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.