A horned bull has fathered six hornless calves after scientists edited its genome in a bid to prevent the need for farmers to de-horn the animals.
For the past two years, researchers at the University of California, Davis, have been studying six offspring of the dairy bull.
The technology has been proposed as an alternative to de-horning, a common practice used to protect other cattle and human handlers from injuries.
Genome editing sees the DNA of an embryo, sperm or egg deliberately altered to influence the characteristics of the offspring.
According to the study published in Nature Biotechnology, none of the bull’s offspring developed horns, as expected, and blood work and physical exams of the calves found they were all healthy.
Researchers also sequenced the genomes of the calves and their parents, analysing them for any unexpected changes.
The data was shared with the US Food and Drug Administration (FDA).
Analysis by FDA scientists revealed a fragment of bacterial DNA, used to deliver the hornless trait to the bull, had integrated alongside one of the two hornless genetic variants, or alleles, that were generated by genome-editing in the bull.
Corresponding author Alison Van Eenennaam, with the UC Davis Department of Animal Science, said: “Our study found that two calves inherited the naturally occurring hornless allele and four calves additionally inherited a fragment of bacterial DNA, known as a plasmid.”
The researchers said plasmid integration can be addressed by a form of screening and selection, which in this case means selecting the two offspring of the genome-edited hornless bull that inherited only the naturally occurring allele.
Dr Van Eenennaam added: “This type of screening is routinely done in plant breeding where genome editing frequently involves a step that includes a plasmid integration.”
She explained that plasmid does not harm the animals, but the integration technically made the genome-edited bull a genetically modified organism (GMO), because it contained foreign DNA from another species – the bacterial plasmid.
She continued: “We’ve demonstrated that healthy hornless calves with only the intended edit can be produced, and we provided data to help inform the process for evaluating genome-edited animals.
“Our data indicates the need to screen for plasmid integration when they’re used in the editing process.”
Researchers did not observe any other unintended genomic alterations in the calves, and all of the animals remained healthy during the study period.
A number of dairy breeds naturally grow horns, but on dairy farms they are typically removed, or the calves “disbudded” at a young age.
Those without horns are less likely to harm other animals or dairy workers and have fewer aggressive behaviours.
The de-horning process is unpleasant and has implications for animal welfare.
Dr Van Eenennaam said genome-editing offers a pain-free genetic alternative to removing horns by introducing a naturally occurring genetic variant, or allele, that is present in some breeds of beef cattle such as Angus.