A causal analysis of diet composition in free ranging cattle in reed-dominated vegetation.

Theoretical and observed diets of free-ranging yearling cattle in an area of reed-dominated vegetation were related to constraints inherent to ruminants (rumination time as reflected in the zone of acceptable crude fibre content of the ration; rumen fill) and in more general terms, to requirements for minerals (Na, Ca, Mg, P), for protein and for metabolisable energy in order to balance the growth rate achieved by experimental animals. Theoretical diets in spring were mainly limited by feeding time; theoretical diets in summer and autumn were mainly limited by rumen capacity. The high proteinto-energy ratio in the forage species caused protein intake to exceed the animals' demands. Mineral intake was in excess of requirements except for sodium. Yearling cattle in our study area maximized their energy intake, with feeding time, rumen capacity and crude fibre content of the forage as important constraints.