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Two studies were carried out with pigs to determine the relationship between back test results and production parameters and between back test results and other factors. In the first study, 823 piglets were tested with the back test at 10 and 17 days of age. Production parameters such as average daily weight gain and lean meat percentage were determined. In the second study, the back test was performed on 566 piglets at 3, 10, and 17 days of age. The number of escape attempts in the back test (back test score) of the mother was known for 364 piglets. Parameters concerning the health of the sow and piglets were recorded, as well as the sow's reaction to piglet removal for testing. The relationships between production parameters and back test scores of the animals were calculated, as well as the influence of birth weight, sex (all males were castrated), parents, and health parameters on back test scores. Back test scores were fairly consistent over successive tests for each piglet. Males had higher back test scores than females, and piglets from sows with low back test scores also had low scores. Finally, a higher back test score was correlated with a higher lean meat percentage and a better carcass grading at slaughter; no relation with daily weight gain was found. It is concluded that there are individual differences in the way pigs cope with a stressful situation, as measured with the back test, and that this coping behaviour is consistent. A positive correlation exists between back test scores at a young age and lean meat percentage at slaughter. The response to stress, and hence back test scores, is assumed to be inheritable.

Disease resistance results from different processes which partly are immunological in nature. Immunological processes can be subdivided into innate (non-specific) resistance (phagocytosis) and acquired resistance. The latter can be subdivided in cellular and humoral immunity. These three compartments of the immune system do not act independently but are complementary. Parameters for each of the compartments have a genetic basis, and expression is affected by many genes, some of which are known. Experimental results suggest that especially aspects of innate disease resistance (the killing capacity of phagocytes) are negatively correlated with humoral immune response. Comparison of pig breeds supports this idea. On the other hand selection experiments show that simultaneous improvement of various aspects of the immune system is possible. Consequently, selection programmes should utilize index selection including parameters of all three compartments. Research in mice and various farm animal species studied genetic aspects of immune parameters. Little attention has been paid to the relationships between immune responsiveness and resistance to specific infections. To support the implementation of selection programmes for disease resistance in pigs, further experiments using e.g. mice as a model and the design of simulation models are useful.