However, all-too-often producers are led down the path of breeding their own replacements by advisers and vets without full knowledge of the consequences of their actions. Even breeding companies have been known to advise this route when it suits their needs.

I am not arguing that it is not a valid route to improved bio-security or that, done correctly, it cannot provide a solution to herd replacements. My aim is simply to inform the reader of the potential consequences of rotational crossing that they may not have considered.

The benefits of homebred rotational cross gilts are:

• A farm closed to live animal imports
• Control of own gilt production quality

However, there are drawbacks to homebred rotational crossing that are due to both homebred gilt production and the rotational cross system of production.

Breeding companies’ programmes are geared to maximising hybrid vigour, as the advantage gained from this can be worth many years of selection for particular traits.

A rotational cross system will lose 50% of the hybrid vigour available in an F1 gilt in the first generation. This will eventually fall to a 33% loss in future generations. This may not sound a great deal, but when you consider that maximum hybrid vigour is worth 1.3 pigs weaned per litter (see Table 1), 50% equates to 3.9 pigs less weaned over the 6 parity lifetime of first generation rotational cross gilts selected, or 390 piglets for a 100 sow herd.

This figure assumes the correct use of dam line semen in the first place. To differentiate between parent sows bred from e.g. Landrace or Large White requires sound recording. Failure to use the correct breed of semen will result in a further drop of 25% (1.95 pigs weaned/lifetime) in the next generation.

Often homebred gilts are seen as a cheaper option used during times of poor cash flow. However, if homebred gilts are properly costed, they can often end up costing more due to poorer lifetime production, loss of some finishing accommodation (hidden cost of having to sell lighter pigs), extra labour used for selection etc., increased genetic lag from the homebred gilts.

There is also a loss of flexibility in homebred gilt production; gilt production is fixed 11 months in advance and the inability to ‘plug’ gaps in production by introducing more gilts than budgeted for also impacts the bottom line. If these costs are added onto the direct costs of loss of slaughter value and the extra genetic cost of dam line semen, the ‘saving’ can look less appealing.

Table 1 Advantage of 100% hybrid vigour in a diverse F1 cross
	Offspring	Mother	Total
Litter Size	+0.30	+0.75	+1.05
Litter Size at Weaning	+0.45	+0.85	+1.30
Sellier, P. (1976) The Basis of Crossbreeding in pigs; a review Livest. Prod Sci 3:203-226

LOOKING AT THE COSTS...
Direct cost of the loss of slaughter value (at AAPP £1.00)	£75
Time for selection, vaccination, tagging	£2
Loss of Hybrid vigour - Value of pig reared @ £20/pig x 3.9 pigs	£78
Additional cost of dam line semen (£30/litter with 3 selected gilts/litter)	£10
Cost of rotationally produced homebred gilts	£165
Cost of purchased F1 gilt	£125
Additional cost (excluding hidden costs referred to in the text)	£40

Source: ACMC - August 2005