Revisiting the Booroola F+ Gene

Part 1 of 3

 Genetic Theory supporting Booroola F+ gene for commercial lamb production

By Robert Leder, DVM

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One of the foundation cornerstones of commercial lamb production is ewe productivity.  While less productive ewes can be profitable provided that the supporting expenses are limited; net profit is income (lambs and wool sold) minus expenses. For a lamb to be sold, it must first be born!  Because prolificacy is only 10% heritable within a breed of sheep; genetic progress is slow.  Genetic exploration was conducted in the US in the 1970’s by MARC and others to increase the productivity of our ewe flock. The searches for prolific genetics led researchers to the Finnsheep breed. Finnsheep were introduced to increase prolificacy of maternal sheep breeds through the use of crossbreeding. Later the Romanov breed was also used through crossbreeding to increase ewe productivity. The prolificacy passed on by the Finn and Romanov is caused by multiple alleles, meaning that multiple genes are responsible for the increase in lamb drop. It is now expected that whatever percent Finn or Romanov blood your ewes have, there will be an equal percent increase in the lambing percentage compared to your starting breed.

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In each of these cases, a population of Finn or Romanov has to be maintained as a genetic source of prolificacy. The practical application of these breeds is to use half Finn or Romanov rams to produce prolific quarter blood maternal ewes.The ewe base breed is commonly Dorset and/or Rambouillet mated to the half Finn or Romanov ram. These crossbred ewes are then mated to a terminal meat sire for the production of lambs.  From a genetic theory stand point, this would be a three breed maternal-terminal crossbreeding system. It employs maximum (100%) maternal heterosis, when these ewes are mated to terminal sires there is maximum (100%) lamb heterosis.

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A logistic problem of this system is maintaining the pool of prolific genes; someone has to breed and maintain pure Finns or Romanovs.Then thses must be bred to purbred maternal breed ewes to produce half Finn or half Romanov rams.   Additionally, undesirable characteristics, such as small size, temperament or slow rate of gain, common with prolific breeds, are transmitted along with the prolificacy genes. These crossbreeding logistic hurdles have been avoided by the production of the Polypay breed, a composite breed that includes one quarter Finnsheep background. Over time though, newly formed breeds lose some of their initial heterosis as the new breed of crossbreds are mated to each other. The genetic base of the composite breed narrows and new genetics must be infused. Heterosis is maximized when matings occur between individuals that are genetically diverse and complimentary to each other.  The productivity increase caused by hybrid vigor is slowly negated by the production of stabilized composites like the Polypay with each subsequent generation.

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If however, prolificacy could be increased by a single gene, and if that gene could be easily indentified on the farm, then the problems of multiple allele crossbreeding could be avoided. This is the advantage of using the Booroola F+ gene for commercial lamb production.   

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A special strain of prolific Merinos was recognized in 1919 in Australia by Bert Seears. Over the subsequent years his extended family bred and maintained the flock.  In 1959 the Seears family gave a ram and sold some ewes from the prolific flock to the government for research.  By 1980 it was determined that the Booroola high fertility was controlled by a single gene. Booroola Merinos were imported to the US in the 1980’s. The specific Booroola F+ fecundity gene was discovered in 2000, and can now be determined by genotyping. F+ gene carrier ewes can be expected to deliver 1.2 extra lambs per lambing compared to non-carriers of the same genetic make up. 

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Genes are the genetic code of all living creatures. Genes come in pairs, one coming from each parent. F+ carrier ewes, those with one gene, are called heterozygous.  The carrier ewe will transmit the F+ gene to half of her offspring. If an animal has two copies of a particular gene it is called homozygous. Ewes that are homozygous will have 2.2 more lambs per lambing compared to non-carriers of the same genetic make-up. Sheep that are homozygous for the F+ gene will transmit the gene to its all of its offspring. 

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On a commercial farm setting, the identification of carrier ewes is very simple.  Most ewes delivering twins at one year of age will carry the gene. Based on this assumption, the accuracy of this “test method” is about 90% specific and about 90% sensitive. Test specificity measures the accuracy of a positive test result; about 10% of ewe lambs delivering twins will not carry the gene. Test sensitivity measures the accuracy of a negative test result; about 10% of ewelambs delivering singles their first year will carry the gene. Subsequent production records help to remove errors of the initial “test method”. Ewes that had a single their first year who go on to deliver triplets or better the next year can be added to the list of F+ carrier ewes.  Ewes that go on to have twins only can be sorted to the non-carrier list. While a small group of ewes will fall somewhere in the middle, two distinct populations can be determined with simple lambing records, the “haves” (carriers) and the “have-nots” (non-carriers). The carrier ewes will pass on the F+ gene to half of their offspring when mated to non-carrier rams.  By limiting selection of replacements to lambs from identified carriers, the gene frequency in the ewe flock can approach 50%.  The maximum prolificacy increase possible using carrier ewes as your F+ gene source would be 60% (50% carriers x 1.2 lambs=.6 = 60%).

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Initial work done the MARC suggested that dealing with all the non-carrier ewes would be a drag on profitability. They favored the multiple allele approach, citing the direct correlation of increase Finnsheep blood to lamb drop. In reality though, a 25% increase in lamb drop is really just one in four ewes having an extra lamb (lambs are born as wholes not quarters). Most commercial shepherds would settle for a 20-30% increase in lamb drop, which means only 20-30% of the flock needs to be carrier ewes. This percentage of carrier ewes can easily be acheived and maintained by selecting replacements from the identified carriers. 

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Another problem recognized by MARC was the difficulty in identifying rams as carriers of the gene. Because rams do not give birth, the only way to determine carrier rams was to progeny test them. That meant the selected rams had to be bred to a group of non-carrier ewes, and the productivity of the offspring would have to be measured. This took several years to complete, and required large populations of non-carrier ewes to be maintained. This was difficult, time consuming and expensive; all not practical for commercial producers. This limitation of the single gene approach of the Booroola F+ gene was significant, and favored the use of multiple allele rams such as half Finn or half Romanov for increasing prolificacy.

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The discovery of the specific F+ gene and the development of the commercial test around 2000 removed this obstacle. Just like the Spider and Scrapie gene tests available, presence of the Booroola F+ gene can now be determined from a blood sample. The gene can be introduced to flocks from F+ test positive rams. While carrier rams will pass the gene to half of their offspring, homozygous rams will pass the gene to all their offspring. The use of F+ carrier or homozygous rams is a much faster way to achieve the desired level of F+ carriers in a flock than selecting replacements from carrier ewes.

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Early work with the Booroola F+ gene focused on infusing the gene into a specific pure breed such as the Dorset or Rambouillet.  By back crossing the carrier ewes to purebred rams after several generations the Merino component disappears, leaving the F+ gene in Dorset or Rambouillet. Sadly, little work was done mixing the F+ gene into a crossbreeding scheme.  

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With prolificacy controlled by a single gene (F+) the inclusion of the Finnsheep or Romanov breeds of sheep is not necessary in the maternal crossbreeding scheme.  Rather, breeds can be selected for what ever traits are needed to match your resources and markets, while maintaining the level of prolificacy desired. By employing a three breed rotational crossbreeding scheme for replacements, such as Dorset, Rambouillet, and East Friesian, 86% of maternal heterosis can be achieved. When the three-way crossed ewes are mated to a terminal sire, maximum (100%) lamb heterosis is realized. The inclusion of the Booroola F+ gene in the flock gives the shepherd greater flexibility selecting the breeds of his/her maternal cross because prolific breeds do not have to be included in the crossbreeding program.  The desired lambing percentage can be targeted by selecting the appropriate replacements from carrier ewes or rams.

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From basic genetic stand point, the infusion of the Booroola F+ gene into a population of sheep dramatically increases the heritability of prolificacy; going from just 10% to nearly 100%. The difference in ewe productivity is then nearly all attributable to the presence or absence of the F+ gene.  While the gene is transmitted to half the offspring from a carrier parent mated to a non-carrier, that frequency and its subsequent production is boost is more than adequate for most commercial shepherds.

 

The following table lists the three crossbreeding systems discussed along with the logistics required and heterosis achieved.

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