Submitted to: Joint Abstracts of the American Dairy Science and Society of Animal Science
Publication Type: Abstract only
Publication Acceptance Date: 4/23/2007
Publication Date: 7/8/2007
Citation: Gregorini, P., Bowman, M., Gunter, S.A., Beck, P.A., Coblentz, W.K. 2007. Effect of herbage depletion on cattle grazing dynamics in wheat pastures. Journal of Animal Science 85(Suppl. 1):496. Interpretive Summary: An interpretive summary is not required.
Technical Abstract: Two complementary experiments were conducted to assess grazing dynamics, intake rate, quality and ruminal degradation kinetics of herbage consumed under three herbage depletion levels. In the first experiment (behavioral), three rumen cannulated steers faced (15 min. grazing session) grazing scenarios simulating the levels of pasture depletion at three sward surface heights (ungrazed, 21 cm, CNTL; medium, 14 cm, MD; and high depletion level, 7 cm, HD). Grazing scenarios were sampled for green leaf and stem mass. Intake rate was determined by rumen evacuations. Grazing dynamic was determined by bite/min, bite depth, eating step/min, eating distance, potential area harvested while grazing, and bites and intake/feeding station. Also, quality of potential herbage eaten was estimated by herbage hand-plucking. In the second experiment (ruminal degradation kinetics) samples of herbage eaten by steers during the grazing sessions of the first experiment were incubated in situ in five rumen fistulated steers. The soluble, degradable and undegradable consumed herbage DM fractions were determined, as well as the DM disappearance rate, lag time and DM effective degradability. Green leaf and stem mass quadratically concave decreased (P = 0.01) from CNTL to HD. Treatment did not affect herbage DMI (675 g, SE 45; P = 0.14) during the grazing sessions; but tended (P = 0.06) to decrease herbage DMI/ feeding station with increasing depletion level. Depletion led steers to a quadratically convex increase in eating steps/ min, eating distance and potential area harvested while grazing (P is less than 0.05). Depletion did not affect bites/min (33 SE 6; P = 0.10); but led to shallower bites (P is less than 0.01). None of the herbage potentially consumed and ruminal degradation kinetics parameters were affected by treatment (P is greater than 0.05). Under these experimental conditions, steers adapted grazing dynamics to sustain a constant nutrient intake. Behavioral adaptations would make nutrient intake rate less sensitive to certain herbage depletion levels.