Dairy bull genetically edited to prevent it growing horns fathers six hornless calves

A genetically modified bull has fathered six hornless calves after scientists edited its genome to prevent the animals from having to be de-horned by farmers.

Researchers at the University of California, Davis, have found an alternative to the routine farming practice of de-horning young bulls – pain-free genetic modification.

Bulls who once would have to go through the unpleasant procedure to have their horns removed either as they start to form, in a practice called de-budding, or later on in life, will now be spared the trauma.

Most bulls are subject to the removal to avoid them posing a risk to other cattle or human handlers.

Many dairy breeds naturally grow horns. But on dairy farms, the horns are typically removed, or the calves ‘disbudded’ at a young age as animals that don’t have horns are less likely to harm other cattle or dairy workers and have fewer aggressive behaviours. 

A naturally occurring genetic variant, or allele, that is present in some breeds of beef cattle such as Angus was introduced into the genome of the cow with horns. 

Thanks to the altered genome none of the bull’s offspring developed horns, as expected, and blood work and physical exams of the calves found they were all healthy. 

The researchers also sequenced the genomes of the calves and their parents and analyzed these genomic sequences, looking for any unexpected changes. 

The findings were published by UC Davis scientists in the journal Nature Biotechnology.

UC Davis researchers 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 findings were confirmed by the U.S. Food and Drug Administration who 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.

Researchers at the University of California, Davis, have found an alternative to the routine farming practice of de-horning young bulls

Unwanted plasmid integration can be addressed by screening and selection in future breeding – in this case, only selecting the two offspring of the genome-edited hornless bull that inherited only the naturally occurring allele for breeding.

Van Eenennaam said: ‘This type of screening is routinely done in plant breeding where genome editing frequently involves a step that includes a plasmid integration.’

Adding: ‘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 say that while the plasmid does not harm the animals it does technically mean the genome-edited bull is classified as a GMO, because it contained foreign DNA from another species, in this case a bacterial plasmid.   

Since the original work for this study was carried out in 2013, initiated by the Minnesota-based company Recombinetics, new methods have been developed so donor template plasmid or other extraneous DNA sequences are not used to transfer the hornless allele.

No other unintended genomic alterations were seen in the calves five of whom were female and one male, and all animals remained healthy during the study period.

Neither the bull, nor the calves, entered the food supply as per the U.S Food and Drug Agency guidance for genome-edited livestock.