**8. Influence of selection for growth on beef cow efficiency**

Growth traits are highly heritable, with heritability's ranging from 0.24 to 0.61, so fast genetic progress is possible when animals are selected for growth rate [59]. Selection for growth is complex, since traits like birth and weaning weight are determined by the animal's own additive genetic merit as well as the maternal component, which can be further separated into an additive genetic and a permanent environmental component [51]. It is well known that selection for a higher growth rate eventually increases the mature size of animals, which is due to the positive correlation between weights at different ages [59]. There is also a negative correlation between mature size and age of maturation, which means that selection for larger size in the long run increases the time taken to reach maturity.

Genetic change in the shape of the growth curve is limited by the degree of genetic flexibility in the shape of the curve, which depends on the degree of interdependence of the size, rate and inflection of the growth parameters [3, 60]. Although theoretically possible, the basic shape of the sigmoidal growth curve as well as the sequence of physiological events remains virtually unchanged. The rate of these processes has however increased remarkably over the past few decades [3]. In fact, selection for increased body weight or growth rate may have an adverse effect on body composition, fertility and survival rate [27]. It was suggested that selection should rather be focused on increased feed efficiency because it may lead to fewer adverse effects. Some researchers also postulated that selection for growth and efficiency may have reached the physiological limits of animals to cope with the demands of maintenance, accelerated growth, development, adaptation and reproduction [3].
