Covering hay is important because it protects the hay from moisture and environmental elements that can lead to spoilage. When hay is exposed to rain, snow, or high humidity, it can absorb moisture, which promotes the growth of mold and bacteria.
Bacteria and mold can reduce the nutritional value of the hay as well as make it unsafe for livestock consumption. Uncovered hay is also more susceptible to damage from UV rays, which can degrade its quality over time.