Regrowth of crested wheatgrass (Agropyron cristatum [L.] Gaertn.) following defoliation

Effects of defoliation of crested wheatgrass (Agropyron cristatum [L.] Gaertn.) on the amount of time required to reach peak regrowth, the lag period for regrowth to begin, regrowth biomass, tiller survival and replacement, and carryover effects of defoliation the following year were investigated. Regrowth of crested wheatgrass was determined during the summers of 1990, 1991 and 1992 in central Saskatchewan following a single defoliation to a 5-cm stubble height at eight stages of growth. Crested wheatgrass regrew 54-130 g m(-2) of biomass when defoliated tillers had less than or equal to 3.6 leaves. Regrowth began accumulating within 3-53 growing degree-days (GDD) and peaked in 705-875 GDD. Rates of leaf development after defoliation (218-252 GDD phyllochron(-1)) equaled or were faster than control (218-330 GDD phyllochron(-1)). Regrowth biomass accounted for 35-76% of total annual production. Total annual production was greatest when plants were defoliated during vegetative growth or at peak growth. In 1991 and 1992, etiolated growth in the spring following defoliation was reduced by defoliation in the previous year. Tiller replacement was not affected by defoliation and averaged 1.2 tillers tiller(-1) (SE = 0.1) in 1991 and 1.5 tillers tiller(-1) (SE = 0.1) in 1992. Two periods of grazing can be expected from crested wheatgrass if it is grazed when tillers have less than or equal to 3.6 leaves; however, the impacts of second grazing must be determined. If crested wheatgrass is grazed late in the growing season, only one period of grazing can be expected, and production will likely be less the following growing season, necessitating a rest period for plants to regain their production potential.