Spare that weed - it may feed cattle or supply rubber

For the Western beef industry, milkweed may be a key to future prosperity. It's a dry-land plant that could provide nutritious cattle feed when ground water for irrigating the usual feed grains or alfalfa runs out.

The showy milkweed (Asclepias speciosam) can also supply natural rubber, sugar , pectin, and other valuable chemicals. It is only one of perhaps hundreds of wild plants that might profitably be domesticated, according to a study recently published by the congressional Office of Technology Assessment (OTA).

There have been a number of such studies in recent years calling attention to the wild wealth which traditional farming has overlooked. Now the OTA has laid out the possibilities for Congress with a view to possible congressional support for research to identify and develop useful new crops.

As the OTA report notes, ''Of the earth's estimated 500,000 to 750,000 species of higher plants, no more than 10 percent have been examined even cursorily for their chemical makeup.'' In fact, only about 300 species (100 in the United States) have been domesticated. Yet, OTA observes, there are probably many other potentially valuable crop plants waiting to be discovered.

The trouble is that many of them won't be waiting much longer. Human population pressure and its accompanying ''development'' are wiping out plant species on a wholesale basis. This is especially true in the tropical forests, which hold most of the plant species, where vast jungle areas are being cleared.

Experts attending a conference on the ''Dynamics of Extinction'' held in August at Northern Arizona University described the trend as alarming. For example, Paul Ehrlich of Stanford University told the conference that ''Earth's biota (both plants and animals) now appears to be entering an era of extinctions that may rival or surpass in scale that which occurred at the end of the Cretaceous.'' That is when the dinosaurs died out.

Recognition of this trend gives a tone of urgency to the OTA report. Yet, the report notes, there are undoubtedly many wild plants that would help solve resource problems now facing many nations. For the United States, there is the question of how to shift from irrigation farming to dry farming as ground-water resources are depleted in semiarid regions. It should be possible to match dry-land plants, such as milkweed, to the farmers' needs and develop a new agriculture suitable for this area.

Two such plants - jojoba, which produces a substitute for whale oil, and guayule, which yields rubber - have already attracted considerable attention. Cultivation of these, and to a lesser extent of milkweed, is being developed in the US on a very limited scale. But this experience has already emphasized a need for extensive research and development to bring a potential crop to the point where farmers can grow it easily and where a profitable market can be found for its products. The OTA report emphasizes that new crop plants and their products won't be integrated into the US farm economy automatically just because , theoretically, they look promising.

OTA does not make policy recommendations to Congress, but it does lay out possible courses of action. Thus, it points out the benefits of commissioning detailed studies to identify specific new crop opportunities and define the research needed to domesticate the plants and develop markets for their products. With this information in hand, Congress could decide whether and how to support this research. It could turn out to be one of the most important agricultural development programs the United States has ever undertaken. Desert sands make fertilizer

Nitrogen, chemically ''fixed'' in a compound such as ammonia, is an essential plant fertilizer. In nature, some bacteria fix nitrogen in the soil and in the roots of legumes. Lightning also makes such fertilizer in the air. But who would have thought that sun-baked desert sands could also do the job?

University of California chemist Gerhard Schrauzer estimates that nonbiological processes promoted by desert sands fix some 10 million tons of nitrogen annually.

Schrauzer and his colleagues at the university's San Diego campus are studying the action of semiconductor materials such as titanium dioxide (rutile) , which exist naturally in many desert sands. These materials are like those used for solar cells. They find that, under the influence of sunshine, these materials can indeed fix nitrogen from the air.

The process has several stages. First, water molecules are split into hydrogen and oxygen. Then the nitrogen is fixed as ammonia using this hydrogen. There seems to be enough water adsorbed on the sand grains to facilitate this process even though the sand is dry to the touch. Arctic 'Eden'

Although far removed from the tropical location of the mythical Garden of Eden, the Arctic may well have been the cradle of many plant and animal species.

According to standard evolutionary notions, most species originated in tropical lands and migrated northward. A research team recently reported in Science, however, that some of these species existed in the Canadian Arctic about 20 million years earlier than had been thought. It now looks as though many species ''probably had their origins in Arctic regions and spread southward ,'' the report says.

The researchers include Leo J. Hickey of Yale University, Robert M. West and Mary R. Dawson of the Carnegie Museum of Natural History in Pittsburgh, and Duck K. Choi of the Korean Geological Survey, who worked on this project while studying at Pennsylvania State University. They point out that, while the Arctic now is a harsh environment, the far northern climate was much milder millions of years ago, with a richer mix of plants and animals than it has today. It should not be discounted as an important setting for evolution.