WASHINGTON (AP) _ Environmentalists say they were heartened by the approval early Tuesday by a House-Senate conference committee of a $40.8 billion farm bill that enhances agricultural conservation.

''It's now an environmental statute,'' Ken Cook of the Center for Resource Economics said of the farm bill. ''We've made it one where it wasn't before.''

Sen. Patrick Leahy, D-Vt., chairman of the Senate Agriculture Committee and the conference committee, said the package was ''a mark of how environmental awareness is changing in this country.''

The committee, which has been working on the legislation for weeks, completed the bill after a 14-hour marathon of negotiations that ended Tuesday at 3:45 a.m. EDT.

The major disappointment for environmental groups watching the lengthy negotiations was the omission of a ban on the export of pesticides that are outlawed or unregistered in the United States.

''They showed who they work for,'' Bill McNichol of the National Coalition Against Misuse of Pesticides said of the conferees. ''They had to make a choice between the economic interests of the chemical companies and the safety of consumers.''

''It was an important provision that will be enacted someday soon,'' said Kathryn Hohmann of the Sierra Club. ''But we look at the other provisions in the bill and we see some gigantic strides.''

Agriculture Secretary Clayton Yeutter said Tuesday he does not like everything in the bill and would have written it differently.

''But, in the end, it is a respectable work product that balances well innumerable sensitive, often divergent interests,'' he said in a statement.

Agriculture Undersecretary Richard Crowder predicted President Bush would be satisfied with the bill.

Leahy had made the pesticide export ban a personal crusade, but pulled it from the negotiating table when conference members tried to water down stringent language he had designed.

The proposal had been intended to stop pesticides that are not used in the United States because of health concerns from showing up on the American dinner table as residue on imported food.

But chemical companies lobbied hard against the plan, saying they would lose billions of dollars in overseas business and be forced to close manufacturing plants in the United States, wiping out hundreds of jobs.

The National Agricultural Chemical Association pointed out that many pesticides manufactured here are unregistered because they are directed at pests that exist only in other countries.

Leahy said he would introduce his plan as separate legislation next session.

Sen. Richard Lugar, R-Ind., the ranking Republican on the Senate Agriculture Committee, called the overall farm bill an ''excellent marriage'' of environmental and business concerns.

''It gives the American farmer a great deal of flexibility and freedom to plant,'' he said. ''It clearly is oriented to make us more competitive.''

Some of the planting flexibility in the bill was forced on the committee by the need to cut the federal budget by $500 billion over the next five years, including $13.6 billion from agriculture.

''You cross your fingers and hope that giving them flexibility will make up for the loss of support,'' said de la Garza.

The bulk of the bill's budget savings would be achieved by requiring farmers of rice, feed grains, wheat and cotton to reduce the acreage protected by the government's support system by 15 percent in each of the five years of the measure.

This so-called ''triple-base'' plan allows farmers flexibility to plant those commodities, experimental crops, industrial crops, or products not already in substantial supply. However, they would not be subsidized by the taxpayers.

The government sends the farmer a check, known as a deficiency payment, when the market price falls below the government-established target price.

The conferees agreed to calculate those payments on a 12-month basis, starting in 1994, instead of the current five months used for most crops. Since current calculations are based on the months immediately after harvest, when market prices are lowest, the government expects to save money by averaging the entire year.

The conference committee also agreed to allow winter wheat farmers, who already are planting their crop, to choose between the triple base or the 12- month deficiency payment for 1991.

Despite the decreases in subsidies these changes would create, the bill still provides $8 billion to $10 billion in support to farmers, said Leahy.

Other savings would come from:

-Setting marketing loans for soybeans at $5.02 a bushel with a 2 percent service charge.

-Putting a 1 percent service charge on loans for sugar, peanuts, tobacco and honey.

-Requiring reduction of 15 percent of acreage for the 1991 wheat crop and 7.5 percent of the corn crop.

Other significant provisions include:

-A $10.10 per hundredweight minimum support price on dairy products with a 5-cent assessment per hundredweight this year, rising to 11-cents in later years.

-A limit of $100,000 on deficiency payments and $150,000 on marketing loans that can be received by a single farmer.

-Continuation of the crop insurance program.

-Forestry provisions designed to save millions of trees on private forest land.

-Incentives to farmers who participate in programs to improve water quality around their farms and underground.

-Strengthening of penalties against draining wetlands for crop production.

-Procedures by which farmers could drain wetlands in the middle of their planting area if they create new wetlands on easements on their property.

-Requirements that records be kept on pesticides that are being used.