About 40,000 km(2) of the Caldenal in central semiarid Argentina are rangelands where the most important economic activity is cow-calf operations. Some information on forage species, mainly regarding taxonomy, ecophysiology, and nutritive value, is available, but detailed studies on composition of free-ranging cattle diets are lacking. The objective of this work was to study the botanical composition and seasonal trends of cattle diets in the southern Caldenal. Diets were studied through microscopic analysis of cattle feces collected monthly in a typical plant community during a 12-month period. Grasses were the bulk of the diets, except in November when more than 50% of the diet was Medicago minima (L.) Grufberp. Highest consumption among the grasses was of Piptochaetium napostaense (Speg.) Hack., one of the dominants in the grass layer. High consumption of Pappophorum mucronulatum Nees, one of the few warm-season grasses in the region, was found during the summer. Calden (Prosopis caldenia Burk.) pods, consumed in late summer and fan, were the only woody fragments found in appreciable amounts. Classification and ordination techniques were used to analyze seasonal trends. In spite of a relatively high homogeneity in the diets, 2 well-defined seasonal trends were detected, one in the fall-winter period and another in the spring. Cattle diet diversity was minimum during the coldest months of the year.
