It appears to be recently planted corn that is growing on this steep hillside. There are actually six people working on the mountainside field pictured here. In this made-to-post, low-res jpeg, they show up as little more than fuzzy specs. Above the left side of the house and a bit above the bottom of the stand of trees that extends towards the middle of the photo, there is a tiny red speck. That is the shirt of a farmer working in the field.
Many of the fields I saw during my trip through the mountainous north of Vietnam were not the lush green one commonly sees in photos of mountain farming in Vietnam. Some fields had crops growing, but many had been recently harvested or were fallow, waiting for the planting of rice and other summer crops.
The rice paddies around my home are serviced by a system of irrigation canals and ditches that ensure the fields have ample water. Sluice gates are positioned in a number of places to cut off, reduce, or re-direct the flow of water. From time to time, I see local farmers using the gates to adjust the flow. There is certainly a water management agency in charge of this activity, though I do not know how it is organized or at what level of local government it exists.
I also do not know how long the irrigation system has existed here, though I do know there are continuing efforts to improve it. In the last couple of months there have been several small projects to upgrade waterways. Sections of the system that had consisted of earthen ditches were dug out and lined with concrete walls and floors. I presume this reduces erosion and water loss from seepage, and increases the flow rate of the water.