The preposition "in" can take both the ablative and the accusative cases. However, it is translated differently in both cases. When in takes the ablative there is a more stationary idea.
in the city
on the wall
they're not moving, they're just sitting there. When in takes the accusative case however more motion is suggested, and in can mean
into the city
against the wall
In the sentence you gave, they are going "into the field" suggested motion/destination requires the use of the accusative.