2023 Limousin Annual Journal

Whether a herd is pedigree or
commercial, introducing polled
genetics needs to be done with
a level of understanding if the
benefits are to be seen.
Polling and horns are controlled by a
major gene and there are two forms
of that gene; the form that causes
polling and the form that causes
horns. The form that causes polling
is denoted by ‘P’ and the form that
causes horns is denoted by lower
case ‘p’.
The gene always comes in a pair of
these forms. An animal inherits one
form from its sire and the other from
its dam. It can therefore have one of
the following combinations:
1) PP
2) Pp
3) pp
There is ‘Polled’ and
there is ‘Polled’!
Animals that are polled
themselves will not always pass
this on to their calves. Here’s
why…
The form of the gene that causes
polling (‘P’) is ‘dominant’ over the
form that causes horns (‘p’).
Dominance determines how the
gene will be expressed (ie what the
animal will look like). Since ‘P’
polling is dominant, animals that
have inherited it will be polled
themselves. In the list above, this
would be Animals 1) and 2). Animal
3) would be horned.
Where both forms of gene are
the same, an animal is said to be
‘homozygous’ for that gene. Animal
1) above is Homozygous Polled, and
Animal 3) is Homozygous Horned. If
the forms of the gene are different,
the animal is ‘heterozygous’. Animal
2) above is Heterozygous Polled.
The Golden Rule
DNA testing will identify the polled
and horned forms of the gene that
an animal has. This information
is available for tested animals in
all Society sale catalogues and for
individual animals on the Society’s
online herdbook www.taurusdata.
co.uk.
How it is interpreted is as follows:
9 Homozygous Polled (or ‘Hom.
Polled’) – animal is polled itself
and all calves will be polled
(since it only has the polled form
of the gene to pass on and it is
dominant)
9 Homozygous Horned (or ‘Hom.
Horned’) – animal is horned itself
and all calves will be horned
(unless the other parent is Hom.
or Het. Polled) since it only has the
horned form of the gene to pass
on.
9 Heterozygous Polled (or ‘Het.
Polled’) – animal is polled himself
BUT will produce some polled
and some horned calves (unless
the other parent is Hom. or Het.
Polled) since it carries both forms
of the gene.
Predicting the Outcome
If the genotypes of the parents are
known, the outcome of a mating can
be easily predicted. In the example
below a Het. Polled bull is being
mated to a Het. Polled cow:
Bull
Cow P p
P PP (25%) Pp (25%)
p Pp (25%) pp (25%)
There is a…
• 25% chance the calf will Hom.
Polled (ie without horns and it will
always pass this to its progeny)
• 50% chance the calf will be Het.
Polled (ie will be without horns
itself but will not always pass this
on to its progeny)
• 25% chance the calf will be Hom.
Horned (ie with horns and will
always pass this to its progeny)
In commercial herds it is possible
that only the genotype of the bull
will be known. If he is Heterozygous
Polled then worst-case-scenario
planning would assume that the
cows are horned, resulting in an
outcome of approximately 50% of
animals being polled and 50% being
horned.
As some farms move towards a lower
cost and labour system, cutting
out a job such as disbudding or
dehorning calves becomes attractive.
Downturn in performance resulting
from the procedure is avoided as is
risk to animals and stockmen from
injury during handling. Industry
commentators are increasingly
indicating that legislation on the
issue is only a matter of time.
Strides have been made in recent
years for polled cattle to match the
commercial performance of their
horned counter parts and the gap
will continue to close as the available
genetics and the tools to identify
them grow.
However, it is important to note that
the polled gene is a dominant gene,
and breeding solely for polled cattle
could lead to a decrease in genetic
diversity. Therefore, it is important
for farmers to carefully manage
their breeding programs to ensure
that their herd remains healthy and
genetically diverse.
From small acorns large oaks grow,
and the growing polled population
within herds and within the breed
will ensure that Limousin continues
to deliver to its customer base.
TECHNICAL
www.limousin.co.uk
ANNUAL JOURNAL 2023 | 139