Rauw et al. (1998) have shown that genetic selection of livestock species for high production is often compromised by physiological and immunological problems. For example, a faster growth rate in broiler chickens runs parallel with an increased incidence of ascites (Scheele, 1996). Breeding for high lean tissue growth rate in pigs prolongs in many cases the weaning to farrowing interval (Ten Napel, 1996). In general, high producing dairy cows are bred later, show more days open and require more services per conception than low producing cows (e. g., Berger et al., 1981). These undesirable side effects of selection may result from an unbalanced resource situation. Resources come from food intake or body reserves. Input constraints (food intake, digestion and absorption) are engaged in series whereas outputs (maintenance, growth and production) are parallel (Figure 1). If the sum of output rates does not match the input, the balance is buffered by body reserves. In the long run, however, energy expenditure must balance energy intake (Weiner, 1992). When ‘genetically forced’ to produce highly, disproportionally many resources may be (re)allocated towards the production trait, leaving the animal lacking in ability to respond to other demands (Beilharz et al., 1993; Rauw et al., 1999). When resources are limited, the body may be economising on the level of protein turnover, because this is a very expensive metabolical process. 

W. M Rauw, P. W Knap, Luis Varona, L. G Raya, J. L Noguera

Proceedings of the World Congress on Genetics Applied to Livestock Production, Volume 2002. Session 10, , 10.03, 2002
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