An extended experimental campaign on the compression behavior of straw bales is carried out. We analyze eight different materials: hard wheat, common wheat, millet, sorghum, corn, rice, oats and barley. The behavior highlighted for all the bales a initial linear elastic behavior followed by an increasing stiffening. The initial slope is found depending on the square of the bale density. A predictive model is proposed to describe the mechanical behavior of a straw bale based on conical spring. Creep and relaxation tests as well as tests with cyclic loads have shown a deeply time dependent behavior.