A decision support system software was developed using an object-oriented expert system approach to provide assistance in managing nutrients and liquid manure for dairy and swine farms from defecation through to field application. This application provides estimates of manure production using a nutrient balance method for swine manure; and number and weight method for dairy operations. This software also recommends appropriate manure field application rates to a single or multiple fields to meet crop nutrient need after giving nutrient credits to previous field applied manure and plowed down old crop. Such recommendation can be made according to various field application rate goals such as satisfying 75% or 100% of N need, P need or using quantity specified by the user. A phosphorus index suitable for Ontario conditions was developed to rate the environmental pollution potential of a field. A rating scheme using the following five criteria: cost, labour, odour, nutrients and environment, is included for both swine and dairy liquid manure management systems.