Mannheimia haemolytica is the principal microorganism responsible for bovine pneumonic pasteurellosis, or shipping fever. We have previously expressed a fragment of leukotoxin, a major virulent factor of M. haemolytica A1, as a fusion protein with green fluorescent protein (GFP) in transgenic white clover and demonstrated that this antigen was immunogenic and elicited toxin neutralizing antibodies in rabbits. These previous results showed that using plants to produce M. haemolytica antigen for use as a vaccine against this disease is a viable strategy. In this present study, we examined the stability of the truncated leukotoxin GFP-fusion protein (Lkt50-GFP) in field-grown transgenic white clover. Transgenic clover expressing Lkt50-GFP was clonally propagated and a confined field trial was established. Western immunoblotting showed that the level of Lkt50-GFP expression in field plants was the same as in transgenic plants maintained under optimal conditions in the greenhouse. We also observed that after harvesting and oven drying at 50 °C, the antigen was still present in the dried clover after 1 year of storage at ambient temperature. As special post-harvest conditions (e.g., refrigeration) are not required, the use of transgenic plants to deliver an oral vaccine against shipping fever appears to be economically feasible.