Inbreeding

Published 1 September 08

Inbreeding

Dairy farmers should be careful when selecting bulls to use in their herds, as levels of inbreeding are rising and could start to impact on production and vigour.

New research from the Scottish Agricultural College and Edinburgh Genetic Evaluation Services (EGENES) has revealed that inbreeding levels for all breeds currently stands at about 2% in the UK. Although that is well below the 5-6% recorded in the US and Canada, UK inbreeding is rising at 0.13 percentage points a year.

At that rate, the UK herd will reach the same level as the US and Canada in around 20 years, leading to depressed performance, poor vigour and higher occurrence of genetic defects.

Although the precise effects of inbreeding depression are difficult to quantify, previous research has estimated a loss of about 15kg of milk per lactation and a 0.4 day increase in calving interval for each percentage increase in inbreeding.

However, there are also plenty of advantages to be had from a low level of inbreeding, as selecting for the best genetic traits enables producers to improve herd performance. Genetic evaluations have enabled us to pinpoint the best genetics in the world, while artificial insemination and, to a lesser extent, embryo transfer, have sped up genetic improvement in herds by concentrating on the very best bulls and cows.

Line breeding has historically been used to 'fix' a certain desirable trait to ensure that it is consistently expressed down the generations. Successful bulls like Blackstar, Chief Mark and Starbuck, which feature heavily in many Holstein pedigrees, are evidence of how this line breeding has developed.

The breeding industry has become truly global, with the best bulls used all over the world. However, as the best bulls and cow families have been highlighted by Genetic Evaluations, this has inevitably led to a shrinking of the breeding pool, as only the best bloodlines are used, raising the level of inbreeding.

It is surprising that the SAC research, revealed that all dairy breeds in the UK have almost identical inbreeding levels at around 2%.  Some geneticists expected levels in the smallest breed populations to be higher, but perhaps greater use of limited proven bulls or on-farm stock bulls has diluted the effect.

Equally, all breeds saw the introduction of foreign genetics in the late 1980s and early 1990s, which has helped keep inbreeding levels down. This is the particularly apparent in the Ayrshire breed since it opened its herdbook to Holstein bloodlines but, conversely, less evident among Friesians which have seen limited imported bloodlines.

Too much inbreeding is detrimental and can lead to significant losses in production, fertility, longevity and health. It also makes genetic defects like Complex Vertebral Malformation more likely to occur. All breeds therefore need to try and achieve genetic progress without incurring unacceptable levels of inbreeding.

As a general rule, commercial dairy producers should avoid matings which lead to the resultant offspring having an inbreeding level of greater than 6.25%. Many breeding companies now offer mating programmes which can be told to avoid inbreeding when they make mating decisions. Producers can also use the table below for simple calculations.

Dairy farmers who milk record can find out the inbreeding levels of all of their milking cows by requesting a herd genetic report from DairyCo. This report now shows the inbreeding level for every animal in the milking herd as well as whole herd levels.

Rather than just sticking to a handful of top sires, producers need to be more aware of inbreeding levels, and monitor them on both an individual and a herd basis. There are many excellent bulls to suit a range of breeding objectives within the top 100 available bull list, which are offspring of 59 different sires, so there's no need to concentrate on only a few related bloodlines.

However, breeders should not be unduly worried by the current position, and should certainly not be striving for zero inbreeding. It is just a case of slowing the rate of increase and finding the right balance for a healthy, productive herd now and in the years to come.

Table 1: Inbreeding levels
Mating Inbreeding %
Sire/daughter 25
Full brother/full sister 25
Half brother/half sister 12.5
Grandsire/grand-daughter 12.5
Grandson/grand-dam 12.5
Uncle/niece 6.25
Son/grand-daughter 6.25
Daughter/grandson 6.25
Full cousins 6.25
Grandson/grand-daughter 3.13
Half cousins 3.13

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