There can be no doubt that PMWS has been one of the hot subjects on pig health over the past few years. Amid the extraordinary amount of information generated by research and practical experience concerning this killer of weaned pigs, particularly fierce debate has revolved around the notion that there might be a breed effect.

In its simplest terms the notion suggests that units hit hard by PMWS could benefit from changing the breed of boar they use for weaner production. Most attention in this regard centres on anecdotal evidence from western and southern Europe of improvements in piglet mortality for herds that have switched to using a boar with Pietrain genes.

Few controlled comparisons have been conducted in European countries so far to investigate whether some breeds really can cope better than others when faced with a PMWS challenge. Within the past year, however, the Danish arm of breeder PIC has been involved in a trial in Denmark to test a change of boar breed towards a greater Pietrain influence, at herds suffering severe death losses due to the wasting syndrome.

The 3 participating herds were selected entirely for the fact that their records over the previous 6 months had shown an exceptionally high mortality rate, explains Dr Jørgen Plomgaard of PIC Danmark. Their post-weaning piglet mortality attributable to PMWS had to be at least 8%. In fact, the worst affected herd was suffering losses as high as 15% and the others had mortality rates in the range of 10-12%.

These were conventional-health units in the size range 450-700 sows, they weaned at about 28 days old and they raised their weaners for sale at 30kg liveweight. Normally 2 of them used Duroc boars while the third relied on a Duroc x Pietrain cross as its terminal sire.

Late in 2004 they began a series of inseminations using Pietrain semen for the mating of experimental batches of 85-100 sows. The semen, representing the PIC L62 Pietrain line and imported from a PRRS-negative stud in Germany, came from 20 different boars to eliminate the possibility of a family effect obscuring any breed-to-breed differences. The resulting litters produced about 1000 Pietrain-sired progeny to compare with the 1000-2000 offspring of the herd's usual boars, all the piglets having been born over the same period of 8 weeks so that environmental influences could also be ruled out.

The death-loss percentages for each herd in the trial are summarised in Table 1. Evidently the herds were less affected by PMWS in this period than in the previous 6 months, given that their average mortality rates were below the 8% threshold set previously well below, in 2 out of 3 cases. However, where the comparison focused entirely on the number of piglets that died on each unit according to the breed of the sire, evidently the differences in genetics did not give a clear-cut effect.

As Table 1 shows, one herd found similar mortality figures for the progeny from the imported semen and from the resident boars. Another recorded a clearly positive benefit for Pietrain offspring. At the third unit, its loss of weaners seemed to be even worse after changing the boar!

"I have to say that the results were a bit confusing," Dr Plmgaard comments. "It would be wrong to make a genetic recommendation based on these results alone. But they did indicate that the breed of the boar is something to be considered when a unit has bad problems with PMWS." PIGI