Improving Beef Cattle Performance on Tall Fescue

The overall goal of the studies described in this dissertation was to improve beef production of cows grazing endophyte infected tall fescue either through management practices or through identifying markers for genetic selection. Experiment 1 investigated differences in spring and fall calving herds grazing endophyte infected tall fescue. This study determined that managing for a fall calving beef herd is the more productive and efficient system. This is due to increased reproductive efficiency as well as traditionally greater market prices at the time of weaning. A spring calving system will have faster growing calves, but the increased weight of the calves is not enough to offset the added value of more calves produced in the fall calving herd. Experiment 2 was a study to validate a single nucleotide polymorphism (SNP) as a potential genetic marker found on the DRD2 gene. Steers with an AA genotype at this SNP have been shown to have greater prolactin levels and this study indicated that cows which have the AA genotype will have their first calf an average of 23 days earlier than cows with the GG genotype. Also when allelic frequency was examined it was shown that spring calving cows had a shift in allelic frequency away from Hardy-Weinberg equilibrium towards the A allele. Experiment 3 used a genome wide association study (GWAS) to confirm the presence of other SNPs that may be used as markers for resistance to tall fescue toxicosis in beef cattle. Twenty four SNPs were identified with nine SNPs associated with birth weight and 15 associated with weaning weight. Some of the SNPs are found within genes associated with production and carcass traits such as average daily gain, acid detergent fiber intake, marbling, and fat thickness. The results of this study are very promising but more research needs to be completed. The SNPs that have been identified need to be validated.

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