Suppose you owned a herd of cattle and the federal government promised to pay four-fifths of the feed bill. What would you do?

a) Sell the herd

b) Grow fruits and vegetables

c) Subdivide the land

d) Laugh all the way to the bank

If you are wondering how you can get such a deal, the Bureau of Reclamation will be happy to provide you with the details. If you are wondering instead how such a perversion of private enterprise can occur, read on.

The Bureau of Reclamation was created in 1902 to open the arid lands of the West to homesteaders. It was a noble experiment to conserve and distribute the scarce water of that region to promote family farms on land where scrub and cactus grew.

But the noble experiment turned into an en-an entrenched bureaucracy that now nourishes a rich, influential constituency of agribusinessmen and industrialists, not family farmers. These constituents are powerfully represented in Washington, as President Carter first learned two years ago when he tried to bring a measure of rational planning to the bureau's works.

Take that cattle herd. A farmer in the Midwest who grows his own forage either trusts nature to provide the rain or sets up an irrigation system. In the deserts of the West, however, farmers are encouraged by ludicrously low, federally subsidized water prices to grow hay and other forms of forage. They can easily afford the six acre-feet of water it takes to raise such crops because the price to them is less than a penny a ton for what is truly liquid gold. That subsidizes a lot of cattle feed.

The Central Arizona Project (CAP) is unquestionably one of the worst ideas ever conceived by the Bureau of Reclamation. In a hard-hitting economic analysis, Dr. Thomas M. Power, chairman of the University of Montana's economics department, calculated that "the project will cost the U.S. taxpayer more than $5.4 billion in subsidies while yielding no positive net return to the nation."

While special interests in Arizona might quarrel with that assessment - after all, federal water helps them prosper - the CAP's effect on other areas needs to be considered. With agricultural oversupplies already driving down the price of many farm commodities, generous subsidies that encourage greater cotton, grain and forage production in Arizona and the arid West "can only be at the expense of farmers elsewhere and the tax-payer who supports farm incomes."

Arizonans are already pumping out 2.2 million acre-feet of water from underground sources that nature will not replenish. Only the CAP, some argue, will help to alleviate this yearly drain on the state's underground supply.

This shortage, however, is entirely man-made. Agriculture absorbs some 90 percent of the state's water but supplies only three percent of personal income. Moreover, three-fourths of Arizona's irrigated land is producing crops that are in surplus nationwide and thus benefit from federal price supports.

The CAP would take 1.2 million acre-feet of water from the Colorado River, lift it over 2,100-foot mountains, store it in dams and move it through canals to Phenix and Tucson, 300 miles and 400 miles respectively. If instead of spending $5.4 billion on the CAP, Congress were to retire some of the irrigated lands under present agricultural set-aside programs, the cost would be on the order of $300 million. This would immediately free 1.5 million acre-feet of water, which is more than the CAP would deliver when it is finally complete.

In 1902, it was in the national interest to encourage homesteading on the Western frontier. Today, that region is teeming with cities and suburbs of tremendous vitality and can afford to pay for its own development. Congress, however, shows little sign of abandoning its flagrantly wasteful, frontier water policy.

President Carter has sent Congress a bill that would require the states to put up five to 10 percent of the cost of future water projects. While that is certainly a small step in the right direction, it leaves out the CAP and other projects Congress has already authorized.

If the CAP were brought before Congress as an isolated issue, it would be hooted off the floor. But water projects and other public works are spread out with geographic calculation and joined in a single legislative package. When the result is paraded before members of Congress, they stand together like d'Artagnan's finest and pledge: "All for one, one for all." CAPTION: Picture, An adequeduct northwest of Phoenix.