The many engine and script changes in Farming Simulator 19 added a lot of features: multi-stage grass drying, and improved rot.
A new multi-step drying process has been added to create hay. When you cut grass wet (indicated by the “Wet Crop” icon in the upper right corner), you need to mow it first. Then it must dry in the sun to become hay. When cutting dry grass (low humidity meaning the "Wet Crops" icon is not displayed), it does not need to take out the tedder, just wait a bit (a few minutes in x120 ) so that it turns itself into hay.
This means that making hay now depends on the weather. Good planning is necessary. The weather forecast will certainly help you because the drying potential is indicated there. The drying potential depends on wind conditions, relative humidity and solar radiation, which means that the grass will dry out more easily in early summer than in winter. Grass is considered wet when its humidity level is at least 20%. The latter can be measured with the hand tool.
If the hay is left in the rain, it will get wet again and eventually it will start to rot. The same goes for dry grass that has been cut. Freshly mown wet grass will not rot until the second night after mowing.
Hay and straw bales rot when they are outside in the rain. However, straw and hay stored inside buildings will no longer rot, as it did in Seasons 17. Grass still rots, as this material is wet. With the new drying system described above, it will take two days instead of a month for rot to start on piles of grass.
Balles
Grass bales will rot and gradually disappear after two days, unless wrapped in such a way that they become silage bales. Wrapped bales do not immediately turn into silage, and require some fermentation time. It takes a third of the season (one period) to completely ferment a bale of silage.
Hay and straw bales exposed to rain will start to rot, so keep them inside sheds. These balls will only rot if the map has been prepared with a mask called Seasons. Placeable buildings by default are prepared with this mask and therefore function correctly.