Santa Gertrudis cattle were recognized in 1940 by the United States Department of Agriculture as an authentic and distinct breed of cattle. They were bred on the King Ranch by the Kleberg family, principally by Robert Justus Kleberg, Jr. He was looking to raise superior heavy beef cattle in the merciless South Texas sun, on meager pasturage plagued by recurrent drought, insect pests, and disease plagues. In 1910 rancher Thomas M. O'Connor gave the King Ranch a half-Brahman, half-Shorthorn bull, which eventually jumped the fence into a pasture of 3,000 Shorthorn cattle heifers. One of the offspring from this foray, a bull calf called Chemmera, later proved to be successful in breeding hardy offspring suited to the range. This sparked interest in the controlled development of a new breed. In 1919 Kleberg mated a Brahman bull, Vinotero, obtained from A. P. Borden, with a blood-red Shorthorn cow of one-sixteenth Brahman cattle blood, a descendant of the original O'Connor bull. The issue was a dark-red male of superior qualities, who, because of his playful behavior, was named Monkey. The uniformly superior animals Monkey sired became the Santa Gertrudis breed; thus Monkey was the "father of the Santa Gertrudis." The breed name Santa Gertrudis derived from that of an original land grant given in the 1760s to Blas María de la Garza Falcón, who named the area that the King Ranch now occupies "Los Cerros de Santa Gertrudis." The breed has three-eighths Brahman and five-eighths Shorthorn blood. The characteristics of the registered animals of the 1990s included a solid cherry-red color with only minor markings on the underline, a high tolerance for heat, and great resistance to biting insects. Santa Gertrudis cattle may be horned or polled (hornless). A mature bull may average 1,700 to 2,200 pounds, while an average cow will weigh between 1,350 and 1,850 pounds. Of particular interest to commercial breeders are the higher weights reached by Santa Gertrudis cattle compared to other meat breeds. At weaning time (eight months) they will average 100 pounds more than European counterparts. Mature Santa Gertrudis steers and cows average 200 pounds heavier than European breeds of the same age. Four-year-old steers on the King Ranch will average 1,400 pounds when finished on grass before going to market, and will outdress European breeds by an average of 3 percent.
In 1943 the King Ranch loaned some Santa Gertrudis bulls to William DuPont to establish a herd in Virginia; the King Ranch purchased the herd in 1966. The first public sale of Santa Gertrudis bulls was held in 1951, when twenty-nine bulls brought $99,000. After the sale the Santa Gertrudis Breeders Association was formed; in the early 1990s it was known as the Santa Gertrudis Breeders International, and its headquarters were in Kingsville. The next spring (1952) the second public sale, this time of twenty-five bulls, brought $212,550, with the top bull selling for $27,000. In 1952 the King Ranch entered into a partnership with Australian cattlemen and developers, shipping 272 Santa Gertrudis cattle to northeastern Australia in a very successful effort to upgrade Hereford, Devon, and Angus cattle in Australia. By the early 1990s these superior cattle had been promoted into a highly profitable enterprise, with thousands of Santa Gertrudis cattle entering the market each year. In 1994 there were 3,500 Santa Gertrudis breeders, and Texas had more than 100,000 Santa Gertrudis cattle.