British White Cows
Background
Robertsonian Translocation (Rob 1;29) (RT) is a chromosomal abnormality first described in Swedish Red and White cattle (a dairy breed) in 1964. RT was subsequently detected in multiple cattle breeds with one of the highest incidences being detected in the UK British White herd in 1975.
Robertsonian translocation Rob(1;29) describes the fusion of the 1st and 29th chromosomes. The most commonly accepted origin hypothesis, supported by subsequent detailed genotypic studies, is the abnormality arose some thousands of years ago in a common ancestor prior to the establishment of the various cattle breeds known today. Cattle with the abnormality appear normal in every respect and RT is only detected by specific analysis of the complete set of an individual’s chromosomes (karyotype).
RT is of concern because it is associated with a reduction in fertility. This effect is due the potential formation of unbalanced zygotes (single cell formed from an egg and sperm cell) when the sire and/or dam are carriers of the abnormality. The resulting embryos are unviable, aborted and the cow continues to cycle until a viable zygote is formed and implanted.
Despite knowing about the high incidence of RT in British Whites since 1975, no action had been taken by either the UK, American or Australian breed societies prior to 2008. In that year, Dr Ian Holmes, made a presentation at a General Meeting. With the assistance of Dr Graham Webb, a geneticist from Adelaide School of Veterinary Science, karyotyping of some Australian herds subsequently confirmed an alarmingly high incidence of RT (over 80% in one herd). In response, the BWCSA formulated a plan that aimed to gradually reduce the incidence of this chromosomal abnormality within the Australian herd. The key components of this plan were:
- Only bulls verified free of RT were to be registered
- The society subsidised RT testing of bulls for a period of two years
- Previously registered bulls of unknown status or carrying RT remained in the herdbook
- Registration of A grade bulls free of RT was reintroduced for a period of two years
- Genotypic parent verification for animals known to be RT free was introduced to allow registration of offspring with determined RT-free status without need for expensive karyotyping.
This plan has now been in place for more than 15 years resulting in a significant reduction in the incidence of RT within the Australian. It is useful to reflect on the approach adopted by the BWCSA in the light of recent published experience describing a similar approach adopted by the breed societies of three indigenous Spanish breeds over the past 20-30 years (1, 2).
Spanish Experience
These two recently published studies from Spain address two frequently raised concerns amongst Australian breeders in regard to the Society’s approach to RT. In particular, whether the gains in reproductive efficiency are worthwhile and secondly whether the approach is having a negative impact on the genetic diversity of the Australian herd.
Jiménez et al. analyse 30 years of data from the Retinta Breed Society and attempt to:
- Describe the evolution in the incidence of RT over 30 years in a population selected against carrier individuals.
- Determine the effect of RT on fertility in beef cattle raised under extensive field conditions.
González-Cano et al. evaluate the effects on genetic diversity of selecting against RT in the Berrenda Negra and Berrenda Roja breeds.
Retinta Cow and Calf
The Retinta breed is an indigenous breed from the south and south-east of Spain known for its longevity and survival in marginal pasture lands in a dry, hot climate. In 1991 as part of an official breeding program aimed at enhancing these desirable traits, the Retinta Breeders Association decided that only bulls tested free of RT would be used and this practice has continued ever since. Jiménez et al. analysed karyotypes (5,721 animals with test results and a further 137,025 determined by breeding), reproductive data and pedigrees, and found a reduction in the incidence of RT from over 15% to around 1% over the 30 years. A slow but steady increase in reproductive efficiency in the breed as a whole was also observed over this period. The RT carrier cows had a small but significant reduction in reproductive efficiency compared with RT-free and RT homozygous cows. The reduction was around 5% and comparable to the effect of advanced maternal age and inbreeding. The authors concluded that exclusion of heterozygous (“59”) and homozygous (“58”) bulls from breeding programs without culling RT-effected cows was able to reduce the incidence of the condition 15-fold and contribute to increased fertility of the breed without notorious reduction in the census or genetic variability of the breed.
Barrenda Colorado cow and calf
The Berrenda en Colorado (BC) and Berrenda en Negro (BN) are respectively red and black varieties of a unique indigenous breed from the south of Spain each with its own breed society. Both breeds are regarded as endangered native Spanish Breeds. They are regarded as a valuable genetic resource with significant cultural, social and environmental value and in particular are valued as the ideal choice for meat production in extensive livestock farming sites of high environmental value.
Currently, there are 3061 registered BC females in 73 herds and 5,390 registered BN females in 140 herds. The breed associations developed breeding programs aimed at improving the low reproductive performance of the two breeds, regarded as a significant impediment to conserving the breed. This program included testing for RT to eliminate carrier sires with the result that the RT incidence declined from 18 to 8% for BC and 47% to 11% for BN between 2009 and 2019.
Concerns were raised about the impact of such an intense selective strategy on the genetic diversity and level of inbreeding of these relatively small breeds. González-Cano et al. analysed SNP genetic markers to evaluate for genetic diversity in carrier and non-carrier populations. They found slightly higher genetic variability in the RT carriers of the BC breed but not the BN breed. Overall, the authors thought it wise for Berrenda breeders to apply gentler selection pressure against RT than had been done with the Retinta breed because of the much smaller herd size. They recommended elimination of homozygous male carriers only, mating of heterozygous carriers with RT-free animals only and the development of easier-to-implement methods for the diagnosis of RT (a SNP-based genotypic method is currently being studied as an alternative to expensive karyotyping).
Barrenda en Negro bull
A reduction in the incidence of RT was thought to contribute positively to the conservation of these two threatened cattle breeds not only by improving their reproductive performance but also by increasing their effective population size in the medium and long term
Implications for BWCSA
The findings reported in these two papers makes fascinating reading for our breed society in reviewing the impact of the initiatives taken to address RT in the Australian BW herd. The information gleaned from these studies involves much larger numbers of animals and much more comprehensive data collection in relation to reproductive efficiency and genetic diversity than is available to our Society. It is thus useful when reviewing our past approach and future directions in managing RT in the Australian British White herd. Importantly, the cattle studied are being raised under extensive grazing conditions and under environmental conditions comparable to large parts of Australia. In addition, at least in the case of the Berrenda breeds, the herd size is relatively small.
Exclusion of bulls carrying RT from breeding led to a dramatic reduction in the incidence of RT over a period of 30 years in the Retinta breed and a significant reduction in the incidence over 10 years in the BC and BN breeds. This confirms that the policies adopted by the BWCSA were an appropriate initiative to gradually reduce the incidence of RT over a period of decades. Furthermore, the studies support the premise that, even under extensive grazing conditions, reduction of incidence of RT contributes in a small but significant way to improving fertility. It is important to remind ourselves that fertility is however multifactorial and highly dependent on other genetic factors, maternal age and especially nutrition and environmental conditions.
These articles also raise concerns about the unintended loss of genetic variability within threatened herds by over-aggressive responses to reducing the incidence of RT. Retaining females carrying RT within breeding programs is one way to reduce this risk. Unfortunately, this requires expenditure of time and money in arranging karyotyping of the offspring of these animals. For this reason, many Australian breeders made a decision to aggressively cull RT-carrier females despite this not being mandated or encouraged by The Society. These actions may have had the unintended consequence of reducing genetic diversity within our herd. The development of cheaper, easy genotyping methods of detecting RT, as currently being investigated by researchers led by Dr Alessandra Iannuzzi at the Institute for the Animal Production Systems in the Mediterranean Environment (ISPAAM) in Italy, may make this option of retaining RT-carrier females more attractive in the future. To assist in these efforts, the BWCSA has recently started discussing options to collaborate and share RT data with ISPAAM.
Efforts by BW breeders in Australia to enhance genetic diversity within our herds whilst reducing the incidence of RT are also challenged by the lack of action in regard to RT by sister societies in the UK and America. Although, a number of breeders in those societies are concerned about RT and its possible contribution to observed fertility issues, the expense and limited access to karyotyping, together with reputational concerns about publicising a potential genetic abnormality that impacts fertility, have meant that no action has been taken at a breed society level overseas. The BWCSA, taking these issues into account, recently modified its regulations creating an appendix register to facilitate use of overseas sperm and embyros from untested or known RT-carriers but with the proviso that offspring would only be eligible for registration in the Australian herdbook if confirmed RT free.
Overall, the BWCSA can be proud of its achievements over the past 15 years in addressing the reputation risk for the breed posed by RT. It needs to continue to evaluate progress via annual censuses and ensure that reduction in the incidence of RT does not come at the price of reduced genetic diversity within the Australian herd. Technical progress in evaluating animals in RT may greatly assist these efforts in the future.
References and links
González-Cano R, González-Martínez, Ramón M et al. Exploring the effects of Robertsonian Translocation 1/29 (Rob(1;29) on genetic diversity in minor breeds of Spanish Berrenda cattle via genome-wide analysis. Animals 2024, 14,793 https://doi.org/10.3390/ani14050793
Jiménez JM, Morales R, Molina A et al. Effect of the rob(1;29) translocation on the fertility of beef cattle reared under extensive condition: A 30-year retrospective study. Reproduction in Domestic Animal, 57, 349-356. https://doi.org/10.1111/rda.14073
