Following this publicity, a family of Merino breeders (the Seears brothers of 'Booroola'), who had been selecting for multiple births for some years, offered CSIRO a Merino ram born alive in a litter of five. The offer was accepted and a dozen ewes born as triplets or quadruplets were purchased. Next year another quintuplet ram was donated, as was a sextuplet ewe in the following year.

The Seears brothers had selected only on the ewe side, and had bought their rams from an outside stud. As a result their flock produced 170-180 lambs born per 100 ewes joined. With subsequent selection on both sexes in the CSIRO flock of 'Booroolas', however, this was raised to well over 200 lambs per 100 ewes.

The fact that success had followed selection on only one sex led to the theory, since proved experimentally, that the Booroolas' high fecundity was due to a single pair of genes, or closely linked block of genes, and experiments during the 1980s have been devoted to finding the best ways of using this gene commercially.

The Booroola is among the few sheep breeds in the world with high fecundity, and is the only one with a white, Merino-type fleece. It has become well-known throughout the world for this reason. Manipulation of the Booroola gene can itself lead to increased lamb production. Research into the physiological reasons for its high fecundity may lead to more techniques which can be used in practice to raise the all-important reproduction rate.

In addition, and at an early stage, scientists had also investigated the physiological mechanisms which control reproduction in sheep, in the hopes that field methods could be developed for the manipulation of their timing and extent.

These research studies began in Cambridge, where many young Australian scientists received their post-graduate training under the supervision of Dr. (later Sir) John Hammond. One of these, Dr. (later Professor) T. J. Robinson, showed that the natural steroid hormone, progesterone, dominated the control of the oestrous cycle in sheep. Coincidentally and independently, two other groups, one in the United States and one in France, made the same basic discovery. When he returned to Australia in 1951, Robinson and his colleagues continued to develop our understanding of the role of progesterone. In essence, it was found that sheep do not shed eggs or mate with the ram while under the influence of progesterone coming either naturally from the ovary or artificially from injections, but when the ovary ceases production, or injections are withheld, the animal ovulates and mates. When injections to a large number of sheep are withdrawn at the same time, all of them breed simultaneously. This fundamental knowledge clearly had great potential for controlled breeding.

There remained, however, at least two major problems before this knowledge could be put to use by graziers. The progesterone had to be injected no less frequently than every second day for about 12 days, requiring an input of labour which was unacceptable under Australian conditions. This might have been overcome by the use of some form of continuous injection but for the second problem, the amount of progesterone needed was excessive. About this time a large number of synthetic progesteronelike substances were being produced in response to successes in the field of human contraception. It was deduced that a product was needed with two distinct properties; first a high potency to reduce the dose to manageable amounts, and second, the substance had to be cleared quickly from the system of the animal so that, upon withdrawal, animals would ovulate and mate after a short and predictable period.

Australian Academy of Technological Sciences and Engineering