Response of beef cattle selected for tolerance to tall fescue toxicosis
Tall fescue (Schedonorus arundinaceus (Schreb.) Dumort.) infected with the endophytic fungus, Epichlooë coenophiala [(Morgan-Jones & W. Gams) C.W. Bacon & Schardl, comb. nov.] produces ergot alkaloids that result in a livestock disorder known as tall fescue toxicosis. Tall fescue cultivars have been developed that do not contain the toxic endophyte. These novel endophyte cultivars have been used to alleviate tall fescue toxicosis. Recently, genetically testing cattle using the dopamine receptor D2 (DRD2) gene has been identified as a way to determine tolerance to tall fescue toxicosis. Results will be presented from three research projects involving cow-calf pairs on pasture, heifers in the GrowSafe feeding system, and rumen fluid in a continuous culture. Research on cow-calf pairs grazing toxic Kentucky 31 tall fescue pasture show that cow pre-calving weight and calf 205-d adjusted weaning weight were greater for animals with tolerant genotype. Evaluation of heifers in the GrowSafe feeding system showed that there was no difference in dry matter intake as a percent of body weight between the tolerant heifers fed Kentucky 31 and the tolerant heifers fed BarOptima. Research in the continuous culture showed that there were very few responses in fermenter characteristics due to genotype when toxic or novel endophyte-infected tall fescue was fed. These results indicate that providing animals with novel endophyte-infected tall fescue is the best way to prevent tall fescue toxicosis. However, the DRD2 gene shows promise as another possible way to mitigate the effects of tall fescue toxicosis in cattle.