There's no logistical reason that agricultural activities couldn't be done at night. Plants do most of their photosynthesis during the day, but humans aren't involved in that particular process. The rest, we do when we're awake just because that's the time we're awake. We can't guard the fields all the time, even during the day, and the techniques we use to guard the fields when we're away should work just as well for a nocturnal species.
Certain activities may be more dangerous, due to reduced visibility. Injuries may be more common. If the workers use torches on overcast nights, there may be some risk of fire in the fields. But these are inconveniences to be overcome, rather than insurmountable obstacles.