Sexed semen Fixed Time AI Trial 2015–2016 Results

By Dr Jon Kelly


In 2015/16 WVC conducted a trial on 19 herds and 1780 dairy heifers to determine if conception rates using frozen sexed semen could be improved by altering the timing of AI, when a 12 day Fixed Time AI program was used. The reason the trial was conducted was because in a previous project by WVC, fertility using frozen sexed semen was improved when AI was delayed more than 50hrs post CIDR pull. Also, the sexed semen processors have proposed that fertility using sexed semen may be increased by timing AI closer to ovulation than what is usually necessary with conventional(non-sexed) frozen semen.


To investigate if this effect could be achieved in Dairy heifers, the following trial protocol was followed, with age, weight, Body Condition Score(BCS),heat at the time of AI as well as timing of AI assessed for their effect on fertility.


The key to the trial was altering the time of AI on Day 12.

This was done by having half of the heifers AI’ed at approximately 48hrs post CIDR pull(Day10), while the other half of the heifers were AI’ed 3 hrs later. If more than 1 sire was used, then the sires were also equally distributed between the early and late groups. Timing of AI was deliberately instructed to begin “approximately” 48 hrs from CIDR pull as this would ensure that insemination would occur over a wide range of times. This was achieved with AI occurring from between 47 – 53 hours post CIDR pull (Day 10). Logistically, this means AI was done between morning and afternoon milking!



The average conception rate across all heifers in the trial was 45%.

Body Condition Score ranged from 3.75 – 5.5(1-8 dairy scale) had no effect on fertility.

Age ranged from 12 -16 months, and also had no effect on fertility.

Signs of heat showed a huge improvement in fertility with those heifers conception rate 50% compared to 18% to those who showed no signs of heat.

Critically, the weight of the heifer, and breed (with X-bred’s more fertile!), contributed significantly to those heifers that showed heat at the time of AI. The bigger the heifer was, the more likely she was to show heat, and thus more likely to get in calf to frozen sexed semen.


Finally, was fertility increased when we delayed the timing of AI?

AI on day 12 was done from between 47-53 hours post CIDR pull on Day 10.

If CIDR pull on day 10 was done at 11am, then this means AI occurred between 10am and 3pm.

We found no difference at all in conception rates between these times!

So, while disappointing on one hand that we could not improve the fertility of frozen sexed semen by simply changing the timing of AI, the trial result reinforces that the WVC 12 day FTAI program is extremely robust and highly flexible. As long as AI occur’s sometime between morning and afternoon milking you will not compromise your result with frozen sexed semen.

Importantly, the most significant factor that effects fertility in heifers is their body weight at joining.

Achieving critical body weights is directly in your control. It has far longer lasting benefits for production and longevity in the herd, than simply achieving better fertility at joining.

Thank you to all the farms involved, Phil Keegan, Zoetis, Sexing Technology, Swans vets(BVDV testing) and the staff at Warrnambool Veterinary.