The American Association of State Highway and Transportation Officials (AASHTO) specifications provide simplified formulae to determine Live Load Distribution Factors (LLDFs) for highway bridges. The formulae for the AASHTO code-specified LLDFs have been developed, considering the effect of typical highway trucks. In addition to highway bridges, there are a large number of bridges located on secondary roadways where farm vehicles having varying configurations and weights frequently travel. Unfortunately, LLDFs for the bridges loaded with farm vehicles are not well known. In this study, hence, two bridge types, including steel girder bridges with plank decking and timber girder bridges with plank decking, were selected to determine LLDFs of the bridges under the effects of farm vehicles. The procedure adopted include the AASHTO code-specified formulae, field testing, finite element modeling, and analytical simulations of all the bridges. Field testing of each bridge was conducted with four different farm vehicles and a five-axle highway truck used as a benchmark for exploring highway truck-induced LLDFs. Commercially available Finite Element Analysis (FEA) software was utilized to generate analytical models of all the bridges, and the models were calibrated with field data. To consider the effects of vastly different farm vehicles, information on 121 existing farm vehicles were collected and used as input loads in the models to compute analytical LLDFs for the bridges. The analytical LLDFs resulting from 121 farm vehicles were used to establish statistical limits representing deterministic values for LLDFs for each bridge. The field, analytical, and statistical LLDFs were compared to those obtained from the AASHTO specifications. Results showed that the AASHTO LLDFs were, in some cases, inadequate for the timber girder bridges, while those were, in most cases, adequate for the steel girder bridges.