NITROGEN AND ATRAZINE ON SHORTGRASS - VEGETATION, CATTLE AND ECONOMICRESPONSES

Application of nitrogen (N) fertilizer and atrazine [6-chloro-Nethyl-N -(ethyl-N'- (1-methylethyl)-1,3,5-triazine-1,3,5-diamine] have each in creased grazeable forage on shortgrass prairie, but their effects are unknown when applied in combination. Therefore, a 9-year study was con ducted to determine effects of N and atrazine applications on 1) herba ge production, 2) steer gains, and 3) profitability of grazing on shor tgrass prairie in north-central Colorado. Treatments were 1) untreated control, 2) atrazine applied at 1.1 kg ha(-1) in the autumn of altern ate years, 3) N applied at 22 kg ha(-1) each autumn, and 4) N + atrazi ne at the rates specified above, Pastures were stocked at 21-41 (contr ol), 27-54 (atrazine), 24-82 (N), or 18-84 (N + atrazine) cattle-days ha(-1) during summer. Pastures were stocked with yearling steers 1979- 1983 and yearling steers and spayed heifers 1984-1985, using put-and-t ake stocking. All treatments increased total October standing crop and blue grama (Bouteloua gracilis [H.B.K.] Lag. ex Griffiths) standing c rop. Nitrogen increased cool-season grass and forb standing crops; atr azine nearly eliminated cool-season grasses but did not affect forbs. Under put- and-take stocking, atrazine and/or N appeared to increase s tocking rate and gain/ha, but not average daily gain or average return s to land, labor, and management. Under optimum stocking rates and gra zing strategies, N or atrazine but not both together might increase re turns.