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Title: Effect of herbage depletion on short-term foraging dynamics and diet quality of steers grazing wheat pastures

Author
item GREGORINI, P - University Of Arkansas
item Gunter, Stacey
item BOWMAN, M - University Of Arkansas
item CALDWELL, J - University Of Arkansas
item MASINO, C - University Of Arkansas
item Coblentz, Wayne
item BECK, P - University Of Arkansas

Submitted to: Journal of Animal Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/24/2011
Publication Date: 10/18/2011
Citation: Gregorini, P., Gunter, S.A., Bowman, M.T., Caldwell, J.D., Masino, C.A., Coblentz, W.K., Beck, P.A. 2011. Effect of herbage depletion on short-term foraging dynamics and diet quality of steers grazing wheat pastures. Journal of Animal Science. 89:3824-3830.

Interpretive Summary: Experiments assessing short-term foraging behavior and diet quality of wheat pasture by cattle were conducted with 3 levels of herbage depletion. The treatments included an ungrazed sward (control), as well as, medium (one-third) and high (two-thirds) levels of herbage depletion. Herbage depletion resulted in cattle moving faster as they grazed and searching more pasture area for desirable forage. As the cattle moved faster while grazing, they consumed less forage per unit of area searched resulting in the intake being less energetically efficient. Neither the quality of forage consumed, nor the ruminal digestion of it, were affected by extent of depletion. In this experiment, steers adapted their foraging behavior and were able to sustain a quality diet in the short-term. These results suggest that behavioral adaptations would make diet quality less sensitive to herbage mass than previously surmised.

Technical Abstract: Two complementary experiments were completed to assess short-term foraging dynamics, diet quality, and ruminal degradation kinetics of herbage consumed by steers with 3 level of herbage depletion. Experiment (Exp.) 1 was a behavioral study in which 3 ruminally cannulated steers were allocated to grazing scenarios simulating 3 levels of herbage depletion. These treatments included an ungrazed sward (control), as well as, medium and high levels of herbage depletion. Grazing scenarios were sampled for sward surface height and amount of green leaf and stem. Foraging dynamics were determined through measurements of bite rate, bite depth, eating step rate, eating distance, potential area consumed while grazing, and bites and intake per eating step. Also, quality of potential herbage consumed was estimated from hand-plucked herbage. In Exp. 2, ruminal degradation kinetics of DM for samples of herbage consumed (masticate) by steers during Exp. 1 were assessed in situ within 5 rumen cannulated steers. The immediately soluble, degraded, and undegraded DM fractions were measured. The DM disappearance rate and lag times were determined from a nonlinear regression model, and the effective degradability of DM was calculated. Herbage depletion resulted in increased eating steps/min, as well as the potential area harvested while grazing (P < 0.05) and reduced herbage intake/eating step (P < 0.05). Neither the herbage potentially consumed, nor the ruminal degradation kinetics, were affected by extent of depletion (P > 0.05). Under these experimental conditions, steers adapted their foraging dynamic and were able to sustain diet quality in the short-term. These results imply that behavioral adaptations would make diet quality less sensitive to certain levels of herbage depletion.