Fundamental changes in the milk and peanut-support programs, an end to the target-price program that supplements farmers' incomes, and slight increases in grain price supports are features of the 1981 general farm bill the administration was scheduled to announce today.

Details of the package were to be disclosed by Agriculture Secretary John R. Block in testimony today before the House Agriculture Committee and Wednesday in the Senate.

According to sources, however, the administration did not intend to send a formal bill to Congress for some days, in an apparent effort to avoid entanglement with the committees over line-by-line details.

In that vein, Deputy Secretary Richard Lyng told a House subcommittee yesterday that the administration, while agreed on broad details, still has not prepared legislative language on the portion of the bill regulating the food-stamp program, which will undergo substantial cuts.

The peanut, milk and grain sections of the bill are expected to cause more unhappiness among farmers, who are increasingly irritated about an agricultural economy in which production costs are rapidly outstripping income.

Reliable sources reported these details about the package that Block and administration officials have put together:

The 1981 price support for wheat would rise to $3.20 a bushel from the present $3; corn would go from $2.25 to $2.40. Farmers have urged higher support levels. They may borrow from the government at those rates, using their crops as collateral when market prices are below that.

The milk price support would continue, but be based on 70 percent of parity rather than the present 75 percent in an effort to reduce costly government purchase of excess supplies and to hold down production. Small dairy farmers particularly in the Northeast, are likely to be hit hard by the proposal.

The peanut support program also would continue, but in sharply different from. The administration would wipe out the acreage allotment system, which gives land owners control over production and, by some estimates, increases the cost of peanuts by about 20 percent because of rental fees that farmers must pay to allotment-holding landowners.

The target price system, under which farmers receive direct payments for the difference between market prices for their commodity and their production costs, would be ended. Block has termed the payments "subsidies" not in step with the administration's free-market and budget-cutting attitudes. n

Lyng reiterated details of the food stamp cuts yesterday, aimed at slicing $2 billion from the program in fiscal 1982 and eliminating at least $1 million from the rolls. Among the new details: an end to food-stamp aid at drug and alcohol abuse centers, homes for battered women and children, and homes for the blind and disabled.

On another front yesterday, reacting to President Reagan's decision to continue the embargo on grain to the Soviet Union, the American Agriculture Movement announced a call for a voluntary farmers' embargo to stop delivery of agrcultural products.

AAM Chairman Marvin Meek said the withholding of products from the market will be an attempt to highlight farmers' demands for higher prices to meet escalating production costs.