing heights increased productivity and favored tall-growing species that had higher harvest indices.