Does Genotype Play a Role in Resistance to Fescue Toxicosis in the Ovine?

The objective of this study was to evaluate the effect of genotype in ewes consuming ergot alkaloids found in endophyte-infected tall fescue seed. Genotype at the DRD2 gene has been associated with serum prolactin concentrations in beef cattle grazing E+ tall fescue. Genomic DNA samples were obtained from 32 Suffolk ewes and used to initially screen the ovine DRD2 gene for Single Nucleotide Polymorphisms (SNP). One SNP in exon I, g270a, was discovered and investigated further due to favorable allele and genotypic frequencies. Allele frequencies at the g270a SNP were 0.64 for G and 0.36 for A; genotypic frequencies were 0.47, 0.34, and 0.19 for GG, AG, and AA genotypes, respectively. A genotyping assay was designed, tested, and found to be 100 percent concordant with sequencing results. Pregnant ewes (n = 32) were fed one of two dietary treatments: endophyte-free tall fescue seed (E-; 0.0 µg ergovaline + ergovalinine/g) or endophyte-infected tall fescue seed (E+; 4.14 µg ergovaline + ergovalinine/g) during two time periods: d 35 – d 85 (T2) and d 86 – d 133 (T3) of gestation in a 2 x 2 factorial arrangement. Endophyte-infected seed was fed at a level of 1772 ug of ergovaline + ergovalinine/hd/d for E+ treatments and an equal weight of endophyte-free seed for E- treatments. All ewes were pair-fed across treatments to maintain equal DMI. Data were analyzed with dietary treatment (E+ or E-), time of gestation (T2 or T3), genotype (AA, AG, GG), and all interactions in the model. Significance was set at P < 0.05. Prolactin concentrations during gestation were higher for AA and AG than GG genotype regardless of dietary treatment or gestation time. Lamb fetal weight was reduced due to E+ exposure during T3 but did not differ by ewe genotype or any interactions with treatment or gestation time. A subsample (n = 16) of ewes were used to assess changes in cortisol and thyroid hormones (Th3 T3 and T4). Cortisol, T3, and T4 concentrations were elevated in AA genotype compared to GG. Dietary treatment, gestation time or interactions with genotype had no effect on cortisol, T3 or T4. Genotype at the ovine DRD2 SNP appears to be associated with circulating hormone concentrations in the ewe, regardless of dietary fescue treatment or stage of gestation.