Final answer:
The most important factor to consider when using silage in a horse's diet is the quality, as it directly impacts the horse's health.
Step-by-step explanation:
The key consideration for using silage in a horse's ration is b) Quality. Quality is crucial when it comes to feeding horses with silage because poor quality silage can lead to various health issues such as colic, respiratory problems, or even nutritional deficiencies. When selecting silage for horses, it is essential to ensure that it is of high nutritional value, well-fermented, and free of mold and harmful bacteria that could detrimentally affect the horse's health.