The Justice Department yesterday responded to two lawsuits challenging a government program of spraying marijuana plants with the herbicide paraquat.

In a 34-page brief, the department said paraquat is used routinely to kill weeds on farmland used for growing vegetables and has been registered with the Agriculture Department and the Environmental Protection Agency as safe for that purpose since 1967.

One suit, filed by the National Organization for the Reform of Marijuana Laws, claimed that the herbicide spraying program by the Drug Enforcement Administration inflicts "cruel and unusual punishment" on marijuana smokers.

The second suit, filed by the Sierra Club, Friends of the Earth and National Coalition Against the Misuse of Pesticides, claimed that the government has not followed its rules governing protection of the environment.

"Paraquat is registered for use as a weedkiller and harvest aid chemical in the United States," the Justice Department said. The brief quoted reports from Chevron Chemical Co., which manufactures paraquat, in saying that 4 million gallons of the chemical were used in this country in 1981 and that about 8 million acres of farmland were sprayed last year.

According to the brief, the chemical is safe if used in accordance with its instructions, which say it may be sprayed on unplanted fields or under certain other conditions.

A hearing scheduled today by U.S. District Court Judge June L. Green on whether to issue a temporary restraining order against the spraying is expected to be postponed until Monday.