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Characteristics of estrus before and after first insemination and fertility of heifers after synchronized estrus using GnRH, PGF₂, and progesterone¹

A. M. Richardson^*, B. A. Hensley^*, T. J. Marple^*, S. K. Johnson and J. S. Stevenson^2,*

^* Department of Animal Sciences and Industry, Kansas State University, Manhattan 66506-0201, and and Northwest Research and Extension Center, Kansas State University, Colby 67701-0786

² Correspondence:
Weber Hall 254 (phone: 785-532-1243; fax: 785-532-7059; E-mail:
jstevens{at}oznet.ksu.edu).

Our objectives were to determine fertility of heifers after synchronization of estrus using PGF₂, preceded by progesterone (P4), GnRH, or both, and to examine the variability of estrual characteristics in heifers before first and second AI. Dairy (n = 247) and beef (n = 193) heifers were assigned randomly to each of three treatments: 1) 50 µg of GnRH (injected i.m.) administered on d -7 followed by 25 mg of PGF₂ (i.m.) on d -1 (GnRH + PGF; modified Select Synch protocol); 2) placement of an intravaginal progesterone (P4)-releasing insert on d -7, PGF₂ on d -1, and insert removal on d 0 (P4+PGF); and 3) 50 µg of GnRH plus a P4 insert on d -7, followed by 25 mg of PGF₂ on d -1, and insert removal on d 0 (P4+GnRH+PGF). Characteristics of estrus were examined before first AI and before the next eligible AI (18 to 26 d later), including duration of estrus, number of standing events, and total and individual duration of standing events. In addition, all heifers were checked visually at least twice daily for estrus. Blood samples were collected on d -7, -1, and 0 for determination of P4, and pregnancy status was diagnosed by ultrasonography 27 to 34 d after AI. Rates of detected estrus were less (P < 0.05) in dairy than in beef heifers, and greater (P < 0.05) in heifers treated with P4. Pattern of conception and pregnancy rates among treatments differed between beef and dairy heifers (treatment x group interaction; P < 0.05). In dairy heifers, conception and pregnancy rates were greatest with P4+PGF, followed by P4+GnRH+PGF and GnRH+PGF, respectively. The opposite was observed among treatments in beef heifers. Administration of P4 without the preceding injection of GnRH produced the lowest pregnancy rates in beef heifers. Of the quantified sexual behavioral characteristics during the synchronized estrus, the number of standing events and total duration of standing events were greater (P < 0.01) than those observed during the next eligible estrus before second AI, whereas duration of estrus was unaffected.

Key Words: Beef • Cows • Dairy Heifers • Estrus • Fertility • Progesterone

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