Mice were selected for growth from 3 to 9 weeks of age on a normal protein diet (N) containing 19-3% protein and a reduced protein diet (R) containing 5 1 % protein. On each diet there were 3 high (H), 3 low (L) and 3 unselected control (C) lines. After 6 generations of selection, half of the mice in each line were tested on each diet. Responses were obtained when selecting for both increased and decreased growth on both diets. The realized heritabilities from within-family selection were 33 and 26% for the divergences on the normal and reduced protein diets, respectively. Consistent genotype-environment interactions were found when all lines were tested on both diets in generation 7. Performance on each protein level was best improved by selection on that protein level. Further, the correlated response was significantly less than the direct response when selecting on both diets. The estimates of the genetic correlation between growth on the two protein levels were low, r N = 016 from selection on the normal protein diet and r B = 0-51 from selection on the reduced protein diet. Selection resulted in a change in environmental sensitivity in the lines, dependent on the diet and direction of selection. The average of the divergences on the two diets was not dependent on the selection environment.

Previous experimental Salmonella infection studies in Denmark have shown that some p1gs remain 8 faecal culture negative and seronegative despite oral maculation w1th 10 c.f.u. S. Typhimunum and housing in highly contaminated pens, suggesting that some pigs are genetically res1stant to Salmonella Our study tested the following hypothesis: The Salmonella-negative status 1n certam p1gs 1s due to genet1c res1stance, related to a single gene The resistance gene was supposed to have a low frequency and to be recessive and that full resistance only would appear if both alleles were recess1ve. A challenge study was conducted to test this hypothesis. The pigs used were three bred crosses of Duroc boars and LY-dams. We infected 600 Salmonella-negative p1gs w1th approximately 10 9 S. Typh1munum via the feed at 15-20 kg live we1ght. On day 15, 22 and 29 post inoculation, p1gs were blood sampled and the sera were examined for Salmonella antibodies us1ng the Danish Mix-ELISA. From seronegative pigs and pigs with low antibody levels individual faecal samples were cultured qualitatively for Salmonella. In total , 7% of the 600 p1gs developed no or very low antibody levels, mdicat1ng genetic resistance; those p1gs were selected for the next study phase. In phase two, 22 resistant female pigs were mated with 17 resistant males. The1r secondgeneration offspnng compnsed 183 pigs, wh1ch were challenge 1nfected as described above The results showed that 7% of second-generation pigs had low antibody levels. Hentabillty was estimated to be 0 13. Our study shows that resistance to Salmonella has a genet1c background and IS most probably ruled by several genes.