The limitation to the production of highly nutritious hay is the extent of drying required to restrict microbial biodeterioration. Management decisions on haymaking, including the best time to harvest and the use of mechanical processes that cause physical losses of the crop, are governed by the need to dry out the crop rapidly. The process of microbial biodeterioration is the conversion of a structured system of high energy to a nonstructural system of low energy. The energy with which water is held by the plant tissue in relation to a water potential gradient, either evaporative, or generated by microorganisms, is a function of the structure of the plant tissue. The xerophytic growth habit of fungi indicates their capacity to destroy the structure of dried plant tissue. Fungal growth within plant tissue where an evaporative potential gradient exists, will result in the rapid desiccation of the tissue. In a dense bale, where an evaporative potential gradient does not exist, microbial hydrolysis of the plant structural polymers results in water more available for microbiological activity and colonization by less xerophilic microorganisms. Saprophytic fungal growth is restricted at higher water contents due to interactions with bacteria; as the plant material dries the metabolism of bacteria is restricted. A biological control strategy to inhibit fungal growth and therefore alleviate the need to dry the crop extensively was proposed; the inoculation of freshly harvested plants with bacteria that produce persistent antibiotic compounds. The methods developed to select a microbial inoculant for hay preservation are reported in this thesis. To assess the efficacy of an inoculum treatment it was necessary to quantify the fungal community of the drying plant tissue. A fungal community occurs within a habitat where fungi interact spatially and temporally. The relationship between the physical and chemical properties of the plant tissue and its approximate moisture characteristic curve (appendix 2) was used to identify tissue that could be sampled as a habitat. A fungal habitat was defined as plant tissue which dried uniformly under thin-layer drying conditions. Model field drying and storage systems were developed. The sampling and cultural parameters of a particle plating technique for the determination of fungal species' occurrence and abundance within the habitat were investigated. The parameters were then applied in controlled experimental systems. Fungal communities were not detected in either experimental system. It is suggested that the drying conditions were not adequate to control the metabolism of native bacterial populations that competed successfully with fungal colonizers. To determine the significance of changes in the fungal community due to a bacterial inoculum treatment, a statistical randomization program was developed. The technique was illustrated using a preliminary data set. The nature of the treatment effects on the fungal community in terms of fungal species' occurrence and relative abundance was assessed by correlation and similarity indice analyses. Approximately 200 bacteria were isolated from unspoilt hay as potential biological control agents. Thirty strains were selected for broad spectrum antibiosis to fungal isolates from drying lucerne in a dilute, lucerne macerate, aqueous suspension, culture. The ecological parameter for bacterial activity on the plant epicuticular wax surface was considered to be surface activity. Culture media was formulated to include waxes in the lucerne suspension as a model epicuticular wax substrate. The surface activity and antibiotic properties of six bacteria was determined and the relationship between these properties in culture analysed. Effective antibiosis was considered to be a function of the diffusion of antibiotics through the plant cuticle. The capacity of bacterial products to interact with the plant cuticle was assessed as a change in the discontinuity properties of the cuticle in drying experiments. Microscopy techniques were used to investigate the fungal and bacterial colonization of the fungal habitat. An image analysis technique for cleared and stained leaves, was developed to directly assess total fungal colonization. The requirements for optimising this technique are discussed. Nomarski interference microscopy was used to investigate the staining anomalies detected from image analysis, and to add a vivid dimension to the description of the fungal colonization of plant tissue. The bacterial colonization of the surface of drying plant tissue was investigated using environmental scanning electron microscopy and stereo-light microscopy.