Comparison of gene editing versus conventional breeding to introgress the POLLED allele into the US dairy cattle population

Abstract

Disbudding and dehorning are commonly used cattle management practices to protect animals and humans from injury. They are unpleasant, costly processes subject to increased public scrutiny as an animal welfare issue. Horns are a recessively inherited trait, so one option to eliminate dehorning is to breed for polled (hornlessness). However, due to the low genetic merit and scarcity of polled dairy sires, this approach has not been widely adopted. In March 2018, only 3 Holstein and 0 Jersey active homozygous polled sires were registered with the National Association of Animal Breeders. Alternatively, gene editing to produce high-geneticmerit polled sires has been proposed. To further explore this concept, introgression of the POLLED allele into both the US Holstein and Jersey cattle populations via conventional breeding or gene editing (top 1% of bulls/year) was simulated for 3 polled mating schemes and compared with baseline selection on lifetime net merit (NM$) alone, over the course of 20 yr. Scenarios were replicated 10 times and the changes in HORNED allele frequency, inbreeding, genetic gain (NM$), and number of unique sires used were calculated. Gene editing decreased the frequency of the HORNED allele to <0.1 after 20 yr, which was as fast or faster than conventional breeding for both breeds. In the mating scheme that required the use of only existing homozygous polled sires, inbreeding reached 17% (Holstein) and 14% (Jersey), compared with less than 7% in the baseline scenarios. However, gene editing in the same mating scheme resulted in significantly less inbreeding, 9% (Holstein) and 8% (Jersey). Also, gene editing resulted in significantly higher NM$ after 20 yr compared with conventional breeding for both breeds. Additionally, the gene editing scenarios of both breeds used a significantly greater number of unique sires compared with either the conventional breeding or baseline scenarios. Overall, our simulations show that, given the current genetic merit of horned and polled dairy sires, the use of conventional breeding methods to decrease the frequency of the HORNED allele will increase inbreeding and slow genetic improvement. Furthermore, this study demonstrates how gene editing could be used to rapidly decrease the frequency of the HORNED allele in US dairy cattle populations while maintaining the rate of genetic gain, constraining inbreeding to acceptable levels, and simultaneously addressing an emerging animal welfare concern.

Strategies for Sustainable Use of Indigenous Cattle Genetic Resources in Southern Africa

“…Gene editing is not ethically accepted by many countries today [47], but offers a faster, cheaper, healthier, more-efficient, and sustainable animal production opportunity compared to conventional breeding programs [48,49]. The technology holds potential for cattle production in future, but researchers must be aware of its challenges when applying it at a large-scale and integration in breeding schemes [47,48,50]. The success and future application of this novel technology will be largely influenced by decisions around the regulatory framework and governance of genome editing for food animals [49].…”

Optimized Genetic Testing for Polledness in Multiple Breeds of Cattle

“…Economic sustainability and animal welfare have driven recent progress in modern livestock production systems especially in efforts to minimize or eliminate undesirable traits such as the presence of horns in cattle. Genetic dehorning is being progressively adopted as the noninvasive approach to breed hornless cattle through genetic selection (Carlson et al 2016;Mueller et al 2019). Genotype-phenotype relationships of horn growth are however complex (Medugorac et al 2012) limiting the informativeness of current assays for early detection for horn status, and the presence or absence of polled alleles (polledness), in some breeds.…”

Genome editing approaches to augment livestock breeding programs

“…Using genome editing, the allele has been introduced into dairy genetics in a proof of concept experiment (Carlson et al, 2016). If this allele could be edited into the genome of a number of elite dairy sires, then artificial insemination could be used to rapidly disseminate that dominant polled allele through the dairy cattle population (Mueller et al, 2019), and eliminate the need for physical horn removal, which is an animal welfare concern.…”