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Single nucleotide polymorphisms in the XKR4 and DRD2 genes influence adjusted birth and 205-day weights of calves grazing endophyte-infected tall fescue
Tall fescue (L. arundinaceum Schreb.) is the most prevalent forage in the Southeastern United States due to the presence of the endophytic fungus N. coenophialum. The fungus enhances the persistence of tall fescue, but decreases the productivity of cow-calf herds grazing it. Single nucleotide polymorphisms in the XK, Kell blood group complex subunit-related family, member 4 (XKR4) and Dopamine Receptor D2 (DRD2) genes both yield the genotypes AA, AG, and GG. The A allele of both XKR4 and DRD2 has shown to increase serum prolactin concentrations in Tennessee beef cattle herds grazing endophyte-infected tall fescue. We evaluated the relationship between genotypes of dam and calf and adjusted 205-day weight (A205) and adjusted birth weight (ABW) in a well-managed fall-calving beef herd in Missouri. The ANOVA model included XKR4 and DRD2 genotype (SAS 9.3, Cary, NC). Genotype and allele frequencies for XKR4 were AA=0.67, AG=0.30, GG=0.04, A=0.82 and G=0.18 for the dam; AA=0.64, AG=0.34, GG=0.03, A=0.81 and G=0.19 for the calf. Since the G allele was sparsely represented in the population only AG and AA animals were used for the XKR4 analysis. Genotype and allele frequencies for DRD2 were AA=0.23, AG=0.46, GG=0.31, A=0.46 and G=0.54 for the dam; AA=0.25, AG=0.50, GG=0.25, A=0.50 and G=0.50 for the calf. Dam genotype for DRD2 influenced ABW such that calves from AA and GG animals had lower ABW than those from AG animals (p<0.0001). This is similar to previous findings in a larger herd of animals from Missouri. The AA genotype for the dam at XKR4 was associated with higher ABW when compared to AG dams (p=0.08). Calf genotype for XKR4 influenced A205 such that AA animals were heavier than AG (p=0.05). The AA genotype for the calf at XKR4 was associated with higher ABW when compared to AG calves (p=0.07). No association was observed between calf genotype for DRD2 on ABW or A205. Similarly, no association was observed between dam genotype for XKR4 and A205. Fall-calving dams are grazing fescue at its peak infection level while in mid-gestation; having a beneficial allele for ABW is important for healthy calf weights. As fall-born calves are grazing endophyte-infected tall fescue prior to weaning, calf genotype at these loci may influence A205. Taken together with previous work, these results indicate a potential for their use as genetic markers for increased productivity of beef cattle grazing endophyte-infected tall fescue.
Keywords: XKR4, DRD2, fescue toxicosis