To identify and quantify the influence of sward structure on the intake of grazing animals requires an understanding of sward-animal interactions at the bite level. We tested the hypothesis that bite dimensions on vertically heterogeneous swards are determined by structural characteristics of the top leaf stratum, independent of those of a bottom layer of pseudostem or stem. Dallisgrass (Paspalum dilalatum Poir.) microswards were constructed by hand in a factorial combination of two lamina lengths (5 and 8 em) and two sward heights (8 and 16 em). Tillers consisted of a top segment of lamina and a bottom one of pseudostem (Exp. l) or stem (Exp. 2) of length equal to the difference between sward height and lamina length. Treatments were replicated over three steers (Bos taurus) of 750 kg avg. body weight. Bite area and depth increased with sward height in swards with pseudostem but were not affected by lamina length. Pseudostems were not a barrier to defoliation and did not affect bite dimensions. Steers restricted grazing of tillers with stem to the upper lamina horizon. Bite depth was limited by stems only when lamina length was less than half of sward height. Bite area was reduced by the presence of stems because tillers bent at ligule height, restricting the animals to gather fewer tillers in each bite than when tillers bent at the base. Bite weight was reduced severely in swards with stem, relative to those with pseudostem. Results confirmed the importance of stem horizons and their relative positions in the sward as determinants of bite weight. P ROGRESS IN UNDERSTANDING the sward structural determinants of instantaneous (dry matter) OM intake rate (IIR) by grazing animals requires better knowledge of sward-animal interactions at the individual bite level. Within a limited range of sward conditions, grazing animals can adjust for sward-induced reductions in bite OM weight by increasing biting rate and/or daily grazing time (Chacon and Stobbs, 1976;Hodgson, 1985;Penning et al., 1991). Beyond these E.R: Flort:s, Universidad Nacional Agraria,

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provided excellent research assistance. DISCLAIMER: This Staff Discussion Note represents the views of the authors and does not necessarily represent IMF views or IMF policy. The views expressed herein should be attributed to the authors and not to the IMF, its Executive Board, or its management. Staff Discussion Notes are published to elicit comments and to further debate.

La Puna, una combinación de pajonales, césped de puna y bofedales de alta montaña, cumple un rol clave en la economía y seguridad alimentaria por los bienes y servicios que provee a la sociedad. La estructura y función de este ecosistema será seriamente afectado por la degradación de tierras y el cambio climático. Modelos ecológicos predicen cambios en la cobertura vegetal, productividad y capacidad de carga de los pastizales, una situación que definitivamente afectará la economía local y la disponibilidad de alimentos. En este escenario urge desarrollar políticas de conservación y manejo sostenible de pastizales que contribuyan a mitigar las emisiones de gases de efecto invernadero generados por la actividad ganadera. Estas políticas deberán desarrollarse bajo un marco institucional que incluya la asignación de recursos económicos a través de la creación de mecanismos de retribución de servicios ecosistémicos que involucren el compromiso y participación de decisores políticos, productores y la población.