Animal Health & Welfare

The need for dehorning and the associated welfare and labour costs and risk of infection can be avoided by the use of a naturally polled bull breeding programme.

Polledness, where the animals do not grow horns, is controlled by a single gene.  This gene has two alleles or versions controlling its expression, one from each parent.  In most western breeds, the polled allele (P) is dominant over the horned allele (p), meaning that, if these two alleles are found together (Pp), the animal will be polled. 

Backcrossing Programme 

Polled strains of breeds which are normally horned can be established by crossing to animals from a polled breed followed by back-crossing to the original breed. This could be achieved without the use of genetic markers.

The following outlines the steps involved in a backcrossing programme to introduce the polled gene from breed B, to a horned breed, breed A:

1. Mate horned cows of the preferred breed A to bulls of a polled breed B. All the resulting calves will be heterozygous polled animals (Pp), with 50% of their genes from breed A and 50% from breed B. Heterozygous simply means that the two alleles are different (Pp - producing polled animals), whilst homozygous means that the two alleles are the same (PP - also producing polled animals, or pp - producing horned animals).  As homozygous PP bulls will never produce horned calves, this is the combination of alleles to aim for. 
2. Mate the crossbred females back to bulls of the preferred breed A. This will result in calves of two genotypes with respect to polledness; on average half of the calves will be heterozygous polled animals (Pp), and half homozygous horned animals (pp). The calves will now average 75% breed A genes and 25% breed B genes.
3. Backcross only the polled (Pp) animals to breed A again. About half of the calves will be heterozygous polled and half will be homozygous horned animals as before. The calves will now average 87.5% breed A genes and 12.5% breed B genes.
4. Repeat step 3 until the proportion of genes from the preferred breed is high enough (e.g. overall performance is close to that of pure breed A animals, or the proportion of breed A genes is high enough to allow pedigree registration).
5. Mate graded-up heterozygous polled animals to each other. This will produce three genotypes of calves: 25% PP (homozygous polled - the desired genotype), 50% Pp (heterozygous polled) and 25% pp (horned) on average.
6. Progeny test polled bulls by mating them to horned cows. Those which are homozygous polled will never produce horned calves. On average, heterozygous polled bulls will produce horned calves from 50% of matings to horned cows. About seven polled calves (and no horned calves) would need to be produced from matings to horned cows to be 99% sure of detecting a heterozygous polled bull.
7. Select and breed from homozygous polled bulls only.

However, the introduction of the polled gene from a polled breed of cattle to a horned breed can also provide a good example of Marker Assisted Introgression (MAI). Since some of the steps involved in the back-crossing program are time consuming and expensive, they could be abolished by using a molecular marker to distinguish between homozygous and heterozygous polled animals.

Advantages

• No mutilation required
• Economic and labour benefits