Charles Ellicott Commentary Exodus 9:19

"Now therefore send, hasten in thy cattle and all that thou hast in the field; [for] every man and beast that shall be found in the field, and shall not be brought home, the hail shall come down upon them, and they shall die." — Exodus 9:19 (ASV)

Gather your cattle. —The peculiar circumstances of Egypt, where the whole country was overflowed by the Nile during some months of each year, caused the provision of shelter for cattle to be abnormally great. Every year, at the time of the inundation, all the cattle had to be “gathered” into sheds and cattle-yards in the immediate vicinity of the villages and towns, which were protected from the inundation by high mounds. Thus it would have been easy to house all the cattle that remained to the Egyptians after the cattle plague, if the warning given here had been generally heeded.