In treatment one, cows that exhibited estrus had a 65% A.I. pregnancy rate, cows not exhibiting estrus at TAI had a 45% A.I. pregnancy rate. In treatment two, cows exhibiting estrus and were A.I. bred at 60 to 66 hours had a 63% A.I. pregnancy rate. Cows that were not in estrus, that underwent delayed breeding at 20 hours post GnRH, had a 50% A.I. pregnancy rate.
Once again the researchers used ESTROTECT patches as a way to evaluate if the cow was in heat at 58 hours post CIDR removal. The cows were broken into four subgroups; half of all cows both with patches rubbed off at 58 hours post CIDR removal and not rubbed off were inseminated. The remaining half of both subgroups was inseminated 18 hours later at 76 hours post CIDR removal.
Table 2. Split-Time A.I. using conventional semen among cows
Delayed TAI in cows that had not shown heat based on the ESTROTECT patch at 58 hours post CIDR removal resulted in a 6.1% numeric improvement to pregnancy rates (45.6% vs. 51.7%) compared to those inseminated at 58 hours.
COWS A.I. PREGNANCY RATE Treatment 1 (fixed-time A.I.)
Treatment 2 (split-time A.I.)
65% (220/339)
63% (214/338)
Estrous Non-Estrous
45% (60/133)
50% (70/141)
Total
59% (280/472)
59% (254/479)
Table 3. Split-Time A.I. using conventional semen among cows
COW A.I. PREGNANCY RATE
The P value of cows not in heat at 66 hours (treatment 1) compared to cows not in heat at 86 hours (treatment 2) was .26
The researchers saw a numeric advantage to delayed TAI for cows, however, not a statistical advantage. In addition, they did not observe an increase to overall pregnancy rates between the two protocols. Jordan Thomas explained that generally, the difference between two pregnancy rates must have a P value less than .05 to be considered statistically significant. Therefore, although there may appear to be a numeric advantage in delaying insemination of non-estrous cows, the data set does not offer a statistically significant improvement over a conventional Fixed-Time A.I. approach. At this time the University of Missouri is not recommending the use of Split‑Time Artificial Insemination in cows. Matthew Markwood and Dr. Jack Whittier from Colorado State University also wanted to study the effects of delayed breeding on mature cows. In the spring of 2013 they identified 1101 mature beef cows at five ranches in Colorado, Wyoming and Nebraska and synchronized them using a variation of the 7-Day CO-Synch + CIDR Protocol. Instead of TAI occurring at 60 to 66 hours and delayed breeding at 20 hours as it had in the University of Missouri trial, they performed TAI at 58 hours and delayed breeding at 18 hours for those not in heat. 7-day CO-Synch + CIDR® with TAI at 58 hours GnRH
GnRH
PG
AI
CIDR® 0
.. 58 hr .. 7
treatment day
10
Modified 7-Day CO-Synch + CIDR GnRH
Day 0
76 hours
Difference
Rubbed off
161/249 (63.7%) 157/252 (61.0%)
- 2.7%
Not rubbed off
145/308 (45.6%) 154/291 (51.7%)
+ 6.1%
The P Value of cows not in heat bred at 58 compared to cows not in heat bred at 76 hours was .13
While the data from Colorado State University showed a numeric advantage to delayed breeding in mature cows, it too, did not show a statistical advantage. However, Dr. Whittier’s group intends to further study the positive trend they observed with their protocol. More research is needed in cows to fine tune the protocol and find if a true statistical advantage does exist. Dr. Whittier points out that a 5% potential increase in pregnancy rates in non-estrus cows (2 to 3% overall increase) is worth some level of risk, and given that the downside risk is low, may be a management strategy many producers will consider this A.I. season. Dr. Whittier also noted that while pregnancy rates may be improved by using a delayed breeding strategy in mature cows, there are some other management considerations to be made because of the suckling calves. Dr. Whittier suggests that calves be separated from the cows when cows are worked through the chute for GnRH injection at 58 hours and then placed with the cows that were inseminated at that time. This allows for the second group of cows that are to be A.I. bred at 76 hours to be held separately until A.I. without the need to re-sort their calves. ESTROTECT patches do add some additional upfront costs, and delayed breeding causes need for some additional time and labor. However, the chance to increase conception rate on the first day of the breeding season can more than make up for expenses in the form of more A.I. calves and a heavier, more uniform calf crop! Sources:
PG
Thomas, J.M. 2013. Split-time artificial insemination: delayed insemination of non-estrous beef cows and heifers to optimize fertility in fixed-time artificial insemination protocols. MS Thesis. Univ. of Missouri, Columbia.
GnRH
CIDR ®
58 hours
AI
Whittier, J. and Markwood, M. 2013. Increasing timed-A.I. pregnancy rate in beef cows by delaying insemination in cows not showing heat by 58 hours after CIDR removal when using the 7-day CO-Synch + CIDR protocol.
.. 18 hr .. Day 7
58 hours
76 hours
67