Pregnancy rates in MGA-prostaglandin synchronized heifers bred Pregnancy rates in MGA-prostaglandin synchronized heifers bred at estrus or inseminated at fixed times at estrus or inseminated at fixed times

The objective of this project was to study the effects on pregnancy rates of inseminating estrus-synchronize d heifers either at observed estrus or at a fixed time. In April, 1994, 574 yearling crossbred heifers, located on six Kansas ranches, were inseminated to achieve this objective. Her d size ranged from 38 to 293 head. The heifers were synchronized with the MGA-prostaglandi n (PGF) system. Heifers were either inseminated 12 hr after the onset of estrus or, if not showing es trus, 72 hr after PGF. Pregnancy rates of 491 heifers bred on estrus averaged 56.6% (39.2 to 80.4%). Pregnancy rates for the 83 fixed-time-inseminated heifers averaged 39.8% (15.6 to 56.5%) . Heifers that responde d to the MGA-PGF synchronization system with a standing heat had higher pregnancy rates than those that were inseminate d at a fixed time. However, fixed-time inseminations used in conjunction with inseminations made at estrus increased the total number of heifers bred to proven AI sires.


Summary
The objective of this project was to study the effects on pregnancy rates of inseminating estrus-synchronize d heifers either at observed estrus or at a fixed time.In April, 1994, 574 yearling crossbred heifers, located on six Kansas ranches, were inseminated to achieve this objective.Her d size ranged from 38 to 293 head.The heifers were synchronized with the MGA-prostaglandi n (PGF) system.Heifers were eit her inseminated 12 hr after the onset of estrus or, if not showing es t rus, 72 hr after PGF.Pregnanc y rates of 491 heifers bred on estrus averaged 56.6% (39.2 to 80.4%).Pregnancy rates for the 83 fixed-time-inseminated heifers average d 39.8% (15.6 to 56.5%) .Heifers that responde d to the MGA-PGF synchronization system with a standing heat had higher pregnancy rates than those that were inseminate d at a fixed time.However, fixedtime inseminations used in conjunction with inseminations made at estrus increased the total number of heifers bred to proven AI sires.

Introduction
Proper management of replacement heifers is essential, because they represent the future profitabilit y of the herd.Utilizing estrus synchro nization and artificial insemination (AI) can increase the prop ortion of heifers bred early in their firs t breeding season and, consequently, increas e their lifetime productivity.Although several synchronization pr ograms and insemination regi mens are available, a common practice utilizes estrus detection, with inseminations based on the AM-PM rule (i.e., a female first showing estrus in the morning is inseminated that evening).
However, fixed-time insemination s also can be used, when all female s are inseminated at a specific time in a synchroniz ation program, whether or not estrus has been detected.This method reduces labor, but pregnancy rates are often variable.Our objective was to evaluate the effects of inseminatin g estrus-synchronized heifers at observe d estrus plus a fixed-time insemination for those not detected in estrus, in order to maximiz e the total number of pregnancies achieved with AI.

Experimental Procedures
In April, 1994, yearling cro ssbred heifers on six Kansas ranches were used in an estrous synchronizatio n and artificial insemination program .Herd size ranged from 38 to 293 head and averaged 96 head.Each herd was assigne d two or three experienced AI technicians befo re the breeding season.Heifers were synchroni zed with an MGA-prostaglandin (PGF) system in which animals were fed MGA at .5 mg/head/day for 14 day s , then given a PGF injectio n 17 days after MGA withdrawal.Heifers were observed for signs of estrus and were inseminat ed artificially 12 hr after the first detected standing heat.Animals showing no signs of estrus were inseminated 72 hr after PGF.All females were pregnancy tested 30 days after insemination by real-time ultrasonography, with a 7.5 MHz intrarectal probe.
At location A (Table 1), 293 heifers were assigned randomly to receive the prostaglandin injection on eithe r day 16 (PM) or day 17 (AM) after MGA withdrawal.Heifers then were evaluated for response t o synchronization, onset of estrus, first-service pregnancy rate, and total pregnancy rate.

Results and Discussion
Table 1 shows the rates of detected estrus and first-service pregnancy rates at each location.Based upon detection of estrus, 85.5% of the 491 heifers responded to synchronization.The first-service pregnancy rates of those heifers ranged from 39.2 to 80.4%, with an average of 56.6%.The remaining 83 heifers, either not responding to the estrous synchronization program or not d e tected in heat, were inseminated at 72 hr after PGF.Pregnancy rates range d from 15.6 to 56.5%, with an average of 39.8%.These data suggest that heifers responding to the MGA-PGF synchronizatio n system with a standing heat have higher pregnancy rates than those that are inseminate d at a fixed-time.However, fixedtime AI used in conjunction with inseminations made after detection of estrus can increase overall pregnancy rates.A primary goal of a synchronization-A I program for replacement heifers is to maximize the number of pregnancie s to proven AI sires in the first 21 days of the bre eding season.This allows heifers to calve earlier in the calving season, breedback earlier in the following breeding season, and wean heavier calves in the fall.Figure 1 illustrate s the percentage of heifers detecte d in estrus at location A after receiving injections of PGF on either day 16 or 17 after MGA withdrawa l .Of the 131 heifers receiving a day-16 inject ion, 81.7% were detected in heat before 72 hr, and first-service pregnancy rate was 62.6%, whereas that of the 24 remaining time-inseminate d heifers was 12.5%.Of the 137 heifer s receiving a day-17 injection, 95.6% were detected in heat by 72 hr, and first-service pregnanc y rate was 56.5%, whereas that of the 6 time-inseminated heifers was 33%.At the completion of a 60-day breeding season with clean-u p bulls, overall pregnancy rates were 96.2 and 94.2% for day-16 and -17 prostaglandi n injections, respectively.These data suggest that heifer s may respond better to estrous synchronization wh e n injected with PGF 17 days following MG A withdrawal.However, overall first-service pregnancy rate (estrus and time-inseminated heifers) and total pregnancy rate were similar between injection times.