The physico-chemical and microbiological process of round baled wet hay was studied under storage conditions. It appeared that the microbial population dynamics resulting in the replacement of the field flora by a mainly thermophilic and xerophilic storage flora did not depend on the initial temperature peak. The determinism of heating phenomena was then studied under laboratory conditions, taking as a basis both the measurements of the respiratory intensity of cut plants and the experiments on sterilized and contaminated hay conservation. The role of the plant respiration in the initial thermogenesis and that of microorganisms in persisting heating and partial loss of organic matter were respectively defined. The experiment conducted on liquid and solid preservatives did not prove satisfactory because the field treatment was insufficiently homogeneous (such was the case of propionic acid). The negative effect of heterogeneity increased with marketed products and sodium chloride because of inappropriate required doses and limited inhibitory action. Ammonia introduced by fumigation in covered stacks allows a far better conservation but requires specific care when dispensing the treated hay to the livestock.
