@StarBrooks It helped me throw a question mark at the end.

Why will further increases of grain production be extremely difficult?

Because Most usable farmland is already being farmed with near-maximal efficiency -- ok good

Beacause Worldwide demand for grain has been increasing steadily -- Yeah it doesn't answer my question.

Because a worldwide grain shortage is likely? Doesnt answer my question -- however the original question answers this question -- "Because increases of grain production will be extremely difficult