WASHINGTON (AP) _ If Popeye had been a sweet potato, he wouldn't have dared say, ''I yam what I yam and that's all I yam'' around certain people in the Agriculture Department.

''Sweet potatoes and yams come from different biological and geographical origins,'' says USDA economist Amy Allred. ''However, the darker varieties of sweet potatoes are often erroneously called yams.''

In fact, the United States produces virtually no yams of its own. Sweet potatoes, yes, around 12 million hundredweight a year, or roughly 545,000 metric tons.

But that's far down the world's production list, with global output in 1987 at nearly 133 million tons. China is the biggest producer at 113 million tons.

Yam production is much smaller at about 24 million tons, with most grown in Africa. Nigeria, with a 1987 production of 16 million tons, is the leading producer.

Allred says in a report by the department's Economic Research Service that sweet potatoes have been cultivated since prehistoric times in two widely separated parts of the world, tropical America and the South Pacific islands.

Early artifacts show yams were used for food in West Africa prior to 50,000 B.C, and have been grown in Southeast Asia since 3,000 B.C.

Allred was amused when the label from a supermarket can of ''yams'' was read to her in a telephone interview. The label said yams in large letters, with ''sweet potatoes'' in finer print.

''Traditionally, people call them yams,'' she said. ''It's pretty common.''

But somewhere along the line, the yam-sweet potato question was settled partly, at least, in North Carolina, the leading U.S. producer. It is now the North Carolina Sweetpotato Commission, with no mention of yams in the official title.

North Carolina produced 4.59 million hundredweight of sweet potatoes last year, followed at a distance by Louisiana, 2.47 million, and California, 1.31 million. Other commercial sweet potato states include Alabama, Georgia, Maryland, Mississippi, New Jersey, South Carolina, Tennessee, Texas and Virginia.

Allred's report included further information about the differences between sweet potatoes and yams:

- There are a thousand species of sweet potato, which has a two-leafed embryo and thus is called a dicot. It also is a member of the morning-glory family. The flesh varies from nearly white to almost orange in color.

- In the United States, per capita sweet potato consumption reached a high of 6.4 pounds in 1982. Comparatively, Americans eat an average of about 119 pounds of regular potatoes annually, measured on the basis of farm weight.

- Most domestic use of sweet potatoes is in candied or baked dishes, but they are also used in puddings, pies and even ice cream. The industry in recent years has promoted new products, including fresh fried sweet potatoes.

- Yams have one-leafed embryos and are called monocots. They can be prepared in the same way as sweet potatoes. Most are consumed in the countries that produce them, often turned into flour and then into various staple dishes. Yams also are a source of a steroid compound used as a base in cortisone and hormonal drugs.

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WASHINGTON (AP) - A 10-year boom in U.S. aquaculture production slowed last year as catfish farmers eased back on expanding their operations, according to the Agriculture Department.

Overall, U.S. production of edible and non-edible aquaculture products increased only 40 million pounds last year, compared with a 1987 growth of more than 100 million pounds, the department's Economic Research Service said Wednesday in a preliminary report.

Despite the throttling back in catfish production, total 1988 output grew 5.2 percent to a record 295 million pounds, the report said. Catfish growth was more than 32 percent in 1987 and 11 percent in 1986.

Catfish production was accompanied by a 23.6 percent increase in prices paid to farmers by processors in 1988. Those price averages ranged from $1.61 per pound in California to 71 cents in Louisiana.

''However, higher feed and energy costs led to a 20 percent to 25 percent increase in production costs, which meant that net returns (to farmers) were only 5 percent to 10 percent higher,'' the report said.

Trout producers surveyed by the agency sold 58.9 million pounds with a value of $63.6 million between Sept. 1, 1987, and Aug. 31, 1988, the report said. Producers also sold more than 361 million trout eggs worth $2.8 million.

Production of pen-raised salmon was estimated at six million pounds in 1988, a 70 percent increase from 1987. The report said domestic salmon production ''will probably continue to see large gains'' in the next few years.

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WASHINGTON (AP) - The Agriculture Department says private exporters have sold the Soviet Union an additional 450,000 metric tons of U.S. corn, the second big purchase this week.

The department said Wednesday that delivery is planned for the current marketing year, which will end Aug. 31. An order for 550,000 tons of corn was reported on Tuesday.

Officials said the latest sales brought total corn sold to the Soviets for 1988-89 to almost 11.2 million tons. Additionally, sales include 4 million tons of wheat, 450,000 tons of soybeans, 1.4 million tons of soybean meal, and 507,400 tons of grain sorghum.

A metric ton is about 2,205 pounds and is equal to 39.4 bushels of corn or 36.7 bushels of wheat or soybeans. The 450,000 tons of corn would be about 17.7 million bushels.

Corn prices have been averaging about $2.59 per bushel at the farm level, according to the latest USDA five-day average, meaning that the latest sale could have a farm value of around $46 million.