DRD2 and XKR4: A genetic approach to fescue toxicosis.

Tall fescue (Lolium arundinaceum Schreb) is the most prevalent forage in the Mid-south United States due to its observed hearty nature and ability to withstand heavy grazing. This is due to a symbiotic relationship with an endophytic fungus (Neotyphodium coenophialum). This fungus produces ergot alkaloids, which can bind to many adrenergic, serotonergic, and dopaminergic receptors. One of the primary functions of dopamine is to inhibit prolactin response. Decreased growth, reproduction, lactation, and depressed serum prolactin concentrations are commonly observed in beef cattle grazing endophyte-infected tall fescue. These symptoms are collectively referred to as fescue toxicosis, and are estimated to cause over $600 million in losses to the beef industry annually. Polymorphisms in the Dopamine Receptor D2 (DRD2) and XK, Kell blood group complex subunit related family member 4 (XKR4) genes may modulate these responses. The objective of this study was to assess associations between each single nucleotide polymorphism (SNP) and serum prolactin concentration, hair coat score (HCS), body condition score (BCS), adjusted birth weight (ABW) and adjusted 205-day weight (A205) in fall calving beef cattle grazing endophyte infected tall fescue in Missouri. The results indicated that dam genotype for DRD2 and XKR4 combined were associated with serum prolactin concentrations (p = 0.04). Serum prolactin concentrations were greater in cows having 2 copies of the ‘A’ allele for DRD2 and at least 1 ‘A’ allele for XKR4. Dam genotype for DRD2 was associated with HCS and BCS (p = 0.0024 and p = 0.011, respectively). The association between dam genotype for DRD2 and hair coat score was similar to what has been seen previously. Dam genotype for DRD2 and XKR4 differed in ABW (p < 0.0001 and p = 0.027, respectively), such that dams with the ‘AG’ genotype for DRD2 and ‘GG’ genotype for XKR4 had increased ABW. Calf genotype for XKR4 was associated with A205 (p = 0.011); calves with the ‘AA’ genotype had increased A205 compared to calves with the ‘AG’ or ‘GG’ genotype. These results suggest that the use of these genetic markers could improve cow-calf production in beef cattle grazing endophyte-infected tall fescue.