Timing is everything

When is the perfect time to inseminate a cow after she shows signs of heat? Judith Roelofs, lecturer in fertility and reproduction at HAS University of Applied Sciences Den Bosch, has been carrying out a study to answer that question and knows only too well that timing is everything when it comes to AI success.

“For successful fertilisation, semen and oocyte must meet at the right time,” she says. “There must be viable semen cells at the entrance to the oviduct when ovulation takes place.”

To determine this exact moment, insight into both the fertility cycle of cows and the life cycle of semen are key.

As part of her PhD research at Wageningen UR, Dr Roelofs looked closely at the heat behaviour of cows. Oestrus length and the extent to which animals show it varies significantly from cow to cow. On average, cows are in heat for about 11 hours and during that period they show behaviours including chin resting and sniffing an average of 25 times, mounting six times and actual standing heats just four times. And the period in which cows show standing heat lasts for not longer than about five hours.

“This means standing heats are often missed in practice. Producers have a lot more to do besides watching cows day and night.”

Ovulation occurs about 25 hours after the start of standing heat, and around 31 hours after the first signs of heat. This oocyte remains viable for about six hours after ovulation. So during this six-hour period there has to be viable semen in the oviduct for fertilisation to occur.

In order to be able to calculate the optimum time to inseminate, knowledge about the life cycle of semen is also needed. “Semen is not yet ready for fertilisation at the time of insemination,” explains Dr Roelofs. “It must first undergo a maturation process in the cow, which takes between six and eight hours. After that, the semen cells are still fit enough for about 24 hours to be able to fertilise an oocyte. One insemination introduces millions of semen cells so if some of die there are always enough left to do the job.”

Semen cells have a considerably longer lifespan than oocytes, which in practice means producers rarely tend to AI cows too early. More typically they’re serving cows too late, according to Dr Roelofs. Based on this, she advises to inseminate as soon as possible after standing heat has been observed. “It is quite possible oestrus is in the latter stages when producers first spot the signs of a cow bulling. Waiting too long to AI after spotting oestrus will be too late.”

Producers who do not wait until they see standing heat before AI can use the rule of thumb that the optimal time to AI is about six hours after observing the first signs of heat.

Figure 1 shows optimal Ai times for conventional and sexed semen. “Producers using heat detection sensors will also have an advantage here. Most systems will flag up the optimal AI timing,” adds Dr Roelofs.

Good timing is even more critical to success when using sexed semen. “Semen cells that have undergone the sexing process have a shorter lifespan,” Dr Roelofs explains. “We don't yet know the exact lifespan or viability of sexed semen. But if we assume that the lifespan is 12 instead of 24 hours, then the optimal timing for AI with sexed semen is about 12 hours later than for conventional semen. In practice, this means that it is best to wait a little longer to inseminate – about 12 hours after observing standing heat and about 18 hours after the signs of oestrus are seen.”