Hormonal control of sheep ovulation a possibility

Press, 26 November 1982, Page 20

Hormonal control of sheep ovulation a possibility

New Zealand had an estimated sheep population of 72 million in June 1982.

This represents a dramatic rise over the last two decades from 47.1 million sheep in 1960, but the rapid expansion in flock numbers has not been matched by a rise in reproductive efficiency.

Figures released by the Meat and Wool Boards’ Economic Service show that only once in the last 31 years (1980) has the national lambing percentage exceeded 100 per cent. In eight of the past 12 years it has been below 95 per cent.

Improvement in the overall lambing percentage appears most likely from the introduction of either better farm management practices or an increase in ewe fertility — producing more lambs from fewer ewes.

Their appear to be two options for increasing the over-all fertility of the national flock. The first — at present the source of much debate — involves the importation of more prolific breeds such as the Finnish Landrace or Booroola.

The second option is to eliminate the season when ewes are infertile. (Most breeds do not ovulate during spring and summer, which means that mating is impos-

sible from September to February). Scientists at Wallaceville Animal Research Centre have now found the main causes of this seasonal infertility and are looking at methods by which ovulation could be controlled.

This has exciting implications at a time of growing pressure for an increased yield of lambs per ewe. Dr Ken McNatty and a team of scientists have been researching the ovulation processes of Romney ewes. Dr McNatty says ewe fertility is controlled by a series of hormone releases. Seasonal infertility in sheep is due to inadequate production of a hormone from the pituitary gland, caused in turn by a reduced output of a hormone from the hypothalamus gland. He says the pituitary contains two key hormones — Luteinising Hormone (LH) and a follicle-stimulating hormone. Measurement of LH secretion from the pituitary has revealed differences in activity between the breeding season and the anoestrous (non-fertile) interval.

This has been confirmed by injections of LH to ewes out of season. Following the injections the ewes ovulated, showed normal oestrus be-

haviour, and some became pregnant. The hormone from the hypothalamus gland, which researchers found to regulate LH secretions in ewes is tf?e gonadotrophic hormonereleasing hormone (GnRH).

Dr McNatty says. the structure of GnRH has been isolated very recently. “It appears that the release rate of GnRH is partly controlled by another area of the brain which releases morphine-like compounds which partly anaesthise the hypothalamus. As the release of these compounds goes up, the GnRH release rate goes down,” he says. Dr McNatty has found that during spring and summer, release of GnRH seems to occur once every 3.5 hours rather than once every two hours as in the breeding season.

“By isolating GnRH and injecting it at the pulse frequency of two hours, we have been able to induce oestrus in ewes,” he says. The Wallaceville team is ow looking into ways of inducing oestrus in ewes at will. The problem is to find an efficient way of speeding up the GnRH-reieasing pulses of the hypothalamus

from once every 3.5 hours to once every two hours in the off season. Farmers in the Northern Hemisphere could have a big advantage when it comes to one of the possible solutions. Dr McNatty says light frequency appears to be an important factor, and the hypothalamus pulse rate can be regulated by light. Stock in the Northern Hemisphere are wintered inside, where it could be possible to manipulate the onset of oestrus by careful control of lighting. Although .New Zealand’s horticulture is not suited to this method, Dr McNatty is looking at another use of light.

“We have wondered whether it is possible to set the yearly rhythms at birth,” he says. “At present there is an experiment in progress at Wallaceville with light controlled rooms designed to mimic Northern Hemisphere conditions.”

Dr McNatty sees other possible solutions in two categories — those using drugs to bring on oestrus, and the alternatives, based on development of new pasture management regimes. ■ GnRH can be readily syn-

thesised in the laboratory and the quantity needed to restore normal fertility to seasonally anoestrous ewes is very small - about 0.2 mg per month. “The problem is not making it or even putting it into the sheep, but in delivering it in a pulsatile form,” Dr McNatty says. "The GnRH must be received in the pituitary gland in regular pulses equivalent to its usual delivery in oestrus ewes.” The need is therefore for a capsule which will release the GnRH in regular waves at the correct frequency. Dr McNatty does not see this as an impossible problem. “Finding some sort of pumping system which will release the GnRH in waves is quite complex, but definitely not insurmountable,” he says.

Another possible answer using drugs is to control the release of the compounds (endorphins) which produce the partial anaesthetising effect on the hypothalamus. A compound called “naloxone” which is given to morphine addicts could be used in a capsule for release at a slow and constant rate to suppress the endorphins and lower their anaesthetising ef-

fect on GnRH production. Dr McNatty describes this approach as being still in the realms of pure science, but it is another possiblity. It is possible that, oestrus could be induced out of season by pasture management. The Wallaceville research team has not yet begun to look in this direction, but it is one which Dr McNatty feels should be investigated.

“We know that the nutritive qualities of various pastures have some degree of control on the ovaries of sheep,” he says.

“There is a need for research into the effect on a sheep’s reproductive system of seasonal changes in the nutritive qualities of pastures.”

"Further research is also needed into the effects of ram introduction on ewe fertility. Scientists at the Whatawhata Hill Country Research Station have found that exposure of out-of-season ewes to a ram will sometimes induce ovulation. There are indications that the ram’s pheromones are the possible trigger, although ovulation in the ewes only lasts for a short while and pregnancies are rare.

Dr McNatty feels it may be possible to “distil” the ram pheromones into an aerosol spray. Combined with some form of drug treatment this could be a feasible alternative.

Scientists now understand how the ewe’s ovary reacts and are beginning to understand how the responses occur, Dr McNatty says. He says the research also has implications for cattle breeding.

“There is quite a long period after calving when beef cattle are infertile. This seems to be because of some interaction between the calf and the mother — cows in milk production without their calves may breed.” Scientists at the University of Nottingham have shown that injection of LH every two hours will bring cattle with calves back into season.

Dr McNatty says the Wallaceville team hope to offer a wide range of alternatives for inducement of ovulation in out-of-season ewes. He would like to be able to offer farmers a choice of management systems or drug-re-lease programmes. Whatever the final outcome, the prospects for an eventual increase in the fertility of the national flock appear extremely promising.