Saving Time and Money with Polled Bulls
Take advantage of the mutation situation!

We are now into bull buying season. Dehorning calves at birth or later is a nuisance and it has been proven that horned cattle are an economic disadvantage both in the cowherd and the feedlot. While we know that dehorning earlier is better for man and beast, there is a method that reduces both stress and labour to zero - taking advantage of a favourable genetic mutation, the polled factor. Polled calves are a nice convenience, but were once thought to be inferior for other traits. Fortunately, seedstock producers have made great improvements with polled cattle in the popular breeds, particularly the exotic breeds where horns were the norm. From a genetic perspective, it is possible to find polled bulls that are equal to if not superior to horned bulls for virtually any trait that can be measured, as evidenced by data from BIO and breed associations. Most bull buyers now look to invest in a polled bull. However, all polled bulls are not created equal with respect to their specific genetic makeup for polledness.

In wild cattle (from which all domesticated breeds are descended) the presence of horns is the natural state. However, wild individuals occasionally demonstrate the polled condition due to a genetic mutation. It is this mutation that was selected for in the creation of polled breeds of cattle.

The polled trait is due to the gene for horn condition which has two alleles (alternative forms). Each progeny gets one allele for horn condition from the bull and one allele for horn condition from the cow. The polled form is dominant, and denoted as P. The horned form is recessive and denoted as h. An animal with a mixed pair of alleles (Ph) will show as polled, since the polled allele dominates over the horned allele.

All polled breeding stock carry at least one P allele (it only takes one P to make the animal exhibit the polled trait), and so will produce progeny that are, on average, at least 50% polled. The percentage of progeny which will show polled depends on whether the polled parent carries the allele pair PP or Ph, and also whether the other parent is horned (hh) or polled (either hP or PP).


Table 1. Terminology of Horns and Scurs

  • Horned - exhibits horns attached to the head; both copies of the horn condition gene carry the allele for horns (h//h)
  • Polled - no horns attached to the head
  • Clean Polled - polled with no scurs
  • Polled/Scurred - polled but with smal appendices that look like small horns
  • Double Polled - both parents of the animal are polled
  • Single Polled = heterozygous - only one copy of the horn condition gene has the polled allele, the other carries the horned allele (P//h)
  • Homozygous Polled - both horn condition genes carry the polled allele for horn condition (P//P)

So when you mate a homozygous polled bull (P//P) to a horned cow (h//h), all of the calves will be heterozygous polled (P//h). They are polled but carry also the horned allele. This can be shown in a Punnett square (Table 2.).

Table 2.

Cow
Bull
P
P
h
P//h
P//h
h
P//h
P//h

However if the bull is Heterozygous polled (P//h) when mated to horned cows (h//h) then only half of the calves would be polled (Table 3).

Table 3.

Cow
Bull
P
h
h
P//p
h//h
h
P//p
h//h

When a heterozygous bull (P//h) is mated to heterozygous cows (P//h), then in theory 25% of the calves will be horned, while 75% will be polled (Table 4). Of the polled calves, 1/3 will be homozygous polled (P//P). So when you hear a bull is double polled (having both parents showing the polled trait), it is not certain to be homozygous polled.

Table 4.

Cow
Bull
P
P//h
P//h
h
P//h
P//h

Unfortunately, it is not possible to determine if a bull is homozygous polled (P//P) just by looking. If you know for certain that both parents are homozygous polled, then the bull must be the same. Things are said like "it is peaked polled so it must be homozygous polled" or "it has a long sheath so it must be homozygous" but neither are necessarily true. Fortunately, there is now a DNA test available to determine if an animal is homozygous polled.

Scurs

The inheritance of scurs is not as simple. Scurs are horn like growths that appear on polled cattle and are generally loose and moveable, at least at young ages. Part of the confusion is the fact that scurs vary in size from being almost as large as horns to being so small that they are hard to detect, more like a scab. It appears there are additional genes affecting the size of scurs and some evidence that there are different genes that cause scurs. Scurs are only expressed on polled cattle and not all horned cattle carry scurs.

The common theory of how scurs are expressed depends on the gender of the animal. For a cow to show scurs, she must have 2 alleles for Scurs (Sc//Sc), whereas a bull will have scurs if it has only one allelle for scurs (Sc//ns). The following table (Table 5) is the result of some Candian research by Dr. Sheila Schmutz. This suggests that all scurred bulls are single polled.

Table 5. Inheritance of Horns and Scurs*

Genotype

Cows

Bulls

P//P Sc//Scsmooth polledsmooth polled
P//P Sc//nssmooth polledsmooth polled
P//P ns//nssmooth polledsmooth polled
P//h Sc//Scscursscurs
P//h Sc//nssmooth polledscurs
P//h ns//nssmooth polledsmooth polled
h//h Sc//Schornedhorned
h//h Sc//schornedhorned
h//h sc//schornedhorned

* key to short forms
P = polled allele
h = horned allele
Sc = scurred allele
ns = allele for no scurs

As well, research in the Charolais breed has found a recessive gene for scurs so that even bulls must have 2 scur genes in order for scurs to be present.

Bull showing scurs.
Figure 1. Bull showing scurs.

So if you wish to buy a bull whose calves will not have to dehorned, ask some questions about his pedigree with respect to both horns and scurs. Ask if the bull has been DNA tested if it is double polled, but you are not certain if it is homozygous polled.

Be sure to consider all of the other economically important traits when investing in a bull. Buying a bull is the most important decision with respect to the genetic improvement of your herd! But remember to take advantage of the mutation situation and consider buying polled breeding stock when possible. They will save time and money in the long run, with dividends being paid if you retain their female polled progeny in your herd.

Click here to view other Virtual Beef articles


For more information:
Toll Free: 1-877-424-1300
E-mail: ag.info.omafra@ontario.ca