Suppression of annual bromes impacts rangeland: Animal responses

Presence of annual bromes (Bromus spp.), introduced annual weedy grasses, c an alter seasonal patterns of forage production and quality and require man agement changes for efficient use of infested rangelands. We determined bio logical impacts of the presence of brome by comparing livestock performance on brome infested rangeland to similar sites on which brome had been suppr essed by autumn application of atrazine [6-chloro-N-ethyl-N'-(1-methylethyl )-1,3,5-triazine-2,4-diamine] at 0.56 kg ha(-1) in 1992 and 1993. Each trea tment was randomly assigned to three, 12-ha pastures. Vegetation was measur ed for 5 months (May to September) each year from 1993 to 1995. Each pastur e was stocked with 8 crossbred steers of British breed origin (Bos taurus) from mid-May to mid-September 1993 and 1995 and to mid-August 1994. Initial body weights averaged 329 kg SD = 31 in 1993, 273 kg SD = 14 in 1994, and 272 kg SD = 21 in 1995. Brome suppression and environment influenced plant species in diets, diet quality, and livestock performance. Brome suppressio n reduced percentage of annual grasses in diets from 14% to 10%. Annual gra sses were replaced in the diet by a variety of forb and grass species {west ern wheatgrass [Pascopyrum smithii Rydb. (Love)], and blue grama [Bouteloua gracilis [H.B.K.] Lag. ex Griffiths]}, with specific replacement depending on year and month. Steer gains were increased from 0.92 to 1.04 +/- 0.02 k g head(-1) day(-1) (P < 0.02) and from 69 to 81 +/- 2.8 kg ha(-1) (P < 0.05 ) with brome suppression. This experiment demonstrated that improvement in livestock performance can be expected with the suppression of annual bromes on semiarid rangelands.