Would a Yellowstone eruption destroy global agriculture?
An eruption of the Yellowstone super volcano would cause agricultural disaster over a very large area. Its global effect would be significant, but not totally devastating.

The resulting climate cooling could last up to a decade. The temporary climate shift could alter rainfall patterns, and, along with severe frosts, cause widespread crop losses and famine.
But a Yellowstone megablast would not wipe out life on Earth. There were no extinctions after its last three enormous eruptions, nor have other supereruptions triggered extinctions in the last few million years.

Eruption of super volcanoes is rare which makes it difficult to ascertain what exactly would happen when the Yellowstone super volcano eventually erupts - which won't be any time soon.
Yellowstone last erupted 174 000 years ago, which was a minor eruption. Lava from Yellowstone won't be the problem, but the ash and gases will be.
The last major eruption, 630 000 years ago, covered much of western and central United States with ash. The maximum thickness of the ash from that eruption is 660 ft. The eruption was a double eruption, 170 years apart. The amount of ash thrown into the atmosphere with each eruption caused the ocean to cool by 3 degrees Celsius with each eruption.