WASHINGTON (AP) _ Objections are being raised to a proposal by Senate Agriculture Committee Chairman Jesse Helms to cure a budget-busting farm bill by lopping off more than $3 billion in farm income subsidies.

Helms proposed the idea Wednesday as part of a list of cuts needed to wrestle the emerging legislation into line with congressional budget targets. The North Carolina Republican contended the committee could meet its obligations on the budget, ''and still provide generous, even massive, assistance to farmers.''

Topping the list of proposed cuts was a reduction in the proportion of each farmer's crop land that would be eligible for the income supports. Instead of paying the farmer based on his entire production, the subsidy would apply only to the wheat, corn, cotton and rice needed to meet domestic and export needs and provide an adequate reserve.

For 1986 that would be about 80 percent, meaning farmers would see a 20 percent drop in their subsidy checks. The proposal drew criticism immediately.

''I don't understand why, when we couldn't freeze entitlements and we couldn't freeze defense, we are not only freezing but cutting agriculture,'' said Sen. Mark Andrews, R-N.D.

Added Sen. David Boren, D-Okla., ''We are in a situation where we need a 10 to 20 percent improvement in net farm income'' rather than a reduction. He called Helm's list of savings ''penny-wise and pound-foolish.''

Helms and his staff hinted that if the committee doesn't accept the across- the-board approach to cost cutting, he will trot out proposals to apply a ''needs test'' to farm program benefits, making them much like welfare payments.

''Despite the difficult economic conditions in the agricultural sector, there is immense wealth in the sector, and most of the payments in our farm programs end up going to farmers with very large net equity and income,'' Helms said.

Committee staffers said they had prepared ways to cut ''hobby farmers'' - those who don't depend on farming for their livelihoods - and large, wealthy producers out of the subsidy programs.

''The net equity of American farmers averages about six times that of the average American family,'' Helms argued. Constituents ''expect that what assistance we can provide should be directed to those who need it most.''

The Senate committee has been working for months on a new long-term farm bill to replace the law that expires at the end of this month. It has worked out most of the details of a new bill, but the measure exceeds congressional spending guidelines by some $9 billion over the next three years.

In a meeting Tuesday with Republican congressional leaders at the White House, President Reagan reiterated his unwillingness to sign any legislation that exceeds the three-year, $39 billion budget target for farm and farm credit programs.

Other items on the proposed savings list included a 50-cent cut in the dairy price-support level on Jan. 1, 1986, saving $700 million; freezing target prices at current levels for one year, allowing for reductions of 5 percent a year thereafter, to save $2.7 billion; slow the rate of implementation of a new farmland conservation reserve and have farmers share the cost of planting cover crops, to save $500 million; and cut export credit, for a $1.3 billion savings.