Pregnancy rates in virgin heifers and suckled beef cows after Pregnancy rates in virgin heifers and suckled beef cows after synchronized ovulation using PGF2", norgestomet, and GnRH synchronized ovulation using PGF2", norgestomet, and GnRH

One disadvantage of most estrous synchronization programs is their i nability to induce cycling in prepubertal heifers and anestrous suckled beef cows. Suckled cows and virgin heifers were treated with PG F , norgestomet, 2 " and GnRH t o induce ovarian cyclicity in prepubertal heifers and anestrous suckled cows as well as to synchro nize estrus in cycling females. The treatment consisted of two injections of PGF (day –14 and 0) plus 100 µg of GnRH 2 " and a 6 mg norgestomet implant on day –7. The implants were removed 24 h after the second injection of PGF (day 0). An injection 2 " of 100 µg of GnRH was given 30 hours after implant removal. The treatment group was inseminated at estrus or 18 hours after the second injection of GnRH. Pregnancy rate in the treated female s was higher than in the controls that had r eceived PGF injections 14 days 2 " apart and were bred a t estrus (65.1 vs 48.1%). The treatment also succe ssfully induced a fertile ovulation in previously prepubertal heifers and anestrou s cows (treatment vs. control; 67.7 vs 20.0% ). We conclude that treating beef cattle with PGF , norgestomet, and GnRH induced 2 " ovarian cyclicity and increased pregnancy rates in prepubertal heifers, anestrus cows, and cycling females.


PREGNANCY RATES IN VIRGIN HEIFERS AND SUCKLED BEEF COWS AFTER SYNCHRONIZED OVULATION USING PGF , NORGESTOMET, AND GnRH
2" "

Summary
One disadvantage of most estrous synchronization programs is their i n ability to induce cycling in prepubertal heifers and anestrous suckle d beef cows.Suckled cows and virgin heifers were treated with PG F , norgestomet, 2" and GnRH t o induce ovarian cyclicity in prepubertal heifers and anestrous suckled cows as well as to synchro nize estrus in cycling females.The treatment consisted of two injections of PGF (day -14 and 0) plus 100 µg of GnRH 2" and a 6 mg norgestomet implant on day -7.The implants were removed 24 h after the second injection of PGF (day 0).An injection 2" of 100 µg of GnRH was given 30 hours after implant removal.The treatment group was inseminate d at estrus or 18 hours after the second injection of GnRH.Pregnancy rate in the treated female s was higher than in the controls that had r eceived PGF injections 14 days 2" apart and were bred a t estrus (65.1 vs 48.1%).The treatment also succe ssfully induced a fertile ovulatio n in previously prepubertal heifers and anestrou s cows (treatment vs. control; 67.7 vs 20.0% ).We conclude that treating beef cattle with PGF , norgestomet, and GnRH induced

Introduction
Estrou s synchronization improves reproductive efficiency by reducing the length of the breeding an d calving season and increasing calf weaning weights, because cows calve earlier.However , most estrous synchronization programs do not induce cycling in prepubertal heifers and anestrous suckled beef cows.Treatments that involv e single or multiple injec-tions of gonadotropin-releasing hormone (GnRH) given at 10-to 1 2-day intervals and(or) implant s of norgestomet have been used to "jump-start " these acyclic females.The effect of GnRH is to induc e LH and FSH release and ovulatio n of follicles that are of preovulatory size and function.The effect o f the norgestomet is to prime the hypothalamic-pituitary axis for the release of endogenous GnRH, LH, and FSH necessary for follicular growth.In both prepubertal heifers and anestrous suckled cows, the norgestome t implant prevents the short luteal phase that follows the first pubertal or postpartum ovulation.That short luteal phase prevents the continuation of pregnancy, even if fertilization occurs.Therefore , our objective was to determine the effect of a treatment consisting of PG F , 2" norgestomet , and GnRH on inducing ovarian cyclicity in prepubertal heifers and suckled cows, as well to synchronize estrus in cycling females before one fixed-time insemination.

Experimental Procedures
Purebred Angus, Hereford, and Simmental heifers and suckled cows were assigned to two treatments : 1) two injection s of PGF 14 days 2" apart (control); or 2) two injections of PGF 2" (days 0 and 14) plus 100 µg of GnRH and a 6 mg norgestomet implanted on day -7 (Figure 1).The implants were removed 24 h after the secon d injection of PGF (day 0).A second 2" injection of 100 µg of GnRH was given 30 h after implant removal.Three blood samples were collected (-21, -14, and -7 days) before the second PGF injection to determine cycling 2" status.Contr ol females were inseminated 12 to 16 h (AM-PM rule) after first detected estrus until 80 h after the second PGF injection, 2" when all remaining females were inseminated.The females in the treatment group were inseminate d either at estrus or a t 18 h after the second injection of GnRH (48 h after implant remova l or 72 h after the second PGF 2" injection).Pregnancy status was determined at day 34 to 35 postservice by intrarectal ultrasonography.

Results and Discussion
Pregnancy rate was greater (P<.05) in the treated females than in the controls (65.1 vs 48.1%).No differences in pre g nancy rates were detected among breeds or parity groups (heifers, primiparous, and multiparous cows).
An interaction (P<.01) occurred betwee n treatment and insemination type.Control and treated females inseminated at estrus had similar pregnancy rates (61.5 vs 62.8%), whereas control and treated females inseminate d at a fixed time were markedly differen t (7.7 vs 66.7%, respectively).An interaction also occurre d between treatment and cycling status (P<.05).
The treatment successfull y induced a fertile ovulation in previousl y prepubertal heifers and anestrous cows (Table 1).Furthermore, the treatment numerically increased pregnancy rates in all cycling females in each of the parity groups.We conclude that treating beef cattle with PGF , norgestomet, and GnRH induced the second inje ction of PGF when 6 mg of norgestomet was implanted.Implants were removed 24 h after the 2" second inj ection of PG F .An injection of 100 µg of GnRH was administered 30 h after implant removal .When 2" 2 concentration of pro gesterone in serum on days -21, -14, and/or -7 (second injection of PG F ) exceeded 1 ng/mL, 2" estrous cycles were assumed to have been established; otherwise, heifers were defined to be prepubertal and cows to be anestrous .Based on ultrasonically determined presence of uterine fluid and embryo on day 34 or 35