DEFOLIATION OF A NORTHERN WHEATGRASS COMMUNITY - ABOVEGROUND AND BELOWGROUND PHYTOMASS PRODUCTIVITY

A defoliation study was conducted on a fair condition, clayey range si te that is potentially dominated by northern wheatgrass (Agropyron das ystachyum (Hook.) Scribn.) in south-central Saskatchewan. Vegetation w as subjected to a factorial experiment with an initial defoliation in early-May, June, July, or August and repeated at 2- or 6-week interval s until mid-September in the same plots for 3 years. An undefoliated c ontrol was also included. Herbage removed, residual live, dead, total, and root phytomass were measured. Defoliation reduced all yield compo nents, with the exception of herbage removed. Residual live grass was reduced 37, 57, and 46 %, respectively, in first, second, and third ye ars; the sedge and forb components of live residual phytomass generall y were not affected by defoliation. Compared to control, dead phytomas s was reduced 77% in the first year, 67% in the second, and 52% in the third year across treatments. Total herbage yield across defoliation treatments ranged from 68 to 83% of control. Total live phytomass (her bage removed + residual live phytomass) in defoliated plots equaled co ntrol. Herbage removal was greatest when initially defoliated in early July and thereafter at 2-week intervals. When defoliated at 6-week in tervals residual live and dead phytomass were generally greater than w hen herbage was removed biweekly. Yields were higher when the first de foliation was delayed and repeated at 6-week intervals. Generally, roo t phytomass was not different among defoliation treatments, but total belowground phytomass was reduced 30% in the 0-30-cm depth after 3 yea rs of defoliation. This northern mixed prairie ecosystem is sensitive to herbage removal. Maximum forage yield can be obtained if grazing is deferred until after peak growth in July.