In a way, they already do. A field of grass sits there all day soaking up energy from the sun and storing it chemically. A grazing animal can then come along and absorb weeks of accumulated energy in a matter of minutes.
Chlorophyll photosynthesis extracts 3%-6% of the total energy from sunlight. If we figure on any given day the cow gets the equivalent of about six hours of peak sunlight, it works out to less than two million joules of usable energy each day.
If we could equip cows with solar panels, which can be several times more energy-efficient than photosynthesis, we could improve that number—but not by much.
By contrast, about 3% of the world's surface area is cultivated, which means that (given rough estimates of geographic distribution of farmland) our crops easily intercept over a thousand times more sunlight than our cattle—which is why grazing is a good strategy.
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