Due to a decline in grain production, China is expected to rapidly surpass Japan and become one of the world's top grain importers.
Literal
Grain-production [attributive-の] drop [because-of-によって], China [topic-は] rapidly Japan [object-を] surpass world's-leading [attributive-の] grain-importer [to-と]-become [thing-こと] [subject-が] is-predicted-[passive].
Economic news analysis. ~によって here is causal 'due to, because of.' 上回る ('to exceed, to surpass') is used for quantitative comparisons. 世界有数の ('one of the world's top, leading in the world') is a set attributive. ~となる (formal 'become'). 予想される ('is predicted') is the passive-impersonal for forecasts and expectations.