REGROWTH AND REST REQUIREMENTS OF NORTHERN WHEATGRASS FOLLOWING DEFOLIATION

Degree-days required for standing crop and above-ground net primary pr oduction of northern wheatgrass (Agropyron dasystachyum [Hook.] Scribn ., syn. Elymus lanceolatus [Scrib. & Smith] Gould) mowed to a 5-cm stu bble to recover to levels similar to an unmowed control were determine d in southwestern Saskatchewan, Nine, single mowing treatments and an unmowed control were established from early May through late August in 1991 and 1992 on a clayey range site at 2 locations, Green and dead p hytomass and above-ground net primary production were determined for 2 to 3 years following mowing, Degree-days required for recovery of gre en and dead standing phytomass on mowed plots decreased linearly and w ere highly correlated (r(2) = 0.64 to 0.99) with the number of days pl ots were mowed after 1 May, Regardless of mowing date, green phytomass did not recover to control levels the year of mowing, Each day mowing was delayed past 1 May reduced the number of degree-days required for total recovery of green phytomass on mowed plots by 15.7 in 1991 and 17.7 in 1992, Degree-days required for recovery of standing dead on mo wed plots were reduced 17.6 in 1991 and 15.8 in 1992, Degree-days requ ired for recovery of above-ground net primary production declined line arly (r(2) = 0.67 and 0.99) as mowing was delayed after 1 May, More de gree-days were required for the 1991 than 1992 plots, At least 2 and s ometimes 3 growing seasons were required to accumulate enough degree-d ays to allow full recovery of green and standing dead phytomass and ab ove-ground net primary production on mowed plots, For optimum sustaine d production, a grazing system should be used on northern wheatgrass-d ominated rangeland with a 2 year rest period applied to paddocks after grazing.