I don't think it's right to tag large areas with landuse:grass. The wiki page https://wiki.openstreetmap.org/wiki/Tag:landuse%3Dgrass says it's for "smaller areas of mown and managed grass for example in the middle of a roundabout, verges beside a road or in the middle of a dual-carriageway." But this area of many square miles includes some agricultural use, and some natural wood. The wiki page suggests some alternative tags to use to the land cover more accurately in smaller areas.