Researchers study sire distribution of FTAI versus natural service

A group of Kansas State University (KSU) researchers, including KSU Professors Dr. Karol Fike and Dr. David Grieger, alongside Graduate Students Esther McCabe, Ashley Hartman and Devin Jacobs, recently conducted a study to determine the relative percentages of calves sired by either natural-service sires or fixed-time artificial insemination (FTAI) within the same estrous period.

“Use of FTAI followed by the immediate exposure of females to bulls for natural service can be a useful management strategy for commercial cow/calf producers to limit labor and time related to bull turnout and increase pregnancy rates earlier in the season,” the researchers note. 

“Considering influence of bull fertility, as well as the time to and length of estrus in females, the expectation for outcomes of natural-service sire versus artificial-insemination (AI) sire parentage is relatively unknown,” they continue. “Our objective was to determine the relative percentages of calves sired by either a natural-service or FTAI sire within the same estrous period.”

The study and results, titled “Sire Distribution of Calves in a Beef Herd with Use of Fixed Time Artificial Insemination Followed by Immediate Bull Exposure for Natural Service in Cows and Heifers,” was published in the Kansas Agricultural Experiment Station Research Report volume nine, issue one.  

The study

According to the research report, this study took place over the course of two consecutive years on a commercial Angus operation in Kansas using cows and heifers from a single producer.

 During this time, heifers and cows were synchronized and inseminated using the seven-day CO-Synch + controlled internal drug release (CIDR) FTAI protocol, in which heifers were AI’d 52 to 56 hours after the removal of the CIDR and prostaglandin injection and cows were AI’d 60 to 66 hours later. The same AI technician was used both years.

Immediately following insemination, all of the females were exposed to natural-service bulls for 90 days, heifers to one sire and cows to another. Both bulls passed a breeding soundness exam before they were exposed to females. 

After calving, DNA was collected from a random subset of calves born in the first 21 days of the calving season for parentage analysis. 

In year one, 59 calves were born from heifers in the study, and in year two, calves born from heifers totaled 82. Additionally, calves born from cows totaled 89 in the first year of the study, and during the second year, 102 calves were born from cows.

During the first year of the study, among calves born from heifers, the percentage sired by natural service was 5.1 percent, or three of 59. Among calves born from cows, the percentage sired by natural service was 14.6 percent, or 13 of 89. 

During the second year, the percentage of calves born from heifers and sired by natural service was 9.8 percent, or eight of 82, and the percentage of calves born from cows and sired by natural service was 20.6 percent, or 21 of 102.

Results and implications

“The percentage of calves born from natural-service sires in year one was less than in year two for both cows and heifers,” the researchers point out. “Although natural-service sires sired varying percentages of calves, it is unknown if those calves were additional pregnancies early in the breeding season or if other factors influenced fertilization and resulted in fewer AI-sired calves.”

They refer to previous studies which have shown pregnancy rates increased with FTAI along with immediate exposure to natural-service sires in heifers when compared to natural service alone and increased conception rates of AI compared to natural service in cows. 

“These studies, however, did not assess parentage of calves to determine if they were sired by an AI sire or a natural-service sire,” they noted.

Therefore, the researchers conclude if commercial producers use FTAI followed by immediate bull exposure in heifers, natural-service sires may sire five to 10 percent of the calves born early in the season. In cows, producers may expect 15 to 20 percent of calves born early in the season to be sired by natural service. 

“This data provides estimates of the parentage of calves from AI and natural-service sires with use of FTAI followed by immediate bull exposure,” they say. “This strategy can reduce time and additional steps related to bull turnout and increase pregnancies earlier in the breeding season.”

Hannah Bugas is the managing editor for the Wyoming Livestock Roundup. Send comments on this article to roundup@wylr.net.