Association of DRD2 with Gr owth and Performance Traits in Cattle Grazing Endophyte-Infected Tall Fescue
Jennings, K., C. J. Kojima, S. Pratt, and C. Burnett.  2016.  Journal of Animal Science, Vol. 94 Suppl. 1.

Abstract:
Fescue Toxicosis (FT) negatively impacts growth and fertility of beef cattle grazing tall fescue infected with an endophytic fungus. The fungus produces chemical compounds including ergovaline that contribute to the heartiness of the plant but are detrimental to the animal. A single nucleotide polymorphism in the dopamine receptor D2 gene (DRD2) has been associat- ed with serum prolactin concentrations and hair coat scores in cattle grazing endophyte-infected tall fescue (E+); cattle with the GG genotype had decreased serum prolactin concentra- tions and increased hair coat scores. Separately, heifers with the AA genotype calved sooner than AG or GG heifers. In this study, DRD2 genotype-phenotype associations with growth and performance of bulls grazing either E+ or non-toxic en- dophyte-infected (NTE) tall fescue were examined. The bulls remained on treatment from late February through mid-June. Urine samples and semen samples were collected as well as body weights, scrotal circumferences, and body condition scores, in April and again in May. Genotypes for DRD2 were obtained from sperm pellets of each bull using a bovine Taq- man assay. Associations of DRD2 genotype with each pheno- type were evaluated using mixed model analysis of variance in SAS. Significance was noted for P < 0.05; trends where 0.10>P>0.05 were also noted. Main effects of treatment, month, and genotype were assessed as well as their interac- tions. Genotype was found to be associated with body weight such that AA bulls have a higher body weight (BW) than GG bulls. A direct effect of month as well as a month*treatment interaction was observed such that BW increased from April to May in both treatment groups but slightly more in the E+ rela- tive to NTE animals. An association between DRD2 genotype and scrotal circumference (SC) was noted such that AA bulls had greater SC than AG bulls but not GG bulls. No other ef- fects on SC were observed. As expected, urine concentrations of ergovaline were greater in E+ bulls relative to NTE bulls. Urine concentrations of ergovaline were also found to be as- sociated with DRD2 genotype; concentrations were almost two-fold greater in GG bulls than in either AA or AG bulls. A genotype*treatment interaction was observed such that the GG bulls grazing E+ fescue exhibited the greatest urine con- centrations of ergovaline. These findings further support the utilization of DRD2 genotype as a selection tool to enhance the performance of beef cattle grazing E+ tall fescue.