They don't care which brand you buy - they just want you to buy it

Not many Americans think about it. But they are being urged to drink more milk, use more cotton or more wood products, eat more meat, poultry, eggs, orange juice, rice, nuts, seafood - and lately, of course, those controversial urgings to drink coffee.

More intense selling of generic products, according to Department of Agriculture's Economic Research Service, is adding up to a sizable advertising bill - about $84 million in 1982. It's being paid out by commodity groups on the theory that in broadly used items - eggs, oranges, seafood - it's the quantity consumed that counts and not necessarily which brand name. Commodity producers have found out that a universal statement such as, ''Orange juice isn't just for breakfast any more,'' is stimulating to sales of all brands on the market.

Generic advertising, the Economic Research Service notes, can do four jobs: ( 1) It can counter competition, emphasizing milk, for example, vs. other beverages; (2) it can increase public awareness of less well-known foods (avocados, kiwi fruit); (3) it can improve consumer knowledge of reputed nutritive elements (fewer calories, more minerals); and (4) it can point up new product uses.

Expenditures for this type of advertising by producers are aimed not only at consumers in general but also at specific groups in the marketing chain. Often, their ads dominate trade publications. They sponsor trade shows and expositions and go after outlets that can push generics - persuading discounters, for instance, to feature cotton products.

About 85 percent of generic advertising funds come from mandatory producer agreements, allocating a portion of the price per item or unit to go toward advertising.