This year, the cows just won't come home for the once-formidable dairy lobby.

Well, that's one way of looking at the milk lobby's recent troubles, including an embarrassing defeat this week in the Senate, a defeat caused by the votes of senators who have received generous milk-industry contributions in the past. There is no doubt that the dairy interests are weaker in Congress today than they've been in years.

And yet, this defeat has left the dairy industry with the government's most generous price support program. Only wheat farmers will enjoy anything remotely as grand (about $3.6 billion for milk, $3.3 billion for wheat over the next five years), if levels approved this week by the Senate become law. There are many more wheat farmers than dairymen.

Put another way, while the Reagan administration scrimps to save money by the tens and hundreds of millions by cutting social programs, dairy farmers next year will collect about $1.1 billion from the federal government, if the Senate-passed bill, the one the dairy interests "lost," becomes law.

And the dairy industry retains large federal price supports even though the government is sitting on huge and growing mountains of surplus dairy products which are unlikely ever to be sold for human consumption. There are now about 1.5 billion pounds of surplus butter, cheese and nonfat dry milk in government warehouses.

Despite all that, the unlikely alliance of consumer organizations and the Reagan administration that has been fighting to moderate milk price supports is claiming victory in the votes on the new farm bill, at least so far.

"We felt it was absolutely necessary to take dairy head-on," boasted one Agriculture Department official involved in the fight. "It's a real defeat for dairy," said Ellen Haas, director of the consumer division of the Community Nutrition Institute.

A "defeat for dairy" may be an outdated concept. It harks back to the days, as recently as last year, when the dairy industry was essentially united on a legislative program. The lobby has now split, a development that Patrick B. Healy, longtime secretary of the National Milk Producers Federation, called "a great tragedy" this week.

One of the three giant dairy cooperatives that contribute the most to political campaigns, the Associated Milk Producers, Inc., has decided to drop out of Healy's federation, a move that will cost the federation about $235,000 annually in dues, and conceivably a good deal in clout, too.

One issue that has pushed AMPI out of the federation is disagreement over tactics in Congress this year. Healy has taken a generally conciliatory tack, acknowledging that surpluses and the cost of the price support program are too high and need to be reduced. AMPI has been more militant, seeking to defend the old price support levels as necessary to preserve dairy farming.

When the Senate Agriculture Committee approved its four-year farm bill early last summer, the dairy lobby suffered a modest reversal. The committee bill called for a price support structure that would effectively freeze the current level ($13.10 per hundredweight) for the next year, then provide for semi-annual adjustments upward in a way that would guarantee steady increases every six months to keep up with inflation.

The problem with the Senate committee bill was that it significantly violated the budget limits already approved by Congress, and directly challenged the administration's budget-cutting crusade. So as the summer recess came to an end, the Department of Agriculture produced a new set of proposals for a revised farm bill that would come closer to the budget limits.

Because dairy was the single biggest cost item in the farm bill, it naturally attracted immediate attention. The administration had a new idea that might save an additional $1 billion on milk price supports over the next five years by allowing the government to skip all increases in price supports if big surpluses continue to accumulate.

Ideally, the administration wanted to include this dairy proposal in a new farm package that would contain scaled-down price support programs for all the big commodities. Traditionally, farm bills have represented a package deal--something for wheat, something for corn, something for milk and peanuts and soybeans. The supporters of each--at least in the old days--backed up one another to get the whole bill through.

But this year the temper of the Agriculture Committee was less collegial. Dairy quickly emerged as a potential sacrificial cow, essentially because the cost of the program was so high and the dairy surpluses were so huge. Sen. Rudy Boschwitz (R-Minn.), one of dairy's staunchest supporters on the committee, said in an interview that he was the only member who really wanted to fight for milk supports.

Sniping at dairy was one reason why the Agriculture Committee could not agree with the administration on a comprehensive revision of the farm bill. In the end, the various commodities had to fend for themselves on the Senate floor, a fact that allowed the administration to get cuts in supports for many of them.

Sen. Robert J. Dole (R-Kan.) emerged as the key bargainer with Agriculture Secretary John R. Block and his deputy, Richard E. Lyng. Dole declined to carry their dairy proposal forward himself, but they all prevailed on Sen. Roger W. Jepsen (R-Iowa) to do so. To the delight of friends of Rep. Thomas R. Harkin (D-Iowa), an ardent friend of the dairy industry who plans to run against Jepsen in 1984, the Iowa conservative agreed to lead the fight for lower milk supports. (Harkin's friends figure the generous dairy lobby won't forget this event.)

Jepsen proposed the new administration plan; Boschwitz offered an amendment restoring the original committee scheme. On the key vote, Boschwitz lost, 53 to 41. Among those voting against him and the dairy lobby were seven members of the Agriculture Committee who together had recived a total of $48,000 in campaign contributions from dairy interests since 1976.

The administration hopes the House will buy the Senate bill. But if there is no action by Oct. 1, the farm bill expires and milk price supports must automatically jump to $13.56 per hundredweight. The milk lobby is on the defensive, but it is far from out of the game.