Agriculture Secretary Bob Bergland predicted yesterday that the Chinese will become "regular and significant purchasers" of American grains and cotton, and good customers for American agricultural machinery.

Winding up a 10-day tour of China, Bergland told reporters at a Hong Kong news conference that the two countries have begun to work out details for exchanges in agricultural sciences, technology and statistics which will start next summer.

He said no trade agreement was signed and no specific figures were discussed for grain and cotton purchases, but the general feeling was that "we could expect a general increase." This year, China purchased 3 million tons of U.S. wheat, 305,000 bales of cotton and 1.2 million tons of corn.

Bergland said the Chinese had asked the Agriculture Department to help facilitate contacts with American manufacturers of agricultural equipment and supplies "and we have agreed."

He said China expressed interest in a whole range of items from food processing equipment to agricultural machinery, and he predicted eventual U.S. sales "will be substantial." He noted that the Chinese "are shopping" and that American companies would have to compete with other suppliers.

Bergland said he expects an Agriculture Department-led team of scientists and technicians from various industry associations to begin visiting China early next year to further discuss products and technology best suited to conditions in the People's Republic.

Bergland's delegation was the highest-raking U.S. agricultural group to visit China since the Communist takeover in 1949 and was part of a growing number of U.S. attempts to open trade with the Asian giant despite the absence of diplomatic relations between Washington and Peking.

In recent weeks, American officials and businessmen have disclosed plans to help China develop coal mines and build dams, construct a chain of tourist hotels and offer loans to Peking for trade and developement.

Bergland said he does not know whether increased purchases from the United States will be at the expense of China's traditional suppliers, Canada and Australia.

As a result of talks with leading Chinese agricultural officials, it appears China is giving agriculture "the number one priority in modernizing the country," Bergland said.

The productivity of Chinese agriculture is among the highest in the world, he said, but it is also among the most labor-intensive, which means people can't leave the fields and farms to work on industrial development. Bergland said that mechanizing agriculture, particularly in the north, will help China increase yields and enable it to go ahead with its other modernazation programs.

Delegration sources said the Chinese asked Bergland for assurances that new U.S. grain inspection rules will prevent contamination in exported grain.

China halted grain purchases from the United States in 1974 when tick smut was discovered in a wheat shipment.

During three days of talks in Peking, U.S. and Chinese officials also discussed extension of the U.S. deterred payment credits for China to buy American agricultural commodities.

The Chinese were told that credits for fiscal 1979 already have been allocated and the Carter administration probably would not consider reallocation.

The Chinese did not ask specifically for U.S. credit but said they are aware tht the recently passed Agriculture Trade Act would empower Bergland to extend one- to three-year credit for commodity purchases.

Because there are no official diplomatic relations between China and the United States - and Bergland said the subject "never came up" - the question of agreements involving both governments was a sticky one.

Dr. Rupert Cutler, assistant secretary of Agriculture for conservation, research and education, said an "oral agreement" on agricultural, scientific and student exchanges was reached between nongovernment institutions.