Document Type

Thesis - University Access Only

Award Date

1994

Degree Name

Master of Science (MS)

Department / School

Animal Science

First Advisor

Herley L. Miller

Abstract

Two experiments using treatment of norgestomet alone or in conjunction with GnRH in prepuberal beef heifers was conducted. In experiment one, 143 crossbred beef heifers were randomly assigned to one of three treatments 97 to 120 d before their first breeding season. Average age of the heifers at onset of treatment was 306 d. Heifers in treatment one (controls) were not given any hormone treatment. Heifers in treatments two and three were given a 6.0 mg norgestomet implant for 9 d. Upon implant removal, heifers in treatment three also received GnRH in an implant or injection form. All heifers were bled weekly to monitor progesterone levels. Onset of puberty was assumed to be 5 d before the first serum progesterone concentration over 1 ng/mL, if followed by cyclic progesterone levels. Norgestomet treated heifers were 26.2 d younger (P = .0006) and 21.6 kg lighter (P = .0035) than controls at puberty. Norgestomet plus GnRH treated heifers were 30.9 d younger (P = .0001) and 18.8 kg lighter (P = .0098) than controls at puberty. The effects that hormonal treatment prepuberally had on subsequent pregnancy and calving rates were variable. In experiment two, twelve prepuberal heifers averaging 218 d of age were randomly assigned to the same treatments. On the day of norgestomet removal and GnRH injection, all heifers were catheterized with a jugular cannula. Blood samples were collected every .5 h for the first 24 h following catheterization and every 1 h for the next 24 h. Norgestomet treatment did not alter blood serum progesterone, estradiol or LH hormone profiles when compared to hormone concentrations found in controls. Norgestomet plus GnRH treatment did not alter serum progesterone or estradiol concentrations but did cause an LH peak to occur 2 to 2.5 h following injection. Duration of the peak lasted 3 to 5 h and in some heifers peak heights approached preovulatory levels. In conclusion, prepuberal treatment with norgestomet or norgestomet plus GnRH reduces (P < .001) the age at puberty in beef heifers. The mechanism by which the norgestomet implant causes this event does not appear to include an alteration of progesterone, estradiol or LH hormone profiles in the time immediately following implant removal. However, norgestomet plus GnAH treatment in the prepuberal heifer may cause earlier puberty through an alteration in the release of LH.

Library of Congress Subject Headings

Heifers -- Breeding
Heifers -- Physiology
Luteinizing hormone releasing hormone

Format

application/pdf

Publisher

South Dakota State University

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Rights Statement

In Copyright