Genetics of footrot and a FeetFirst update

In these two videos, NZM’s Production Science Manger, Dr Mark Ferguson, speaks about the genetics of footrot and why we are developing a new genetic test to build more resistance to the disease in New Zealand's fine wool sheep, as well as giving an update on the FeetFirst project (including the development of a genomic breeding value (gBV) for footrot resistance and the expanded central progeny test).

Genetics of footrot

There are three key things that need to come together in order for a footrot outbreak to occur on a farm – there needs to be a host (and, unfortunately, our fine wool sheep are one of the more susceptible hosts in the sheep world), the bacteria that causes footrot (Dichelobacter nodosus), and the right environmental conditions (sufficient warmth and moisture for the bacteria to multiply).

Both the environment and the presence of the bacteria are very difficult to control.  However, through breeding, we are able to select sheep (the potential host) that are less susceptible to footrot infection.  This is why the aim of the FeetFirst project is to build a new genetic test that will enable us to shift the fine-wool sheep population over time to be more resistant to the disease.

In terms of resistance to footrot, there are many attributes that contribute – from the the sex and age of the animal, to the breed of the sheep and the different genetics within that breed, to the interaction between the sheep’s genes and its environment.

Many genes across the sheep genome are associated with footrot resistance.  Therefore, looking at a single gene marker will only ever give us a limited picture of an animal’s genetic resistance to footrot.  

The new genetic test will be in the form of a genomic based breeding value for footrot resistance.  The test looks at the whole genome and tells us which sets of genes contribute more to resistance (or susceptibility).  

The focus is not on finding an individual animal that is resistant to the disease; it is about moving the whole fine-wool sheep population to a state that is more resistant.

This does not mean every animal within that population will be resistant to footrot – it simply means that, on average, the population will be more resistant than it is currently.

FeetFirst update - July 2015

We would like to acknowledge the great support from the industry for the FeetFirst project – our growers who contributed through DNA sampling of their sheep, all the vets who assisted with sampling, and our co-funders Merino Inc. and the Ministry for Primary Industries (through the New Zealand Sheep Industry Transformation (NZSTX) project).

We have collected DNA samples from several thousand sheep on commercial farms – both sheep that did and did not get footrot under a challenge – and analysed the DNA to get a genotype from each animal.

This first dataset allowed us to make an initial prediction of the heritability of footrot resistance in fine-wool sheep of between 20 and 30 percent.  That is, approximately 20 to 30 percent of the variation between individuals is explained by genes.  This represents an opportunity for New Zealand’s fine-wool sheep industry to make some significant gains.

A second dataset, which consisted of sire genotypes only – it was unknown whether the sires were resistant to footrot or not – was compared to the first dataset and we were able to predict the likelihood of those sires (and their progeny) getting footrot (see graph below).

The third dataset we have is the central progeny test data.  A large number of progeny have been bred from a range of industry sires nominated by the stud sector.  We have challenged the male progeny with footrot and scored each of the individual feet.  That information has been utilised to validate the initial dataset and again we found that the predicted heritability of genetic resistance to footrot is between 20 to 30 percent in fine-wool sheep.

The results show a big variation in the genetics out there for resistance to footrot.  Our aim is to help the industry identify the animals that are more resistant to the disease.  This will provide ram buyers with greater access to rams that are more resistant and, over time, make their own flocks more resistant to footrot.