Get reliable science

In southern British Columbia, McLean and Wikeem conducted clipping trials at two upper grassland sites for rough fescue, two lower grassland sites for crested wheatgrass, and one site each in the upper and lower zones for bluebunch wheatgrass in order to assess annual variability. The 15- and 20-cm stubble height approximated utilization of 35 and 20% of the season's growth, respectively, for all species, but percent utilization differed among species at the lower stubble heights. The 5-cm treatment utilized about 80, 75, and 70% of the herbage for bluebunch wheatgrass, rough fescue, and crested wheatgrass, respectively, while the 10-cm height utilized 60, 50, and 45% for the three species. For all species, weekly season-long defoliation to 5 cm resulted in severe mortality ranging from 67 to 95% and vigor of surviving plants was greatly reduced. All treatments that extended to the end of May on the lower sites or late June on the higher sites resulted in heavy mortality of 55 and 79% and vigor reduction. Fall clipping alone (early September) did not impair plant vigor but the effects of fall defoliation, in addition to spring clipping, were inconsistent, both within and among species Rough fescue was the species most adversely affected by the additional September clipping, bluebunch wheatgrass was intermediate, and little to no effects on crested wheatgrass. These results show that late spring is a critical time for defoliation for all three species. The grasses must be given enough time to replenish carbohydrate reserves before dormancy. This can be achieved by early removal of cattle to allow for regrowth or using a grazing rotation that involves spring rest every second or third year. Fall grazing, which should not harm the grasses if spring grazing was early, should prevent the development of large plants with coarse foliage that are unpalatable to cattle.