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Improving your herds through genetics
Understanding how to improve the genetics of pigs in your herd can increase the profitability of your piggery:
- use genetics to improve your pig herd
- which traits to use in selection
- issues around inbreeding
- techniques available to help you select the best replacement breeder pigs.
Learning how to measure herd performance and replace breeding stock will help you to maximize weight gains while maintaining quality.
Understanding genetics in pigs
The first step in improving your pig herd using genetics is to understand some of the concepts about how genes influence the traits and characteristics of your pigs.
How genes influence the traits of your pigs
All cells of living organisms, including pigs, contain genes. These genes drive all the biochemical processes that make up life and determine the characteristics that make up individuals. In other words, individuals vary because they have different genes and because these genes have adapted to different environmental conditions.
Genes are distributed along thread-like structures called chromosomes, which are arranged in pairs. A pig has 19 pairs of chromosomes. One member of each pair comes from the sire and the other from the dam.
The action of genes on pigs can be either simple or complex.
Simple inheritance
A trait that is passed onto offspring by simple inheritance is usually under the control of a single pair of genes. If a dominant and recessive gene are both present, the dominant gene will determine the trait.
An example of this is how pigs inherit either white or coloured coats. Most genetic diseases in pigs are also inherited this way. Genes that cause abnormalities are usually recessive to the normal (dominant) genes with which they are paired, so an animal carrying only 1 defective (recessive) gene is normal.
Not many economically important traits in pigs are passed on by simple inheritance. One exception is stress susceptibility, under the control of the halothane gene. Under high summer temperatures and long transport distances to market, a pig with this gene has an increased chance of stress death, resulting in an inferior quality of meat (pale, soft pork).
Complex inheritance
Most economically important traits are more complex. Each animal in a population will exhibit a trait somewhere along a smooth variation from one extreme to the other.
Economically important traits that are inherited in a more complex manner can be divided into 2 groups:
- performance traits
- growth rate
- food conversion ratio
- carcass leanness.
- maternal traits
- conception rate
- litter size
- piglet survival
- piglet growth.
Growth rate example
Each pig has some positive genes on its chromosomes that increase growth and some negative genes that decrease growth.
In a pig of average growth, positive and negative genes balance. Fast growers have more positive genes and slow growers have more negative genes.
Taking 1 pair of chromosomes from each, we can depict the 3 growth types as faster growth, average growth and slow growth. All these types of animals exist in any unimproved pig herd.
A selection program for growth aims to increase the number of pigs with a high proportion of positive genes by making sure that animals picked as parents have a high proportion of positive genes.
Pig traits and genetics
The theory of quantitative genetics enables you to improve traits under complex inheritance, such as growth rate.
This theory rests on 4 main concepts:
- heritability
- selection differential
- generation length
- inbreeding.
Heritability
Any trait expressed by an individual pig is the sum of the effects of its environment and the effects of the genes it inherited. The percentage of the total variation (i.e. genetic + environment) that is genetic is a measure of the heritability.
- Heritability % = 100 (genetic variation) ÷ (genetic + environmental variation)
Heritability is also the percentage of the parents' superiority that is passed to their offspring (see Figure 1. It is more effective to improve traits with a high heritability than those with a low heritability.
Heritabiity values
Many favourable traits in pigs, particularly growth rate and carcass leanness, have medium to high heritability. The performance of an animal is also a very good guide to the genes it carries for these characteristics.
Trait | Heritability (%) |
---|---|
Back fat thickness | 30–70 (high) |
Growth rate | 20–50 (medium) |
Feed conversion ratio | 20–50 (medium) |
Litter size at birth | 0–20 (low) |
Litter size at weaning | 0–20 (low) |
Computer-based selection
For traits that have low heritability (mainly those relating to reproduction), an animal's performance tells us very little about the genes it carries and passes to its offspring.
As an individual has some genes in common with its close relatives, we could examine the performance of these relatives to learn more about traits that have low heritability. This can be done more easily with a genetic evaluation system like the PIGBLUP.
Reproductive characteristics are very difficult to improve using selective breeding because of low heritability. However, specialist breeders have achieved genetic gains for litter size using best linear unbiased prediction (BLUP) technology, such as PIGBLUP.
Manual selection
Focusing selection on a greater number of objectives, such as litter size, will reduce the gains made in other traits.
A suitable strategy to maintain a reasonable level of performance for most commercial herds is through:
- good management
- a low rate of inbreeding
- the culling of breeding animals with poor reproductive histories
- the occasional introduction of a boar (via artificial insemination) from a herd with a high estimated breeding value for litter size.
In contrast, the higher heritability traits are relatively easy to improve by genetic means. There are now many examples of substantial genetic improvement in these types of traits, particularly growth and leanness. This improvement has occurred mainly through the selection of herd replacements based on growth and fat measurements made on the animals before breeding age.
Selection differential
The selection differential is a measure of the performance superiority of selected stock over the average of the group from which they were selected.
Generation length
The generation length is the time it takes for 1 generation to be replaced by the next.
Inbreeding
Animals whose parents are closely related may suffer inbreeding depression. This has the most striking effect on traits connected with reproduction, early growth and survival. A certain level of inbreeding is unavoidable in a herd selecting its own breeder replacements. However, the benefits of a selection program based on performance testing far outweigh any adverse effects of inbreeding if the level is kept low.
Predicting genetic improvement
The annual improvement in any trait under selection in a breeding program can be estimated using the following formula:
- Annual improvement = [(selection differential × heritability) ÷ generation length] − inbreeding depression.
The conditions for fastest improvement are obvious.
High rate of improvement:
- selection differential
- heritability.
Low rate of improvement:
- generation length
- inbreeding.
Performance testing to select replacement pig breeding stocks
On-farm performance testing to select replacement breeder pigs can be a highly profitable practice.
A breeder replacement plan begins with performance testing of pigs in your herd and involves measuring and recording individual pig characteristics to find the best pigs.
If you are running on-farm, pig herd performance-testing programs and wish to implement reduced-effort variations, read about ideal performance testing conditions and performance-testing program variations to achieve the best results.
Performance testing techniques
- Replace breeding stock uniformly throughout the year regardless of fluctuations in herd performance.
- Replace boars after 6–12 months of work.
- Replace sows after 4–6 farrowings or earlier if litter results are poor.
- Performance test as many pigs as possible for each one selected as a replacement. The minimum degree of choice is 1 gilt per 10 tested (10%) and 1 boar per 20 tested (20%). Have as many litters as possible represented within each performance test group.
- Keep the age of pigs in a performance test group within a 2 to 3-week range.
- Compare pigs only within a live weight range of 15kg.
- When buying in genetic material from seedstock providers, seek information about estimated breeding values. Herds in the national pig improvement program (NPIP) will provide this information.
Replacement plan
Start by creating a clear-cut plan for replacing breeding pigs and stick to this plan regardless of changes in performance during the year. Changes or fluctuations are due more to variations in environmental conditions than the genetic quality of the pigs.
The number of replacement pigs needed each year depends on the turnover of breeding animals and herd size. A herd of 100 sows may require 25–50 female and 10 male replacements per year.
Spreading the selection of replacements uniformly throughout the year in a 100-sow herd, would mean selecting 1 to 2 gilts per fortnight and 1 boar every month. This provides the highest degree of choice over the fullest possible range of genetic types within the herd.
For maximum herd productivity (pigs sold/sow/year) and rapid genetic progress, boars should work for 6 months and sows should produce between 4 and 6 litters before culling. Some animals may need to be kept longer to offset the occasional breeding failure of replacements.
Genetic traits used in selection
Traits used to select for rapid genetic improvement are:
These traits are economically important, have medium to high heritability and are easily measured before breeding age.
Traits with lower heritability, such as litter size, can be selected using information you collect about relatives and a best linear unbiased prediction (BLUP) computer program, such as PIGBLUP.
Selection differential
The selection differential is a measure of the performance superiority of selected stock over the average of the group from which they were selected. It is calculated separately for boars and sows and then averaged.
The selection differential tells you about the number of pigs chosen compared with the number available to choose from (i.e. the degree of choice). The selection differential increases as the proportion of animals selected from the group decreases.
There is not a direct relationship between the proportion selected and the selection differential. Extremely high degrees of choice are not warranted. Reasonable targets for degree of choice are 1 in 10 for gilts and 1 in 20 for boars. If selecting for growth rate alone, this would give a selection differential for boars and sows combined of about 0.1kg/day.
Generation length
The generation length is the time it takes for 1 generation to be replaced by the next. It also equals the average age of parents when their offspring are born. Genetic gains can only occur when 1 generation is replaced by the next. The more often this occurs in a given length of time, the faster progress is made. This results in the need to replace boars and sows at a young age.
There are serious disadvantages in keeping sows for too short a time. First priority must go to maintaining a high output of pigs per sow in the herd. Because the number of piglets reared per litter increases over the first few litters, the optimum age to cull sows is after their fifth litter, though there may be reasons to cull individual sows before this, such as sickness.
The best way to achieve a short generation length is to replace boars frequently. Boars should be replaced before they are 18 months old. If sows are kept for an average of 5 litters, this gives an average generation length for sows and boars of 1.8 years.
Performance testing conditions
Performance testing involves measuring the chosen characteristics (growth rate, back fat) on a group of pigs as they grow to turn-off (bacon weight). Pigs that perform best are selected as herd replacements. The greatest genetic gains are made when both boars and sows are tested, but smaller herds with superior boars can also benefit from testing homebred gilts.
All pigs must be tested under the same growing conditions to ensure that genetic differences in performance are obvious. The effects of variation in feed quality, weather conditions and disease incidence are minimised if performance comparisons are confined to pigs born within 2 or 3 weeks of each other. Even so, pigs born at the same time may grow under conditions where there are pen-to-pen differences in tail biting and scouring. Where these exist, pigs are compared not on their actual performance, but on the margins above or below the average performance of all pigs in the same pen.
Tips for testing
- Genetic variability between pigs being tested and the number tested per pig selected must be high to give the high degree of choice for rapid genetic gains.
- Include as many different litters from as many different sires as possible in a performance group (preferably a minimum of 4 litters). More parents contributing offspring will result in more genetic variation available for selection
- If you have large herds that farrow uniformly throughout the year, you will naturally have groups of pigs that are large enough to give a satisfactory degree of choice.
- If you have smaller herds, batch farrowing (a practice that has other advantages for herd management) will provide sufficient pigs of similar age for testing.
Using performance-testing programs in piggeries
Understanding the effects of variation on improving herd genetics can help you to use your on-farm, pig performance-testing programs to make improvements to your selection and breeding program.
If you are serious about genetic improvement, the ideal option is to use information from best linear unbiased prediction (BLUP) and National Pig Improvement Program (NPIP).
A pig herd with the following characteristics is well placed to carry out a genetic improvement program:
- herd size – 75 sows, 4 boars
- replacements per year – 40 gilts and 4 boars
- performance tests – every 3 weeks
- pigs available per batch – 9 litters with 8 pigs (36 gilts and 36 boars)
- number selected per batch – 3 gilts (per batch) and 1 boar (4 times per year).
This page includes a detailed description of the topics:
- general principles of performance testing variations
- ideal performance-testing conditions
- possible genetic improvement
- variations
- reduced degrees of choice
- actual performance versus pen margin
- mixing sexes
- mixing litters
- homebred or buy in.
General principles of performance testing variations
- Regardless of where you get the boars, failure to performance test gilts reduces genetic gain by about 45%.
- The more matings made to boars from non-testing herds, the more your own efforts at genetic improvement will suffer.
- Using artificial insemination (AI) boars with above-average estimated breeding values considerably enhances your rate of improvement. Several different AI boars should be used to limit inbreeding.
- Adopting several reduced-effort variations can significantly affect the genetic improvement of your pig herd.
Ideal performance-testing conditions
An ideal genetic improvement program will have:
- at least 10 pigs per pen
- pen mates of the same sex
- pen mates born in the same week
- pen mates from several litters by different sires
- fewer than 15kg (live weight) of difference between the heaviest and lightest pen mate
- pigs in the same pen during the 50–90kg growth phase
- breeder selection based on margins above or below the average performance of the pen
- degrees of choice of at least 20 males and 10 females tested for each boar and gilt selected
- low competition for food and high health status.
Possible genetic improvement
Given these ideal testing conditions, genetic theory can predict improvement. Selection is a process of replacing boars and sows with their best-tested offspring. A single generation of selection, taking about 2 years, will increase the value of each pig produced in the herd in all future years. Background knowledge of costs and returns of pig production suggests that, on a value of $150 for a baconer, this improvement is worth $12. Additional generations of selection will further increase the value of each future pig.
Variations
Reduced degrees of choice
The degree of choice achieved can be less than the ideal of 1–in–20 males and 1–in–10 females tested. Sometimes, the best performing gilts are unsuitable for breeding due to unsoundness or anoestrous. Halving the degree of choice to 1–in–10 males and 1–in–5 females reduces the genetic gain to 84% of that possible for the ideal program. In practice, a degree of choice of 1–in–20 for boars and 1–in–5 for gilts can be readily achieved. This gives 91% of the possible genetic gain.
Actual performance versus pen margin
Pigs should be assessed as future breeders not according to actual performance but according to pen margin. This is the difference between their actual performance and the average performance of the pen in which they grew. This simple calculation removes the inaccuracies caused by growth environments, which vary between pens. If we assess pigs on their actual performance rather than pen margins, we may reduce genetic gain significantly.
For example, if all pigs were selected on their own performance, genetic gain could be reduced to 85% of the ideal. Selecting gilts this way would reduce genetic gain to 95%.
Mixing sexes
Pens containing pigs of both sexes can cause difficulties for a performance-testing program. As males and females perform differently, combining them distorts the pen average and the margins above or below the average. It is better to use single-sex pens if enough pigs are available, as this will give the desired degree of choice.
In an ideal herd (as described above), 20 boars would be tested for each 1 selected and 30 gilts would be tested for every 3 selected (1 in 10). If mixed-sex pens must be used, pigs of the same sex in the same pen should be treated as a separate group (if each group contains at least 5 pigs). Sometimes pigs are in such short supply that groups of less than 5 have to be used. The test results of these pigs should be combined with similar small groups of the same sex in other pens. This should be done only if the average growth rates of these groups differ by less than 10%.
Mixing litters
When performance testing, it is good practice to mix pigs from different litters in the same pen. This increases genetic variation and, therefore, the chance of selecting breeders with true genetic superiority. If all pigs within each pen had the same sire but were from different dams, the efficiency would be reduced to 83% of the ideal. However, if all had the same sire and dam (e.g. litter mates), the program's efficiency could be reduced to as little as 56% of the ideal.
Homebred or buy in
Sometimes farmers do not wish to performance test. Gilts are often chosen purely on physical soundness and boars are introduced from herds that may or may not have a performance-testing program. Alternatively, some boar power might be drawn from an AI centre.
Several combinations of home selected and introduced breeding stock were compared (based on 1996–97 figures, i.e. from farm test gilt figures, Wacol Pig Test Station-approved boars). Table 1 shows the genetic gain percentages expected from these combinations. The level of gain expected from our ideal herd, in which all replacement breeders are home tested and selected, is 100%.
The 'untested herd' referred to in Table 1 starts with the same genetic level but is not performance tested and is unlikely to improve. The AI sires are from performance testing herds.
Measuring for pig herd performance testing
Measuring and recording the characteristics of your pigs is an important part of performance testing and helps you to select the best pigs for your breeding program.
Growth rate
This is normally measured from birth to bacon weight and the only information required is birthdate and live weight at bacon. Live weight is divided by the number of days from birth to bacon. Only pigs that differ in live weight by less than 15kg should be compared.
Back fat thickness
The depth of back fat over the eye muscle is the best single measurement of lean meat content. It is taken at the P2 position, which is 65mm down the left side from the midline at the level of the head of the last rib.
To locate the P2 position on live pigs:
- Find the rearmost edge of the last rib on the pig's left side.
- Mark a spot vertically above on the midline.
- From this spot, measure 70mm forward on the midline, and then 65mm down the left side from the midline.
- Mark the P2 site with a felt-tipped pen.
Back fat thickness is conveniently measured using echo sounder/ultrasound equipment designed for this purpose.
Selecting the best pigs
This example is a simple method for selecting breeders on-farm. With some additional effort, such as the provision of accurate pedigree records and computer recording, you can select breeders using your pigs' measurements through service providers, such as through computer programs PIGBLUP and the National Pig Improvement Program (NPIP). These programs consider the pigs' relatives and your herd in relation to other herds.
The selected pigs from any method are then assessed for physical soundness, such as the teat number in females.
Calculating index scores
The best way to use the performance measurements for selecting breeding stock is to combine them into a breeding value index. This simple equation combines growth rate (multiplied by an appropriate factor) and back fat thickness after each is given an appropriate weighting factor. The third item in the equation simply corrects back fat for live weight variation between pigs.
The weighting factors in the index account for variations in costs and returns in the pig industry, and need to be updated periodically as these do change.
The following in an example index that will improve growth rate, food conversion and carcass lean:
- Index score = [60 x daily gain (kg)] − [fat P2 (mm)] + [0.1 x weight (kg)]
In this form, the index can be used to compare only individuals of the same group that were grown under the same conditions. Comparisons among pigs grown in different groups can be made by calculating a corrected score:
- Corrected score = individual score − average score
Results from on-farm performance tests can be used to compare only animals on the same farm. With simple index selection, there is no way of knowing if the best boar tested on farm A is better or worse than the best performer on farm B, as most of the differences in performance between farms are environmental.
Inbreeding
High levels of inbreeding depress performance in reproductive traits. The best way to reduce inbreeding is to use as many different sires (or their semen) as possible in the herd each year. This means a rapid turnover of boars (working life of 6 months), which is easier to achieve in large herds. A recommended minimum number of boars to replace in a closed herd would be 8 per year.
In small herds, it will be necessary to introduce outside boars occasionally to control inbreeding. They must be superior boars drawn only from herds conducting a performance-testing program. Superior across-herd evaluated boars, or their semen, from the National Pig Improvement Program (NPIP) are highly regarded.
Keeping records
On a test record sheet enter the pig's identification number, weight and fat measurements.
Sample on-farm performance test sheet (from birth)
Calculation of index score | |||||||||
---|---|---|---|---|---|---|---|---|---|
Pig no. | Date born | Weight (kg) | Age (days) | Daily gain | Daily gain x 60 | Fat P2 (mm) | Weight x 0.1 | Index score | Corrected score |
1 | 30/9 | 83 | 146 | .57 | 34.2 | 15 | 8.3 | 27.5 | |
2 | 2/10 | 84 | 144 | .58 | 34.8 | 12 | 8.4 | 31.2 | +0.5 |
3 | 2/10 | 80 | 144 | .56 | 33.6 | 14 | 8.0 | 27.6 | |
4 | 2/10 | 85 | 144 | .59 | 35.4 | 11 | 8.5 | 32.9 | +2.2 |
5 | 11/10 | 80 | 135 | .59 | 35.4 | 15 | 8.0 | 28.4 | |
6 | 2/10 | 86 | 144 | .60 | 36.0 | 12 | 8.6 | 32.6 | +1.9 |
7 | 24/9 | 83 | 152 | .55 | 33.0 | 13 | 8.3 | 28.1 | |
8 | 24/9 | 91 | 152 | .60 | 36.0 | 18 | 9.1 | 27.1 | |
9 | 2/10 | 94 | 144 | .65 | 39.0 | 13 | 9.4 | 35.4 | +4.7 |
10 | 24/9 | 94 | 152 | .62 | 37.2 | 15 | 9.4 | 31.6 | |
11 | 24/9 | 95 | 152 | .63 | 37.8 | 16 | 9.5 | 31.3 | |
12 | 2/10 | 81 | 144 | .56 | 33.6 | 14 | 8.1 | 27.7 | |
13 | 30/9 | 91 | 146 | .62 | 37.2 | 14 | 9.1 | 32.3 | |
14 | 11/10 | 86 | 135 | .64 | 38.4 | 12 | 8.6 | 35.0 | +4.3 |
15 | 30.9 | 88 | 146 | .60 | 36.0 | 13 | 8.8 | 31.8 | +1.1 |
Averages: | .60 | 13.8 | 30.7 |
Heterosis in pigs
Members of the same population of pigs (e.g. breed or strain) are usually related and, therefore, somewhat inbred. When 2 populations are crossed, the level of inbreeding in the offspring falls to zero and those traits that suffered from inbreeding depression in the parent populations improve. This improvement is called heterosis.
Heterosis is the recovery of performance depressed by inbreeding in the parent populations. The degree of heterosis for the same trait varies among strains, breeds and environments.
As with inbreeding depression, heterosis is most often seen in traits of low heritability, particularly those connected with reproduction, early growth and survival. It occurs least often in traits of high heritability, such as carcass characteristics. Heterosis is usually greater if the genetic difference between the crossed populations is wide. Crossing breeds should give more hybrid vigour (i.e. improved biological traits in offspring) than crossing strains within the same breed.
Types
Maternal heterosis
Maternal heterosis benefits the individual pig through the hybrid state of its dam. It has the greatest effect when the individual pig is dependent on its dam i.e. from conception to weaning. Because of the economic importance of the number of pigs weaned per sow, maternal heterosis is the most important of the 3 types.
Offspring heterosis
Offspring heterosis benefits the individual pig itself due to its own hybrid state. It affects the pig's growth and survival throughout its life, but mostly after weaning when it is independent of its dam.
Paternal heterosis
Paternal heterosis results from the hybrid make-up of the sire. It shows itself by improving mating success, including through increased libido and conception rate. Evidence for this type of heterosis is limited, but it appears that boars can be somewhat inbred before their ability to produce offspring is harmed.
Calculating
The degree of heterosis is the difference between the performance level of the offspring and the average performance of its parents. It is usually expressed as a percentage of the parents’ performance (average effect).
For example, 1 study found litter sizes of 8.5 and 8.3 pigs for large white and landrace. Sows of the cross between these breeds produced 9.1 pigs per litter, which was equivalent to 0.7 pigs per litter (or 8%) higher than the purebred average of 8.4 pigs per litter. The degree of heterosis was therefore 8% (0.7 divided by 8.4 and multiplied by 100).
This shows that the performance of a crossbred is made up of the average effect of its parents (8.3 and 8.5) and the degree of heterosis appropriate to that cross (8%). It follows that a high value for heterosis may not overcome a poor to average effect of a parental breed.
Values
This table:
- summarises estimates of the degree of heterosis for important pig traits
- lists values from crosses of all breeds and from those involving only large white and landrace breeds
- separates maternal and offspring heterosis.
Key points:
- maternal heterosis benefits only the number of pigs born and weaned
- offspring heterosis benefits growth traits
- paternal heterosis values are not given due to insufficient information
- as the heritability of the traits rises, heterosis values fall and maternal heterosis has no effect on post-weaning growth, efficiency or fatness.
Trait | Heritability | All breed crosses | Large white and landrace-cross | ||
---|---|---|---|---|---|
Offspring | Maternal | Offspring | Maternal | ||
Number born | 10 | 3 | 8 | 2 | 4 |
Number weaned | 10 | 6 | 11 | 5 | 5 |
Weaning weight | 25 | 5 | 0 | 5 | 0 |
Post-weaning daily gain | 35 | 6 | 0 | 5 | 0 |
Food conversion ration | 45 | 3 | 0 | 2 | 0 |
Back fat | 55 | 0 | 0 | 0 | 0 |
Predicting crossbred performance
This example shows how to predict the number of pigs weaned per litter for the three-way cross of a duroc sire over a large white and landrace-cross dam.
If average breed effects for large white and landrace are 8.5 and 8.3 respectively, and maternal heterosis is 5%, then the expected number weaned from a large white and landrace-cross sow mated to any breed of sire is 105(%) x 8.4 = 8.8. When the breed of the terminal sire (i.e. duroc) is different from that of the dam's parents (i.e. large white and landrace), all offspring are crossbred. This slightly raises their chance of surviving to weaning.
Key points for breeders
It’s difficult to accurately predict how much heterosis to expect from a given cross (such as backcross, criss-cross and three-breed combinations). This is because the wide variation in values from the different studies that made up the averages (even between crosses of the same breeds) is not shown. This results from variation in strains of a breed crossed and in the testing environments used.
Any values given for degrees of heterosis are a guide only, and the choice of breeding system depends on management, pig health, required level of recording and cost considerations (such as the maintenance of own purebreds or the purchase of first cross-breeders).
Most Australian producers use combinations of large white and landrace, and some use a terminal sire such as the duroc.
The average effects of the purebreds that made up the cross are easier to predict than the degree of heterosis.
You can request information on average breed effects for growth and carcass traits can be calculated by those who coordinate central genetic improvement programs. Australian programs provide a service that calculates estimated breeding values for individual pigs from within or across herds.
Inbreeding in pig herds
Inbreeding occurs in all closed breeding populations, and is increased by:
- small herd size
- long working life of boars
- performance testing and selection programs.
The effect of these factors can be calculated and remedial steps taken to slow down inbreeding, such as by introducing unrelated breeding stock.
Inbreeding is the mating of related animals. The progeny of these matings have an increased chance of inheriting copies of the same genes from their sire and dam. The level of inbreeding (LOI) is a measure of this chance and theoretically ranges from 0% to 100%. The average LOI rises over time in any interbreeding population (e.g. breed, strain) and the more closely related the parents, the faster this rise will be.
Effects
- Animals whose parents are closely related may suffer inbreeding depression.
- The most striking effect is on traits connected with reproduction, early growth and survival.
- The extent of inbreeding depression varies between different populations (e.g. breeds, strains).
- Inbreeding depression depends on the actual LOI and on the time it takes to reach this level. A rapid rise results in greater inbreeding depression than a gradual rise.
- Any estimate of the effect of inbreeding can only be a general guide.
- One study of several inbred lines of pigs found that an increase of 10% in LOI reduced the number of piglets born per litter by 2.5% and reduced the body weight at 160 days by 3kg. These values are considered high because the rise in inbreeding was rapid in this study.
Calculating
All members of the same breed are related if their pedigrees are traced back far enough. However, when calculating inbreeding, we usually pick a starting point not too far back and assume all parents at that point were unrelated (i.e. LOI is 0%). We then work out the increase in LOI from that point to the present. For example, analysing pedigrees of Australian large whites and landrace born in the 1970s showed that, relative to their first herd book records, the LOI was 7.5% for both breeds. In the 1980s, unrelated genes from overseas animals were infused into our local large whites and landrace and this reduced the level of inbreeding. This allowed the establishment of a herd of large whites or landrace with only 5% LOI.
Breed crossing
When 2 unrelated populations are crossed, the LOI in the crossbred (F1) offspring falls to 0%. When the F1s are crossed among themselves to produce an F2 generation, the LOI rises to 50% of the level in the parent populations.
For example, if both landrace and large white had an average LOI of 5%, an F1 gilt from a cross between these breeds would have an LOI of 0%. Crossing this F1 gilt to an F1 boar would produce F2 offspring with LOI at 2.5%.
LOI would rise further in generations following the F2 (i.e. F3, F4, etc.). This can be slowed in the short term by avoiding close matings.
For example, LOI is increased by 25% in the offspring of full siblings (same parents) and by 12.5% in the offspring of half siblings (one common parent). However, levels of inbreeding in the longer term are determined more by herd size and other factors.
Herd size
In a herd closed to the entry of breeding stock from outside, the rate of increase in inbreeding will depend on the herd's size; being higher for small herds than for large herds.
Selection
A herd selection program increases inbreeding, as animals that perform well are likely to be selected as breeder replacements and are also likely to be related.
Two ways of avoiding an unacceptable rise in LOI are to:
- increase the rate at which boars are replaced
- obtain some boars from outside the herd.
Boar replacement rate
Keeping boars for a shorter time reduces the chance of their mating with daughters and this leads to a reduction in LOI.
Introducing breeder replacements
A way of slowing the rise of inbreeding in a herd is to introduce some breeder replacements. If these are obtained from similar but unrelated herds, we can estimate their effect on a herd's LOI.
Acceptable limits of inbreeding
- A certain amount of inbreeding is unavoidable in a herd selecting its own breeder replacements.
- The benefits of a selection program based on performance testing far outweigh any adverse effect of inbreeding, provided the level is held down.
- An upper LOI limit of 10% over 15 years is suggested.
- When the herd reaches this upper limit, LOI is reduced by outcrossing to stock of other herds carrying out similar programs of selection. Performance results will show which herds these are.
- It is equally important to keep the rate of increase in LOI to a minimum.
- At least 10% of all matings in the herd should be performance-tested AI boars (when these are used to provide the outside matings) to maintain satisfactory genetic improvement.
Replacing breeding stock in commercial pig herds
Commercial pig herd operators differ in the degree to which they rely on purchased breeding stock, although this is the simplest was to improve herd genetics. The disadvantages of buying all your replacement stock include the cost and the potential for adaptation problems and the introduction of disease.
Some operators buy all their replacement gilts and boars while others select gilts from within existing herds and purchase only boars. A third option is to select both male and female breeding stock from within the herd, with either regular use of artificial insemination (AI) or the occasional introduction of a boar to introduce new genes. AI may be cheaper than buying all replacement stock.
The National Pig Improvement Program (NPIP) can give an independent assessment of breeding values.
Buying replacement gilts and boars
A number of breeding companies specialise in selling gilt and young boar replacements. As these breeding companies are performing costly genetic improvement and selection, buying all replacement breeding stock is one of your more expensive options.
The company you choose should have:
- a rigorous selection program that achieves a fast rate of genetic improvement
- a selection program running best linear unbiased prediction (BLUP) software (such as PIGBLUP or PEST).
BLUP software is particularly useful in selecting traits with low heritability such as litter size or number weaned. BLUP can also improve the estimated breeding values (EBVs) for traits with medium heritability such as growth, feed conversion efficiency and carcass quality.
Most breeding companies will only sell crossbred lines as a way of protecting their investment in the nucleus purebred and to exploit hybrid vigour. Gilts produced from crossbred dams may produce a smaller litter size than their parents, as some loss in hybrid vigour might occur. The value of hybrid vigour may be over-promoted compared with maintaining a healthy, well-managed herd. Purebred large white herds in a genetic improvement group consistently produced more pigs born per litter than the average recorded in Pig Stats over a 5-year period.
Issues with selecting gilts from within your herd
In times of financial difficulty, producers who obtain stock from a breeding company may be tempted to select gilts from their own herd to reduce costs.
An important consideration is whether these gilts are suitable for breeding. The prime example is where a producer has been using a terminal sire line that incorporates the stress or halothane gene. Some of the gilts would be carriers of the gene and could produce actual stress reactors (not just carriers) after being mated to carrier boars.
On-farm gilts and purchased boars
It is possible to have a successful breeding program by purchasing boars or using AI while selecting all gilts from on-farm. To reduce the risks of disease through live animal introductions, AI is preferable, with possibly a boar on-hand for backup.
Purebred or mixed semen may be available at a cheaper price. If mixed semen is used, a synthetic herd would be formed from a mixture of different breeds. Mixed semen from large white and landrace could maintain most hybrid vigour for reproductive performance. It may be a good idea to avoid using duroc genes in gilts as this breed has a lower reproductive performance than the 2 white breeds.
One way of reducing the cost of AI is to use contract semen collection. Herd operators can group together and purchase elite boars that would be too expensive for an individual herd operator. The cost of AI can be less than natural mating.
On-farm gilts and boars
If all boars and gilts are bred on-farm, it is advisable to introduce genes periodically to avoid loss of productivity through inbreeding depression. With fewer introductions, it is important to select the very best boars, as their genes will remain in the herd longer. Probably the best source is purebred stock with a national ranking (e.g. high EBV boars from the National Pig Improvement Program).
The frequency of periodic 'top-ups' will depend on the efficiency of your selection program. For larger herds using BLUP, fewer introductions may be required. However, with high selection intensities and the possibility of inbreeding depression, periodic gene introductions from high-scoring EBV boars is recommended.
AI offers a safer method of introducing elite genes because there is less risk of disease than with live animals. AI has become popular for this and other reasons including availability, ease of use and because fewer boars are needed on-farm.
© The State of Queensland 1995–2024
- Last reviewed: 08 Sep 2021
- Last updated: 08 Sep 2021