TAKINGTHERELAXEDVIEWBOOSTSAI PERFORMANCE
TAKINGTHERELAXEDVIEWBOOSTSAI PERFORMANCE
Scientists are closer to
developing successful
cervical AI techniques using
frozen semen in flocks.
Emma Penny reports
FOR flockmasters, use of laprascopic AI is a necessity if they are part of a sire reference scheme, and for top breeders, its a way of using semen from top rams in their flocks.
But there is controversy over the technique – the 1995 Banner committee report on use of advanced reproductive techniques expressed its concern over animal welfare issues, and put the onus on scientists and the sheep industry to come up with welfare-friendly AI and embryo transfer techniques.
Thats what has prompted work at SAC Aberdeen by sheep and reproduction specialist John Robinson.
"In cattle, semen can be deposited in the uterus. However, its not so easy in sheep; the cervix is very tight, and can only be penetrated in perhaps one in a hundred ewes. However, we do know that where we can get a pipette containing semen through the cervix, it increased the rate of conception dramatically."
According to Prof Robinson, the key to progressing cervical AI lies in understanding natural cervical function. "The cervix is an active organ, stimulated by hormones. We know that it changes in the presence of a ram, the outer lip opening and closing, so drawing semen in – that obviously doesnt happen with AI."
Research using tiny cameras inside ewes and behavioural observation also shows that there is an enormous variation in ewe reaction after service.
"Cervical AI is impeded by the tight cervix. However, we know that when ewes are 125-130 days pregnant it is possible to pass something with a diameter almost as wide as a biro pen through the cervix – it is more relaxed, but perfectly intact. The relaxation is due to the oestrodial rise – and thats what we need to simulate for cervical AI to be successful."
Trails have found that it is possible to imitate cervical relaxation using oestrodial-type hormones, but the difficulty with this treatment is that ewes will not superovulate, and their reproductive system shuts down. "This delays ovulation and means conception is difficult," explains Prof Robinson.
Work is now focusing on a battery of possible relaxation agents, selected from those involved in preparing both animals and humans for birth and substances which have the ability to soften collagen. "The relaxation process increases blood flow to the cervix. This initiates an inflammatory-type reaction followed by the release of enzymes which soften the collagen tissue of the cervix.
"We found one agent which allowed better cervical penetration when applied round the cervix, but it hasnt increased conception rates so we are continuing our search."
However, work has shown that best cervical penetration is achieved using a pipette with an offset centre angled tip, which seems to be able to traverse the cervix better than a straight tip.
Another vital part of developing cervical AI is to allow use of frozen semen. Currently, only fresh semen is used with cervical AI, and its limited shelf life means that is rather impractical, particularly where a ram is in widespread use – perhaps as part of a sire reference scheme.
"We know that freezing semen changes it. When a sperm fertilises an egg it has to capacitate, which means its membranes change, and once that has happened, sperm has only a limited life. Freezing also capacitates sperm – giving it a finite life once it has been thawed."
This explains why AI with fresh sperm is less time-critical than with frozen semen. Fresh semen still has to go through capacitation so it has a longer survival time, and any delay in getting through the cervix is less vital than where frozen semen is used.
Identifying a relaxation agent which would allow easier penetration of the cervix means that frozen sperm could be used.
"If we could find a relaxation agent which affected the cervix and could place semen into the womb we would increase cervical AI conception using frozen semen to 70% – but we need to make that consistent for the technique to be commercially acceptable."
CERVICAL AI PROGRESS
• Identify relaxation agent.
• Use of frozen semen.
• Aim for 70-75% conception.