SIDE-EFFECTS OF SELECTION FOR GROWTH IN LABORATORY-ANIMALS

Correlated responses are examined from a biological rather than from a biometrical point of view. Selection for increased body weight in laboratory animals usually, though not always, leads to increases in food intake, gross efficiency and fat deposition, while some aspects of fertility are usually impaired. Increases in fatness are more pronounced in older animals, possibly because selection for early growth requires a high food intake which is not correspondingly reduced as the accretion of lean tissue slows down. Further, fat deposition may be an alternative to heat output, which explains why increases in both fat deposition and in gross efficiency may not be incompatible. It is concluded that many of the adverse side effects of selection for growth are the physiological consequences of increased fatness. In terms of applications to domestic livestock, it is suggested that the undesirable side effects should be controlled managementally by restricting food intake, on the grounds that the simultaneous avoidance of deleterious effects would unduly impair the efficiency of selection for increased growth. © 1979.