Abstract

A pasture (45 ha) in northeastern Oregon was grazed with 30 yearlings (419 kg, Body Condition Score [BCS] = 5.05) and 30 mature cows with calves (499 kg, BCS = 4.65) during August of 2001 and 2002. Sampling dates were d 0, d 10, d 20, and d 30. Forage availability before grazing was 1,039.0 kg·ha^-1 and declined to 332.6 kg·ha^-1 after grazing (p < 0.10). Grasses dominated the pasture (44.5%), followed by forbs (30.7%), grasslikes (15.9%), and shrubs (8.9%). Due to grazing quackgrass (Agropyron repens (L.) Beauv.), western fescue (Festuca occidentalis Walt.), California brome (Bromus carinatus Hook.), and redtop (Agrostis alba L.) exhibited the greatest decline in quantity. Shrub utilization was high from d 20 to d 30 (49 to 58% for willow [Salix rigida {Hook.} Cronq.] and 58 to 74 % for alder [Alnus incana {L.} Moench.]). Forbs decreased (p < 0.10) in moisture late in the grazing period, while shrubs were (p > 0.10) still succulent (63%). Forbs and shrubs were higher (p < 0.10) than grasses in crude protein (11, 14, and 6%, respectively) and digestibility (59, 50, and 42%, respectively). In summary, our results suggest that cattle grazing late-summer riparian pastures will switch to intensive shrub utilization when grasses decline in quantity and quality, and forbs decline in quantity. Land managers need to know the effect of their management on vegetation and if a goal is to protect riparian woody vegetation, our data suggest that late-summer grazing should be light, or avoided when grasses have senesced.

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