The threat of Russian aggression toward Ukraine’s Black Sea ports and ships will force the country to export its enormous quantities of grain by river, road and rail — all of which are fraught with challenges.

After exiting the Black Sea grain export deal earlier this week, Russia announced that all ships headed to Ukrainian ports will now be considered military vessels. With harvests coming in, that means Ukrainian farmers will no longer be able to rely on the route that was used to move about half of last season’s crops.

That adds to pressure on global food markets, and delivers a serious blow to one of Ukraine’s biggest sources of revenue.