ESTRUS DETECTION, FIRST SERVICE CONCEPTION, AND EMBRYONIC DEATH IN BEEF HEIFERS SYNCHRONIZED WITH MGA AND PROSTAGLANDIN

In April, 1996, 1501 yearling crossbred heifers located on seven different ranches were estrus-sync h ronized and artificially inseminated (AI) 12 hours after th ey were detected in estrus. Herd size ranged from 82 to 43 9 head. Of the 1501 heifers, 86.1% were detected in estrus. First service conception rates of those 1292 heifers averaged 58.4% (40.3 to 68.8%). In three herds, ultrasonography was used to diagnose 525 heifers as pregnant a t 30 days after AI. At 60 to 90 days after the breeding season, palpation of the uterus confirmed that embryonic death had occurred in 4.2% (4.0 to 4.8%). First service conception rates varied widely among ranch es. The variation might have been due to factors such as climate, average daily gain, body condition, A I technician, and AI sire. A small percentage of embryos died after the 30-day ultraso und exam regardless of the ranch or management system.


Introduction
Proper management of replacement heifers is critical t o their future production and longevity.Many pro ducers utilize estrus-synchronization systems and AI to increase the proportion of replacement heifers that conceive earlier in the breeding season.Consequently, these femal es produce their first calf early in the calving season and tend to continue to calve earlier throughout their productive life.
Fetal aging by rectal palpation, subsequent estrus activity, or calving dates are used to deter mine whether the calf was sired by an AI or cleanup bull.Although these methods are useful, they very often produce inaccurate conception or pregnancy rates.
Ultrasonograph y can be used to determine the presence of a viable embryo as early as 28 days of pregnancy.In addi t ion, ultrasonography can be used for ovarian and uterine scans and fetal sexing.This technology also can be used to accurately determine conception and pregnancy rates and evaluate the viability of estrus synchronizatio n and AI protocols.Our objectives were to determine the difference in first service conc eption rates among ranches using a common estrus-synch r onization protocol and to estimate t he incidence of embryonic death after a viable embryo had been d etected at 30 days of pregnancy.

Experimental Procedures
In April, 1996, 1501 yearling crossbred heifers from seven ranches in Kansas and Missouri were synchronize d using a common estrus synchronizatio n and AI program.Herd size ranged from 82 to 439 and averaged 214 head.Estrus was synchronized by feeding MGA (.5 mg per head per day for 14 days) and then giving a prostaglandin F injection 17 days 2" after MGA withdrawal.Heifers were observed for signs of estrus and insem i nated artificially by an experienced technician 12 to 16 hours after the first detected standing heat.At 29 to 33 days after AI, ultrasonography was used to establish the presenc e of a viable embryo and to determine first service conception rates.Incidence of subsequent embryonic death was monitored in three herds.A t 60 to 90 days after the end of the breedin g season, pregnancy was reconfirme d by uterine palpation in the 525 heifers that were earlier diagnosed pregnant to AI by ultrasonography.

Results and Discussion
Table 1 shows the variability in detection of estrus and first service conception rates among ranches.Of the 1501 heifers initially synchronized, 1292 (86.1%) were detected in estrus within 72 hours and ha d first service conception rates of 40.3 to 68.8%, with an average of 58.4% .The remaining 209 heifers received a fixed-time AI and(or) were exposed to cleanup bulls.A primary goal of an estrus-synchronizatio n system is to maximize the number of replacement heifers bred to proven AI sires within the firs t few days of the breedin g season.Thus, calves ar e born earlier and have the potential for greater growth and heavier weaning weights.
The variability in first service conception rate among ranches could have been a result of such factors as average daily gain, body condition score, climate, AI technician, and AI sire.
Emb ryonic death was measured in three herds (A, D, G; Table 1).Of the 525 heifers diagno sed pregnant at 29 to 33 days after insemination , 4.2% (4.0 to 4.8%) of the heifers did not have via ble embryos at palpation.Most reports suggest a 5 to 25% embryonic loss between 30 and 60 days after insemination.Therefore, our observations support the notion that, regardless of the management system, a small percentage of embryo s die.Ultrasonography can monitor the success of a breeding progra m and can accurately determine first service conception rates and embryonic death.This technology can enhance the ability of cowcalf producers to make decisions that impact profitability and efficiency of their operation.

Table 1 . Estrus Detection, First-Service Conception, and Embryonic Death of Beef Heifers Synchronized with MGA and Prostaglandin
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