Association of genotype at DRD2 with growth and reproductive performance traits in bulls grazing toxic endophyte-infected tall fescue

Fescue toxicosis can negatively impact growth and fertility of beef cattle grazing tall fescue infected with an endophytic fungus. The fungus produces ergot alkaloids that contribute to the hardiness of the plant, but can be detrimental to the animal if consumed. A single nucleotide polymorphism (SNP) in the dopamine receptor D2 gene (DRD2) has been associated with serum prolactin concentrations and hair coat scores in cattle grazing toxic endophyte-infected (TE) tall fescue; cattle with the GG genotype had decreased serum prolactin concentrations and increased hair coat scores. Separately, heifers with the AA genotype calved sooner than AG or GG heifers. The objective of this study was to examine the DRD2 SNP for genotype-phenotype associations in bulls grazing either TE tall fescue or non-toxic endophyte-infected (NTE) tall fescue. Bulls remained on their respective pasture treatments from February through mid-June. Statistical analysis focused solely on the months of April and May. Urine samples, semen samples, body weights (BW), scrotal circumferences (SC), and body condition scores were collected. Bulls were genotyped using a bovine Taqman genotyping assay (Applied Biosystems, Foster City, CA). Genotype at DRD2 was associated with weight gain such that GG bulls gained less weight than AA or AG bulls regardless of pasture type (P=0.0055). Bulls on TE tall fescue gained more weight than bulls on NTE tall fescue (P=0.0433). Genotype at DRD2 was associated with SC such that AA and GG bulls had a greater SC than AG bulls (P=0.0033). A trend was also observed between scrotal circumference and treatment (P=0.0768) with bulls grazing NTE tall fescue tending to have a smaller scrotal circumference than bulls grazing TE tall fescue. Urine ergot alkaloid concentrations were greater for bulls on TE tall fescue (P<0.001) and were also associated with DRD2 genotype; concentrations were almost two-fold greater in GG bulls than AA or AG bulls (P=0.0046). A genotype*treatment interaction was observed such that the GG bulls grazing TE tall fescue exhibited the greatest urine ergot alkaloid concentrations (P=0.0018). These findings further support the utilization of DRD2 genotype as a selection tool to enhance the performance of beef cattle grazing TE tall fescue.