A novel estrus-synchronization program for anestrous and A novel estrus-synchronization program for anestrous and cycling, suckled, beef cows cycling, suckled, beef cows

27% and pregnancy rates by 46% and 37%, respectively, over the control group, without altering conception rate. Both treatments increase d the estrus, conception, and pregnancy rates of noncycling cows, compared to controls.


Summary
We used four herds at three Kansas ranches to evaluate the potential of two new estrus synchronizatio n strategies to increase estrus expression and fertility of 911 c rossbred suckled beef cows.The treatments included: 1) 100 µg of GnRH and a 6-mg norgestomet ear implant on day -7 and 25 mg of PG F and implant 2" removal on day 0 (GnRH+NORG+PG F ); 2) 2" 100 µg of GnRH on day -7 and 25 mg of PGF 2" on day 0 (GnRH+PG F ); and 3 ) (control) 25-2" mg injections of PG F on days -14 and 0; and GnRH+PGF treatme nts increased (P<.01) 2" the overall percentages of cows detected in estrus b y 49% and 27% and pregnancy rates by 46% and 37%, respectively, over the control group , without altering conception rate.Both treatments increase d the estrus, conception, and pregnancy rates of noncycling cows, compared to controls.

Introduction
Estrus-synchronizatio n programs are popula r and profitable tools for improving the reproductiv e performance of cow herds.Synchroniza tion combined with artificial insemination (AI) improves overall reproductive efficiency by reducing the duration of the breeding and calv ing seasons and allowing increased use of AI sires with superi or genetic potential.Current synchronization programs are designed to synchroniz e estrus in cows that are already cycling a t the beginning of the breeding season.They are not intended to induce estrus in noncycling cows.Therefore, our objective was to test the effect of two new treatments to induce estrus and increase conception and pregnancy rates in an estrous suckled beef cows, as well as to synchro nize estrus in cycling cows.

Experimental Procedures
Four herds of predominantly crossbred cows (n= 911) at three locations were allotted randoml y to two treatments and one control (Figure 1): 1) 100 µg of GnRH and a 6-mg norgestomet ear implant on da y -7 and 25 mg of PGF and implant removal on day 0 PGF on days -14 and 0; (2×PG F ) (control).
2" 2" Three blood samples were collected (days -14, -7, and 0) before the last PGF injection to 2" determine estrus-cycling status.If any one of the three samples containe d $1 ng/ml serum progesterone, then the cows wer e assumed to be cycling.Cows were observed for estrus twice daily (4 hours each) for 144 hours after PG F .

2"
All cows wer e inseminated 12 to 14 hours after first detected standing estrus.Body condition score was assessed at the time of PGF 2" injection , and pregnancy was diagnosed by transrectal ultrasonography between 32 and 51 days after AI.Conception rate was defined as the proportion of cows detected in estrus and inseminate d during 144 hours afte r PGF that 2" becam e pregnant.Pregnancy rat e was defined as the proportion of treated cows that became pregnant.

Results and Discussion
Body condition scores ranged from 1 (thinnest ) to 6.5, with an average o f 4.5 on a 1 to 9 scale.In addition, days postpartum at the onset of the breeding season ranged from 21 to 108, with an overall average of 72 across all herds.The combination of somewhat lower body condition scores, fewer days postpartum, and the lack of spring pasture may have reduced estrus, conception, and pregnancy rates in herds 3 and 4.
The percentages of cows that exhibited standing estrus were greater (P<.05) in the two treatments than in controls.The GnRH +NORG+PGF treatment had 51% and the 2" GnRH+PGF treatment had 27% more cows 2" showin g heat than the control.Although the treatment s had no statistically significant effect on conception rate (Table 1), pregnancy rates were greate r (P<.05) in the two treatments than in the control.
Based on the three blood s amples, 54.8% of the females were cycling at the time of the PGF injection.Within the cycling cows, 2"