The year of the rat has arrived ahead of schedule in China.

State-run newspapers declared today that large parts of the nation are plagued by billions of rats that bite farm animals and chew up more than 20 million acres of prime grain fields, forest and pasture.

The epidemic has reached almost biblical proportions in central China, where one province claims to have a rat for every 70 square feet of farmland. Two state farms in the south saved their crops by killing 106 tons of rats last year. But peasants in the northeast grasslands were less fortunate and expect their hay harvest losses to reach about $400 million.

No health warnings were issued today, but most Chinese recall how bubonic plague transmitted by rats killed a million people in the first half of this century.

Although the next lunar year officially is the one named after the menacing rodent--the current being the year of the pig--Chinese zoologists say this is a peak year for rat reproduction because of the drastic decline in their natural enemies--cats, weasels, owls and snakes.

"The major cause of the serious danger from rats is the disruption of the ecological balance," said ecologist Hou Xueyu, writing in the official People's Daily. The slinky creatures appear to be winning the survival race in ironic fashion: their predators have died from feeding on rats that had eaten poison.

The cycle of nature favors rats because they propagate at much faster rates than their predators. A mother rat is capable of producing up to 100 babies annually.

Rats also benefit from the ecological havoc wreaked by Chinese peasants who poach weasels for their pelts and illegally cut forests for lumber, thus destroying the habitat of owls and snakes.

Many cats were destroyed in the 1970s, along with other domestic animals, by fanatical Red Guards who regarded them as "bourgeois."

Although the rats have been winning the battles so far, their human predators are determined not to lose the war. A nation that imports billions of dollars' worth of grain cannot afford to lose 100,000 tons in one province because of pests.

Tourists, who are becoming an important source of foreign exchange for China, have complained of rats in hotels.

Experts writing in People's Daily and Economic Daily called for a national extermination campaign reminiscent of the extraordinary anti-fly blitzes of the 1950s. Then, school children were provided fly swatters and given daily quotas to fulfill.

The strategy for vanquishing rats involves no human-wave tactics. But suggestions have been made to ban hunting of predators and to halt deforestation, as well as to encourage cat-keeping through incentives. A village in eastern China has banned rat poison and offered a subsidy of $2.50 to every peasant who raises a cat, plus a $1 bonus for each litter of kittens.

Militiamen in southern China armed themselves with bamboo bows and arrows and mounted a rat hunt at a rural commune near Canton. In one day, they bagged over a ton of carcasses. But they were less concerned with ecological balance than a good meal--rat meat is regarded as a delicacy in the south.