For+days+at+a+time+the+animals+had+nothing+to+eat+but+chaff+and+mangels.+Starvation+seemed+to+stare+them+in+the+face.

The famine that the Animal Farm is facing right now is like the famine in USSR due to collectivization and industrialization. Because they were so focused on industrializing, building more factories and etc (the windmill in the book), they barely had time to care for the plantations and crops, which led to famines in the following years.

Like what James stated, this passage is in reference to the famine in Russia due to some of Stalin's policies such as collectivization, industrialization from the Five-Year Plans, called the Holodomor (in Ukrainian). Much like the Five Year Plan of Stalin, Napoleon from the Animal Farm "industrialize" by using the animal work force to build the windmill in the cold winter. As a result, they face famine since they did not have enough food that was harvested and they had spent much of their time building and rebuilding the windmill.