Which caused the prices of the grain from the plains to fall

More grain was produced elsewhere as well as the fact that increasingly more grain was produced around the same demand of grain.

More grains where produced in other areas making supply to exceed demand in market

Explanation:

The prices of the grain fell as production/supply exceeded the demand for grains in the market. as with every goods or service the increase in supply without the similar increase in demand of the goods and services will lead to the sharp fall in the prices of the goods and services in the market.

some of these grains grown in the great plains were : barley, corn, sorghum, soya beans and canola. these grains were grown and exported to other parts of the world. but as other areas began to produce them the market value of these grains began to reduce because the supply increased against demand