**10. Conclusions**

and environmental management hold much promise to optimize cow reproduction [22]. One of two approaches is often recommended when selecting for improved fertility [35] namely:

Step 1. The *direct* approach involves the physical selection for fertility traits. This should include traits such as scrotal circumference, age at puberty, age at first calving as well as calving date and the proportion of heifers in production at a given age. The use of any prospective fertility trait depends on the ease of measurement and the inherent relationship with fertility. Step 2. The second or *indirect* method proposed is to use an array of traits that indirectly affect fertility, such as milk production, growth rate, calving ease and body condition. Selection for optimum combinations of these traits should create a favorable "genetic environment" for fertility.

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

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,

Information on the effects of selection for body weight or growth rate on reproductive fitness in cattle is unfortunately limited [27]. In a fundamental theorem of natural selection in the 1930s it was already postulated that reproductive fitness and body weight will be near the peak of fitness in a natural population [61]. However, when selection for growth takes place, the population is no longer in a natural equilibrium, so the reproductive fitness may in fact

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

52 Ruminants - The Husbandry, Economic and Health Aspects

size in the long run increases the time taken to reach maturity.

accelerated growth, development, adaptation and reproduction [3].

**9. Growth rate and reproduction**

This study investigated the effects of different bioregions in the semi-arid parts of Southern Africa on the growth, size and reproduction efficiency Bonsmara cows. This study employed novel techniques to investigate the influence of production environment on the growth, size and reproduction efficiency Bonsmara cows. Results indicate that bioregions affect the growth, size and reproduction efficiency of beef cows, and provide evidence for the existence of an optimal cow size for different bioregions. Results revealed a complicated relationship between bioregion and the growth, size and reproduction efficiency of Bonsmara cows. The proportion of variation in cow growth traits due to the regional distribution of cows, depended on the physiological stages of growth, e.g., birth, weaning, 12- or 18-month growth stages. In terms of production efficiency, weaning- and yearling weights as well as AFC and RI were influenced most by differences in regional distribution of cattle. Management practices and breeding objectives have a major effect on the efficiency of beef cow production efficiency. The effective implementation of management practices, such as the provision of nutritional supplementation at weaning and yearling age, limits the negative influence of regional effects on cow growth and size.

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Effect of Bioregion on the Size and Production Efficiency of Bonsmara Cattle in Semi-Arid Parts…

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Genetic trends indicate that the efficiency of growth improved remarkably during the past 20 years in the Bonsmara cattle breed. However, improvements in growth and efficiency, were associated with a decline in reproductive characteristics. Reproduction efficiency is the single most important aspect of beef cow efficiency and breeders should guard against indiscriminate selection for growth traits, which may adversely affect reproduction performance, especially since much research endorse the existence of a negative relationship between growth and reproduction traits. Although the common belief is that smaller cows reproduce better in more resource constrained regions, the current data indicate that composite type cows of medium size had the best reproduction rates. The reproductive ability of larger size cows improves markedly with improved management and strategic feed supplementation.
