“I really like this bull because he produces polled calves 80% of the time”. “I had a polled cow inseminated with semen from a polled bull and I got a calf with horns. How is that possible?” “I just acquired a new bull. He has horns, but his grand-parents were polled. I am optimistic that he will produce a certain number of polled calves”.
Do these statements sound familiar? It is very possible, because in the past few years, there has been a real interest from beef cattle producers for polled (hornless) animals. At the same time, they are a reflection of a part of genetics that is often misunderstood and that causes confusion for many. To get a clearer picture, let’s look at the principles affecting the transmission of horns in the bovine species.
Firstly, we have to remember that each animal possesses in its genetic code a pair of genes (2) that will determine if it will have horns or not. During conception, the calf inherits a gene from each parent for this trait. It is also essential to know that the “polled” gene (A) dominates the “horn” gene (a). From these statements, we have three possible genetic combinations:
It will be polled and said homozygous because it possesses two identical genes for that trait. As they are two dominant genes, the animal will only produce polled calves, regardless of the type of cow it will be bred with.
- The animal receives the “polled” gene from one parent and the “horn” gene (Aa) from the other parent
It will be polled as (A) dominates (a). It will be called heterozygous polled because it carries two different genes. This animal will produce no more than 50% of polled calves if it is used solely with horned cows (see table).
- The animal receives two “horn” genes (aa) from both parents
It is the only combination that will produce a horned cow as the dominant polled gene is not present.
To have a better picture, let’s examine the six different possibilities in the following table:
We now understand why it is so important for a breeder wanting to have polled calves to know the difference between the words homozygous and heterozygous. However, how do we determine if a bull is heterozygous or homozygous for the polled trait? It is very simple. If a polled bull is bred solely with horned cows and produces only one calf with horns, he is automatically classified as “heterozygous” as he had to transmit the (a) horn gene to obtain this result. On the other hand, if a bull is bred to at least ten horned cows and that the resulting calves are all polled, the probability that he is homozygous for this trait is 99.9%. There are also DNA tests which will quickly provide information to know the genotype of a bovine for this trait.
What about scurs?
They are partly developed horns that are not solidly implanted on the head. They can be as small as buttons and, in some cases, almost as big as real horns. Contrary to popular belief, scurs are controlled by a pair of different genes than those for horns. Consequently, there is no connection between horns and scurs.
What we must remember however is that the scurs gene is dominant in the bull and recessive in the cow. It will then be very easy to identify the carrying males as they will automatically have scurs. However, a carrying cow, unless she is homozygous, can transmit them to her progeny even if she does not have any herself. Consequently, this gene will be hard to detect in cows, particularly in those that are not polled as the scurs will be hidden by the presence of horns.
With these few basic facts in mind, you will be able to answer your neighbours the next time they approach you with one of the statements presented at the beginning of this article. So, a bull that “dehorns” his calves at 80, 60 or 40 %, does not exist. It is important to know is that a polled homozygous bull will only produce polled calves while a heterozygous sire bred with horned cows cannot do better than 50% in terms of producing polled calves.
It is simply the law of probabilities that applies as one time out of two, the sire will transmit the “horn” gene to his progeny. In the second case, it had to be a bull and a cow that were polled heterozygous. Looking at the table, we can see that in 25% of the cases, such a mating produces a horned calf. Finally, to answer the third statement, we should mention that a horned bull and cow can only produce horned calves if they are bred to horned animals, regardless if they have polled ancestors in their pedigree.
By : Pierre Desranleau, T.P.
Beef Cattle Division