Polymorphisms at DRD2 Gene Affect Physiological Responses in Heifers Fed Tall Fescue Silage.

Livestock consuming the ergot-like alkaloids produced by tall fescue [Lolium arundinaceum (Schreb.) Darbysh.] infected with wild-type Epichloe coenophiala display reduced forage intake and growth. This phenomenon called tall fescue toxicosis costs U.S. livestock producers around $1 billion annually. The recently discovered dopamine receptor D2 (DRD2) gene in cattle may partially control the tolerance of livestock to these alkaloids. Genetic variation at the DRD2 gene involving a guanine/adenine substitution partly determines the animal’s susceptibility to tall fescue toxicosis. The objective was to determine the response of heifers selected for tolerance or susceptibility to tall fescue toxicosis when fed rations of toxic endophyte-infected ‘Kentucky-31’ (KY-31) or novel endophyte-infected ‘BarOptima Plus E34’ (BO+) tall fescue silage (2x2 factorial). Heifers were fed in the GrowSafe feeding system where dry matter intake, body mass, and physiological variables were monitored for 70 days. Three years of data will be presented. Ergovaline levels were approximately 50 ppb in the BO+ and 400 ppb in the KY-31 tall fescue rations (P<0.05). Preliminary results from the first two years indicate there was a year x treatment interaction. In 2013, dry matter intake, average daily gain, and serum prolactin were not significantly different between the tolerant heifers fed KY-31 and BO+ (P>0.92). Results from 2014 did not concur with these findings. A third year of data is being collected and will be presented. The DRD2 gene shows potential as a way to mitigate tall fescue toxicosis if non-toxic rations are not feasible.