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Understanding Scurs

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10/9/2023
In Holstein and Jersey cattle, the trait of being polled or having horns is determined by a single gene. Each animal has two versions of the gene (alleles), one inherited from the dam and the other from the sire. The polled allele (P) is dominant to the horned allele (H). If an animal has two polled alleles (PP), it is homozygous for the polled gene and is polled. If the animal has one polled allele (P) and one horned allele (H), it is heterozygous and will also be polled. However, if the animal is heterozygous polled (PH), it may pass either the polled or horned allele on to its offspring. Since the polled allele is dominant, the only situation when an animal is horned is when it possesses two recessive horned alleles (HH) and is homozygous horned.

Dominant alleles are expressed anytime that allele is present, whether it is heterozygous or homozygous. For example, if you have a bull that is heterozygous polled (PH), and he is mated to a homozygous horned cow (HH), it can be expected that 50 percent of the resulting calves will be polled (heterozygous carriers) and 50 percent will be horned (and homozygous for the horned alleles). If a bull that is homozygous polled (PP) is mated to a homozygous horned cow, all of the calves will be polled, heterozygous carriers (PH).

Scurs

Some polled cattle have scurs. Scurs are incompletely developed horns that are generally attached only to the skin. They range in size from tiny scab-like growths to large protrusions almost as large as horns. Therefore, polled cattle can be smooth-polled or scurred-polled (polled but expressing scurs). Many breeders have the mistaken idea that scurs represent an error in transmitting the horned characteristic. That’s not the case. Scurs are genetically transmitted, but scurs are determined by a different gene separate from the polled gene. In males, the allele for scurs appears to be dominant. So if a bull has scurs, he might be either heterozygous (Ss) or homozygous (SS) for the scurred condition. In females, the allele for scurs is recessive. Only homozygous scurred (SS) cows will develop scurs. Ss and ss cows will be smooth polled.

Scurs can only be seen in polled cattle. In horned cattle, the horn hides the scurred condition, so you cannot evaluate the scurred condition in horned cattle. Not all polled cattle are free of scurs; it appears that only cattle that are heterozygous for the polled/horned genes (PH) will express the scurred trait. This means that both homozygous horned and homozygous polled animals will be free of scurs.

It is probably easiest for breeders interested in polled cattle to ignore the scur trait except that this is an indicator that the animal is heterozygous for the horned/polled gene. Scurs are not seen in homozygous polled cattle. Efforts should be concentrated on the polled gene. When the herd becomes homozygous polled there should no longer be scurred animals.

When distinguishing between animals that express the scurred trait, it is important to remember the following rules:

  1. Only cattle heterozygous for the polled trait (PH) will express scurs.
  2. The scurred allele is sex-influenced and appears to be dominant in bulls and recessive in females.
  3. Only females carrying two dominant alleles for scurred (SS) will express scurs.
  4. Animals may not express the scurred allele, but they may carry the allele and pass it on to their offspring.
  5. A homozygous polled herd will not express the scurred trait.

 

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