WASHINGTON — Congress is set to begin debate today on a $300 billion, five-year farm bill that expands some farm and food programs and shrinks payments to some of the wealthiest farmers — but that still drew a veto threat from President Bush on Tuesday.

The bill boosts spending by about $10 billion, including more money for food stamps, specialty crops and assistance to dairy farmers, but it does so with budget moves that the White House considers gimmicks and hidden tax increases. It cuts off farm program payments to farmers with farm-related adjusted gross income greater than $750,000 but falls well short of the limits sought by the Bush administration and budget hawks in Congress.

For Northern New York, which is dominated by the dairy industry, the bill promises expanded government payments to farmers when milk prices are low and feed costs are high — the first time dairy farmers have seen greater government assistance based on the cost of feed for their cows. Feed costs have doubled in some cases over the past few years.

"I am deeply disappointed in the conference report filed today as it falls far short of the proposal my Administration put forward," Mr. Bush said in a statement. "If this bill makes it to my desk, I will veto it."

With his statement, Mr. Bush appeared to erase any speculation that earlier, more vague veto threats were bluffs.

Mr. Bush cited a strong farm economy and said now is the time for deeper reforms in farm subsidies. Farmers are receiving record-high prices for corn, soybeans and other grains, although those high prices hurt dairy farmers, who are primarily buyers of grain.

In addition, Mr. Bush rejected lawmakers' estimate of $10 billion in additional spending, saying the real additional cost of the bill would be $20 billion, reflecting government payments if it becomes law.

With the threatened veto, the question facing the farm bill is whether enough Republican lawmakers will defy the White House to reach the necessary two-thirds majority for an override. That was not certain, although the package has broad bipartisan support on the House and Senate agriculture committees.

Should the bill fail, Congress could extend current farm programs for an additional year, in order to avoid a return to the basic farm law of the last century, which could automatically take effect. That law required farm-level prices for milk, for instance, to be adjusted for inflation — which would send prices sharply higher than even the record highs farmers have seen lately.

Northeast lawmakers cheered the dairy provisions, which resulted from negotiations on a House-Senate conference committee. The subsidy, called the Milk Income Loss Contract, does not grow as much as supporters sought, but they beat back speculation that the best they could hope for was the status quo or a reduced program.

"The corn ethanol boom has not only caused the price of food to go up, but the cost to feed animals, including dairy cows, has also gone up causing even higher prices for milk," said Rep. Kirsten E. Gillibrand, D-N.Y., one of just two New Yorkers on the agriculture committees. "By helping our local dairy farmers pay for their feed costs, we hope that this will help lower milk prices for consumers in the end."

New York Farm Bureau President John Lincoln praised the agreement, saying in a statement, "We're happy to see that dairy, specialty crop, research, rural development, nutrition, and energy programs important to our farmers are included in this version of the farm bill."

The revised MILC program will boost farmers' payments based on a formula of the prices for corn, soybeans and alfalfa hay, a measure meant to reflect the feed mix given to dairy cows. Farmers will continue to be paid when milk prices fall below a federal target of $16.94 per 100 pounds, but at a slightly higher rate. Lawmakers also approved a slight increase in the amount of milk eligible for payment.

The measure also creates a commission to study the nation's milk pricing system, which is composed of regional marketing orders that set minimum prices farmers must be paid for milk. Among other tasks, the commission must review whether the system fosters competition and benefits farmers, consumers and milk processors — a sore point among farmers who decry the declining number of buyers for their milk.

Lawmakers also agreed to extend mandatory fees for milk promotion to imported dairy products, over the objections of foreign governments and U.S. dairy importers. Critics say the provision could violate trade rules and is unfair because the U.S. blocks imports of beverage milk.

One item that rankles some farmers is the re-establishment of forward price contracting in the dairy industry. That allows farmers or their bargaining cooperatives to negotiate prices well in advance for milk to be delivered, but critics say it has enabled buyers to pay farmers less than the federal minimum price and to coerce farmers into the agreements.

The revised program, which is voluntary, prohibits processors from requiring forward contracts. The new program would run through 2012.