Using old hay as a cover for new hay is generally not recommended, as it can introduce several problems. Old hay is often already degraded, and may introduce moil, seeds, or pests that could transfer to the new hay.
In addition, old hay does not provide an effective barrier against moisture and environmental elements. Depending on its age and condition, it may be more prone to absorbing water, or may simply allow rain to drain through instead of keeping the new hay dry. To protect your investment and the health of your herd, it's better to use clean, durable materials such as high quality hay covers to protect new hay.
