response of russian brome, crested wheatgrass and tall wheatgrass to annual precipitation and grazing management in a semi-arid area

three grass species, agropyron desertorum (agde) and agropyron elongatum (agel) and recently, bromus tomentellus (brto) are mainly used for range improvement as mixed seeding in semi-arid areas of iran. because there is little information about management of these grasses, this study was conducted at the sisab research station in northern iran to evaluate their responses to grazing management and annual precipitation. four grazing management treatments were applied for three consecutive years, from 1999 to 2001: no grazing, and light (20-45% forage removal), moderate (45-65% forage removal) and heavy (65-80% forage removal) grazing by lambs and sheep. the fourth year was considered to be a rest from grazing. data recorded up to fifth years from 1999-2003. the average herbage yield (hy) production across years of agde (1357 kg/ha) and agel (1250 kg/ha) was statistically similar, and higher than that of brto (880 kg/ha). hy decreased with reduction in annual precipitation for all species, especially agel. plant number per unit area declined over the study period in all species. three years of heavy grazing resulted in significant reduction in hy and/or plant number for brto and agde, but the rest year compensated for the negative effect of intensive grazing on dry matter production