Considering the relative sizes of your average farmer, a wolf and a chicken how big is this bag of corn that it won't fit in the boat along with the chicken? Or for that matter, what sort of boat couldn't carry a chicken perched on the bow/stern/mast..? and if the bag of corn is so big that it won't fit in the boat along with either the wolf or the chicken then why is the farmer so worried that the chicken would eat it all if left alone with it?
Given that no barrow/trolley/truck/trailer has been mentioned it is safe to assume the bag of corn is small enough for the farmer to carry, further we can assume it is small enough that the farmer is still mobile enough to prevent the wolf from chasing down the chicken with the inevitable consequenses, which all only adds to the mistery of why the chicken and corn can't fit in the boat at the same time.
There is only one logical conclusion and that is if either the wolf or chicken were left unattended they would wander off, and therefore all three items must be transported at the same time.
So... the farmer puts the corn in the boat, picks up the chicken and throws it towards the opposite bank. (chickens
can fly, they mostly choose not to). The wolf, seeing his chance, jumps in after the chicken, but the farmer (being in a boat) is able to keep ahead of the wolf and simply steers to where the chicken landed and wandered to on the opposite bank. Once the wolf has caught up all are across the river in one go.