Polledness is a trait of increasing importance to beef producers for a multitude of reasons. While animal welfare issues attached to dehorning are a key driver for some producers involved with horned breeds to select for poll genetics, many are discovering the added benefits such as reduced labour costs and increased workplace health and safety.
In beef cattle of European ancestry, the trait of being polled or having horns is determined by one pair of genes. One gene in the pair is inherited from the dam and the other from the sire. The polled gene (P) is dominant to the horned gene (p). If an animal has two polled genes (PP), homozygous, or one polled and one horned gene (Pp), heterozygous, the animal will be polled. However, if it is heterozygous polled (Pp) it may pass either the polled or horned gene on to its’ offspring. The only situation when an animal will be horned is when it possesses two recessive horned genes (pp), homozygous horned.
There are additional genes that affect horn-like growth, scurs, on an animal’s head. Scurs are incompletely developed horns which are generally loose and movable beneath the skin, not attached to the skull. They range in size from small scab-like growths to occasionally almost as large as horns. Because the gene for scurs is transmitted separately it has no effect on the presence or absence of horns. Not all horned cattle carry the gene for scurs and not all polled cattle lack scur gene.
The gene for scurs is expressed differently from the gene for polledness/horns. The way the gene for scurs is expressed depends on the sex of the animal. In males the scur gene is dominant, meaning that if only one of the two genes is for scurs the bull will be scurred. Therefore, it is easy to detect the scur gene in the bull and eliminate it from the herd. In females the scur gene is recessive, meaning that she must possess both genes for scurs for the cow to be scurred. If the cow possesses only one scur gene she will not have scurs herself but has a 50 percent chance of passing the scur gene on to her calf. The smooth polled cow may have the recessive scur gene, resulting in much more difficulty in identifying/eliminating the scur gene from the herd.
In Australia, soon after Limousins arrived in 1980, 99.9 percent of registrations were horned. The poll variant barely existed, but by 2009 the mix was 50 – 50 horned and polled and in 2017, 84.6% of the calves registered were polled or scurred/polled. In 1995 just under 18% of Simmentals were polled. By 2010 it was a 50-50 mix of horned and polled and by 2017, 68.5% of new additions to the herd book were polled. The Charolais breed made slower progress towards polledness in earlier years. In 1995, just 5% of the breed’s registrations were polled and that had risen to only 11% over the next 15 years. But in the period since 2010 registrations of polled Charolais have surged with the proportion of polled calves now over 50%. (Source: www.beefcentral.com)
In Ireland, whilst we lag behind Australian levels of polledness, most breeds have seen an increase in the numbers of polled animals in the past few years. Through genotyping, more animals are being identified as heterozygous and homozygous polled. Without genotyping, there is no way of formally identifying polled animals in the Hereford, Charolais, Limousin or Simmental breeds.
The below graph shows the percentage of polled/heterozygous calves born, expressed as a percentage of overall genotyped calves.
Homozygous polled progeny exist in each of the breeds. Expressing the number of polled calves born per breed is not accurate as not all animals are genotyped. The proportion of pedigree animals genotyped by birth year ranges from 47.23% – 73.92%. (Based on numbers of valid genotypes received of pedigree animals on 27/2/19).
|Percentage of Pedigree Progeny Genotyped (By Birth Year)|
Animals that are genotyped polled receive a bonus in the Terminal and Replacement indexes to reflect the labour and capital cost of disbudding calves. Polledness has an economic weighting of €5.95 in the Replacement Index. All animals, apart from Aberdeen Angus, must be genotyped to establish their polledness status before a polledness contribution can be included in their index. Once genotyped and the polledness status can be properly established, Angus animals will either receive the homozygous (€5.95) or heterozygous (€2.97) value.
- Genotyped homozygous polled animals (all progeny polled) receive the full contribution of €6.
- Genotyped heterozygous polled animals (half of progeny polled) receive half of the contribution of €3.
Polledness (animals being born without horns) has an economic weighting of €7.55 in the Terminal Index. Once genotyped and the polledness status can be properly established, animals will either receive the homozygous (€7.55) or heterozygous (€3.78) value.
- Genotyped homozygous polled animals (all progeny polled) receive the full contribution of €7.55.
Genotyped heterozygous polled animals (half of progeny polled) receive half of the contribution of €3.78.