Influence of endophyte consumption and heat stress on intravaginal temperatures, plasma lipid oxidation, blood selenium and glutathione redox of mononuclear cells in heifers grazing tall fescue

¹ Department of Large Animal Clinical Sciences, Virginia-Maryland Regional College of Veterinary Medicine, Blacksburg 24061-0442
² Department of Animal and Poultry Sciences, Virginia Polytechnic Institute and State University, Blacksburg 24061-0306
³ Department of Biomedical Sciences and Pathobiology, Virginia-Maryland Regional College of Veterinary Medicine, Blacksburg 24061-0442

^* To whom correspondence should be addressed. E-mail: cvmwss{at}vt.edu.

	Abstract

A grazing experiment was conducted to assess the effects of wild type endophyte-infected (E+) tall fescue consumption and elevated ambient temperatures on intravaginal temperatures, plasma lipid peroxidation, and glutathione redox of peripheral blood mononuclear cells. Angus heifers (n = 34) were allotted by BW to 4 blocks consisting of E+ and endophyte-free (E-) fescue pastures. Monthly, in June, July, and August, temperature loggers were fixed into blank controlled internal drug releasers (CIDR) and inserted into a subsample of heifers (n = 16) for 2 d. After 48 h, heifers were weighed, and blood (30 mL) was collected via jugular venipuncture. Peripheral blood mononuclear cells were isolated for analysis of glutathione peroxidase activity, glutathione reductase activity, and reduced:oxidized glutathione. Plasma malondialdehyde was evaluated as a marker of lipid peroxidation, and whole blood Se concentration was determined. Serum prolactin was assayed at the conclusion of the grazing period. Data were analyzed using repeated measures with pasture as the experimental unit, and block as a random effect. Heifer ADG was greatest in August, and least in July (P < 0.001). In August, heifers grazing E+ fescue exhibited greater (P < 0.05) afternoon intravaginal temperatures and temperature fluctuations than heifers grazing E- fescue. In July and August, all heifers had greater afternoon temperatures (P < 0.02) and less reduced:oxidized glutathione (P < 0.0001) than in June. Glutathione reductase activity of all heifers was greater in June (P = 0.03) than in July. Similarly, all heifers exhibited decreased glutathione peroxidase activity (P < 0.0008) in July, whereas whole blood Se was reduced (P < 0.0001) in July and August. No treatment or date effects were detected for malondialdehyde, but serum prolactin was reduced at the end of the grazing period (P = 0.008) in heifers stocked on E+ fescue. Using these markers, differences in oxidative stress were not detected between heifers consuming E+ fescue and those consuming E- fescue. Date effects indicating altered glutathione redox and enzyme activity may have been related to heat stress and nutritional limitations.

Key Words: Cattle, endophyte, fescue, glutathione, heat stress, oxidative stress