BILL E. KUNKLE INTERDISCIPLINARY BEEF SYMPOSIUM: Genetic resistance to the effects of grazing endophyte-infected tall fescue.

Forages are the base source of nutrition for any cow-calf operation. Forage types vary based on soil type and climate. Tall fescue () is the most commonly used cultivated grass for grazing beef cattle in the United States. This cool-season perennial is easily established; is resistant to drought, insects, and nematodes; and has the ability to withstand heavy grazing pressure. Most tall fescue varieties are infected with the endophyte fungus () that is essential for the plant's survival but detrimental to cattle performance. Ergot alkaloids are the generally accepted toxic agents produced by the fescue endophyte. Cattle that consume forages infected with this endophyte can develop fescue foot, fat necrosis, or fescue toxicosis. It is estimated that the beef industry loses over US$500 million annually due to fescue toxicosis through heat stress, reduced weight gain, suppressed appetite, and decreased reproductive performance. Other symptoms include a retained or rough hair coat and increased body temperature, which can be detrimental when animals are located in hot and or humid environments. Different forages and forage systems, feed additives, and animal management strategies have been tested through the years allowing the use of tall fescue in beef production systems while minimizing the adverse effects. An animal genetics approach needs to identify and select animals less susceptible to the ergot alkaloids. Research in this area reports that different cattle within the same herd can respond differently when grazing tall fescue, and evidence exists that breed type may also play a role in genetic tolerance to the negative effects on performance. Some studies have looked at the potential of identifying genetic markers that may assist in the selection of more resistant animals. From these studies, there is evidence that genetic variation does exist for resistance to the ergot alkaloids present when grazing endophyte-infected tall fescue. Forage management coupled with animal genetic selection could allow for more efficient use of tall fescue as a significant forage source in beef cattle systems. These would allow producing more beef per hectare in the United States in response to an increasing population combined with decreasing resources.