U.S. soybean growers are losing market share in the all-important China market because the race to grow higher-yielding crops has robbed their most prized nutrient: protein. Declining protein levels make soybeans less valuable to the $400 billion industry that produces feed for cattle, pigs, chickens and fish. And the problem is a key factor driving soybean buyers from the U.S. to Brazil, where warmer weather helps offset the impact of higher crop yields on protein levels. A decade ago, the United States supplied 38 percent of soybeans to China, the world’s top importer, compared to 34 percent from Brazil. Now, Brazil supplies 57 percent of Chinese imports compared to 31 from the United States.