**7. Effectiveness of selection for reproduction efficiency**

Fertility is a complex trait that has many components [28, 29]. Both male and female traits contribute and show considerable variability. Selection for both male and female fertility is therefore desirable [56]. Although the aim is usually to maximize the reproductive potential of beef cattle, more is achieved by optimizing rather than maximizing reproduction because the gross margin per cow increases parallel with the calving rate, but the margin per cow does not necessarily show the same response [58]. Fertility traits are heritable, but relatively few heritability estimates have been reported for fertility in beef cattle [28, 38]. In a review of fertility traits the heritability estimates for fertility ranged from ≤0.10 to ≥0.60 [38]. Unfortunately genetic improvement of fertility is hampered by a lack of information, low heritability and the delayed expression of the trait.

The heritability's of fertility traits are difficult to estimate because the expression of the reproductive potential is often constrained by management systems [29, 56]. Moreover, the underlying genetic merit for fertility is often not expressed, due to the threshold nature of fertility traits. There are only two outcomes possible for successful reproduction: Whether the cow is pregnant or not, degrees of pregnancy are not observable. The environment has a strong influence on which side of the threshold trait an individual falls [35]. The general consideration is that selection has a limited potential to improve fertility in beef cattle, while improvements in cow 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:

decline when the mean of a population is moved in either direction due to selection pressure [62]. The antagonistic relationship between fertility and milk production in dairy cows and the resource allocation theory support this theory [63]. The general consensus is that selection for increased body weight or growth rate may have an adverse effect on fertility [27] for the

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

http://dx.doi.org/10.5772/intechopen.72713

53

• Increased infertility is the result of the deviation from an optimum body weight that is as-

• Pleiotropic genes with opposite effects on growth rate and fertility may become important

• Major changes in body weight or growth rate may upset the natural homeostasis and endo-

• Selection for increased growth rate may result in indirectly selecting for feed intake and this may lead to the breeding of animals with a predisposition for high feed intake. Glut-

There is therefore a concern that selection for high growth rate might have negative effects on the fertility of cows [64]. However, contrasting results have been published which indicate that cows with a high pre-weaning growth, reared more calves over their lifetime, had lower calf mortalities and also calved earlier than cows with lower pre-weaning growth [65]. In another unrelated study the reproductive performance of Angus females selected for a high growth rate was similar to those of females where there was no deliberate selection pressure at all. The EBV trends obtained and presented in **Figures 4** and **5** for growth and reproduction traits in Bonsmara cows, indicate a negative correlation and warns against excessive selection for growth traits in extensive beef cattle, especially if the natural

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

crine balance which developed in each species over its evolutionary history.

tonous animals can become obese at maturity, which may influence fertility.

following reasons namely:

resources are limited [22].

**10. Conclusions**

sociated with an optimum degree of fitness.

as a result of prolonged selection.

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.
