Year of Publication

2019

Degree Name

Doctor of Philosophy (PhD)

Document Type

Doctoral Dissertation

College

Agriculture, Food and Environment

Department

Animal and Food Sciences

First Advisor

Dr. James C. Matthews

Abstract

Consumption of endophyte-infected tall fescue results in a syndrome of negatively altered physiological systems, collectively known as fescue toxicosis. Another challenge to endophyte-infected tall fescue -based beef cattle operations is that the soils often are selenium (Se) poor, necessitating the need to provide supplemental Se. To test the general hypothesis that different forms of supplemental Se would ameliorate the negative effects of fescue toxicosis, predominately-Angus steers (BW = 183 ± 34 kg) were randomly selected from herds of fall-calving cows grazing an endophyte-infected tall fescue pasture and consuming vitamin-mineral mixes that contained 35 ppm Se as sodium selenite (ISe), SELPLEX (OSe), or an 1:1 blend of ISe and OSe (MIX). Steers were commonly weaned and depleted of Se for 98 d. Steers were assigned (n = 8 per treatment) to the same Se-form treatments upon which they were raised and subjected to summer-long common grazing of an endophyte-infected tall fescue pasture (0.51 ppm ergot alkaloids: ergovaline plus ergovalinine; 10.1 ha). Selenium treatments were administered by daily top-dressing 85 g of vitamin-mineral mix onto 0.23 kg soyhulls, using in-pasture Calan gates. The first project objective was to determine the effect of forms of supplemental Se on whole blood Se, serum prolactin, liver glutamine synthetase (GS) activity, carcass parameters, and growth performance (Experiment 1). In Experiment 1, whole blood Se increased for all treatments from day 0 to 22 and then did not change. Across periods, MIX and OSe steers had greater whole blood Se than ISe steer. Compared to ISe steers, MIX and OSe steers had more serum prolactin. Liver GS mRNA, protein content, and activity were greater in MIX and OSe steers than ISe steers. However, the ADG and carcass parameters were not affected by Se treatments. The second project objective was to determine the effect of forms of supplemental Se on serum clinical parameters of Experiment 1 steers (Experiment 2). In Experiment 2, across periods, MIX steers had more serum albumin than OSe, and ISe steers, respectively. Serum alkaline phosphatase (ALP) activity was greater in MIX and OSe steers. In addition, blood urea nitrogen (BUN), serum sodium, phosphorus, and magnesium concentration were affected by Se treatments. Partial correlation analysis revealed that serum albumin, BUN, and ALP activity were correlated with whole blood Se concentration. The third project objective was to evaluate the hepatic transcriptome profiles of Experiment 1 steers using microarray and targeted RT-PCR analyses (Experiment 3). In Experiment 3, bioinformatic analysis of microarray data indicated that hepatic glutamate/glutamine, proline, arginine, and citrulline metabolism was affected by different forms of supplemental Se. The mRNA expression of critical proteins involved in glutamate/glutamine (GLS2, GLUD1, GLUL), proline (PYCR1, ALDH18A1), and urea (ARG1, ARG2, OAT, NAGS, OTC, ORNT1) metabolism were differentially expressed by Se treatments. Collectively, we conclude that consumption of 3 mg Se/d as OSe or MIX forms of Se in vitamin-mineral mixes 1) increased whole blood Se content, an indicator of greater whole-body Se assimilation; 2) increased serum prolactin, albumin, and ALP, the reduction of which are hallmarks of fescue toxicosis; and 3) altered hepatic nitrogen metabolism, as indicated by changes in key enzymes of glutamate/glutamine, proline, and urea metabolism. However, 4) these positive effects on metabolic parameters were not accompanied by increased growth performance.

Digital Object Identifier (DOI)

https://doi.org/10.13023/etd.2019.028

Funding Information

This work is supported by a United States Department of Agriculture-Agricultural Research Service Cooperative Agreement (James C. Matthews) and by the National Institute of Food and Agriculture, U.S. Department of Agriculture, Hatch project No. 1010352.

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