Masters Theses

Date of Award

12-1987

Degree Type

Thesis

Degree Name

Master of Science

Major

Animal Science

Major Professor

Bert H. Erickson

Committee Members

J. B. McLaren, T. T. Chen

Abstract

A fungal endophyte, Acremonium coenophialum, associated with tall fescue is known to produce factors which are toxic to grazing livestock. Cattle which ingest the affected fescue may exhibit depressed physical condition, reduced reproductive performance as well as decreased pro- lactin (PRL) levels. Decreased PRL release has been shown to delay puberty in some species and may be the cause of reduced reproductive performance in animals grazing infested fescue. Four experiments were conducted on heifers grazing tall fescue pastures with varying degrees of infestation to determine the effect of treatment (fungal endophyte level) on the ability of the hypothalamus, pituitary and ovary to respond to hormonal challenges. An intramuscular injection of Gonadotropin releasing hormone (GnRH) analog was administered when the heifers were approximately 3 and 5 months of age. Jugular blood was collected at 0, 1, and 2 hours after the injection of GnRH and plama was assayed for gonadotropins, prolactin and 17β-estradiol (E2). An injection of 1 mg of 17β-estradiol benzoate at 7 months of age was administered to test the ability of the hypo-thalamus to produce a preovulatory-like LH surge. Samples were col-lected 14.5, 15.5 and 18.0 following the injection of estrogen. To test the effects of fungal-infested fescue on the growth and functional abil-ity of the ovarian follicle, a superfollicular response was induced with 1,000 lU of Pregnant Mare Serum Gonadotropin (PMSG) when the heifers were 9-months-old. Two 12 ml blood samples were collected 5 days post- injection of PMSG and heifers were ovariectomized immediately following the bleedings. Ovaries were quartered, sectioned (10 microns) and representative sections were evaluated for level of follicular develop-ment, germ-cell survival, follicular atresia, number of ovulations and signs of luteinization. GnRH, E2 and PMSG did not significantly affect PRL release (p > 0.10) however PRL levels were significantly less (p < 0.01) in the treatment groups as compared to Controls, except when ambient tempera-ture was high. Follicle-stimulating hormone (FSH) and luteinizing hormone (LH) were not significantly affected by treatment (p > 0.10); however, the LH levels of heifers in the infested fescue groups were lower than those of Controls in all experiments except for the E2 challenge. Those heifers which did respond to 17&beta-estradiol benzoate stimulation with LH levels greater than 5.0 ng/ml were either in the Control or 60% infestation group. Therefore, factors other than toxicants from the fungus must have delayed the maturtion of the hypothalamo-hypophysial axis. Germ- cell survival was adversely affected by the ingestion of fungal-infested fescue (p < 0.02) and an apparent dose-dependent relationship existed. Vesicular-follicle response to PMSG was also adversely affected in all infestation levels as compared to the Controls, however, the difference was not significant (p > 0.10). The results indicate that the reduction in PRL and/or LH levels in heifers grazing fungal-infested fescue may be responsible for decreased germ-cell survival and impaired follicular development and these factors would probably cause a reduction in repro-ductive performance.

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