Submitted to: Arkansas Experiment Station Research Series
Publication Type: Experiment Station
Publication Acceptance Date: 8/31/2006
Publication Date: 12/1/2006
Citation: Gregorini, P., Caldwell, J.D., Bowman, M., Masino, C., Coblentz, W.K., Beck, P.A., Gunter, S.A. 2006. Effect of Herbage Depletion on the Grazing Dynamics and Short-Term Intake Rate of Steers Grazing Wheat Pastures. Research Series 545. Arkansas Animal Science Department Report. 9:130-133. Interpretive Summary: It is often assumed that a negative linear relationship exists between forage mass and voluntary intake by grazing cattle. However, several research studies have suggested that reduced forage mass does not accurately predict intake rate, largely because relative proportions of leaf and stem and the relative accessibility of leaves are not considered in measurements of forage mass. In this study, changes in forage availability and accessibility within small paddocks affected grazing behavior and intake rate. This understanding of grazing dynamics may help to design grazing management systems that allow cattle to harvest as much green leaf as possible with the least effort. Therefore, grazing cattle in short grazing sessions, with small leafy areas (pastures) may be a management alternative to be considered.
Technical Abstract: Reduction of herbage mass may not accurately predict herbage intake rate, as it does not incorporate aspects of availability and accessibility of preferred plant parts. There is little research attempting to understand cattle foraging strategies during pasture depletion. This study aimed to assess grazing dynamics and herbage intake rate under pasture depletion grades, analyzing other components than herbage mass. Three steers were assigned to grazing scenarios that simulated 3 levels of pasture depletion (Treatments: Undisturbed sward, CNTL; high level of depletion,HD, and medium level of depletion, MD). Grazing scenarios were characterized by the amount of green leaf and stem, and their ratio. Intake rate was determined by a rumen evacuation technique. Grazing dynamic was determined through bite and eating step rate, bite depth, eating distance and grazed area, and bites and intake per feeding station. From the CNTL to the HD; green leaf mass, stem mass, and accessibility decreased, but green leaf availability increased. Steers showed a decrease of fresh herbage intake rate (P = 0.04), and dry (P = 0.06) and fresh (P = 0.04) herbage intake per feeding station, as well as a tendency (P = 0.1) to increase bite rate with increasing level of depletion. Depletion led steers to increase foraging velocity (eating steps per min) and grazed area. It seems that concepts other than herbage mass need to be taken into account when changes in herbage intake, grazing dynamics and grazing management are analyzed.