What to select for and why
Profit comes from keeping a productive sow that regularly weans good litters that grow well to slaughter weight with no deaths. That’s the main objective.
It’s also important to make sure pigs have a friendly temperament and have no physical defects.
But you have to sort out which bits of this objective are related to breeding (genetics) and which to feeding and management. You’ll need some simple records to do this.
Keep a record card for each sow and list:
- Sow number
- Her date of birth and breed
- Litter number order, and sire of that litter
- Date of last farrowing
- Number born (dead and alive)
- Number weaned
- Total weight of litter weaned
- Days to slaughter and carcass grades
'Number of piglets born' indicates the sow’s inherent fertility, provided the boar’s fertility is not at fault and she is healthy. Hence the need to record who she was mated to. These are weakly inherited traits but nevertheless important in selection over a long time to find productive females.
Date born for the litter, and days from last farrowing are also important aspects of sow fertility, and include her ability to recover after lactation. This has a low genetic component and is strongly affected by management.
Number of piglets born (dead and alive) is weakly inherited too, but over time can be useful to select those sows that consistently perform well.
Number of piglets weaned, compared with number born alive (ie. deaths to weaning) is again weakly inherited, but over time shows up sows that look after their piglets well.
Total weight of litter at weaning is a very useful all-round indicator of profit as it shows up sows that are fertile, milk well and look after their piglets. It also reflects the ability of the piglets to grow fast and look after themselves! It’s a mixture of traits that are both strongly and weakly inherited, but nevertheless is a useful measure for keeping sows as future dams.
The growth rate of individual pigs from weaning to slaughter is strongly inherited, and is the best measure to use when selecting efficient replacements as parents. Pigs that grow fast generally convert their feed into meat efficiently.
Purebreds or crossbreds
Crossbred pigs (on average) usually show some hybrid vigour in fertility and growth. But don’t assume all crossbreds are better than good purebreds these days.
Today’s top commercial pigs are “hybrids” and have been bred from crossing different selection lines. So they may not be separate “breeds” as such.
It’s important to remember that there is often more variation within a breed than between them. This is because individual animals vary so much both in their genes (genotype) and what they look like (phenotype).
The greatest benefits in crossbreeding occur when the animals crossed are genetically very far apart. Then you will truly see some “hybrid vigour”. Otherwise the crossbred will be no better than the average of both parents. "Hybrid vigour" is when the cross is better than the parents’ average.
Inbreeding and linebreeding
This is where you mate animals that are related - generally called “close breeding”. It’s mating animals that have common ancestors.
You can see this in a pedigree where you find the same individual ancestor on both sides (sir and dam) of the pedigree.
If the level of inbreeding increases rapidly (eg by mating very close relatives like sons back on dams or half to half sisters) then this would be called "inbreeding".
If inbreeding was less intense (eg half cousins or more distant), it would be called "linebreeding".
Mating relatives will concentrate their genes - good or bad. If inbreeding levels are kept low there will be few problems.
Once it builds up, you may see genetic defects appearing, and if you keep on inbreeding, you’ll see “inbreeding depression”, where fertility and survival get worse.
Breeding pigs need to be able to walk, and remain healthy, so they must have good feet and legs, and be able to eat properly.
Breeding females must have at least eight functional teats on each side.
All breeding stock should have a good friendly temperament.