Regrowth and Rest Requirements of Northern Wheatgrass following Defoliation

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Degree-days required for standing crop and above-ground net primary production of northern wheatgrass (Agropyron dasystachyum [Hook.] Scribn., syn. Elymus lanceolatus [Scrib. &Smith] Gould) mowed to a 5-cm stubble to recover to levels similar to an unmowed control were determined 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 phytomass and above-ground net primary production were determined for 2 to 3 years following mowing. Degree-days required for recovery of green and dead standing phytomass on mowed plots decreased linearly and were highly correlated (r2 = 0.64 to 0.99) with the number of days plots 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 mowed plots were reduced 17.6 in 1991 and 15.8 in 1992. Degree-days required for recovery of above-ground net primary production declined linearly (r2 = 0.67 and 0.99) as mowing was delayed after 1 May. More degree-days were required for the 1991 than 1992 plots. At least 2 and sometimes 3 growing seasons were required to accumulate enough degree-days to allow full recovery of green and standing dead phytomass and above-ground net primary production on mowed plots. For optimum sustained production, a grazing system should be used on northern wheatgrass-dominated rangeland with a 2 year rest period applied to paddocks after grazing.