REPRODUCTION is reviewed in the context of fitness and presents an overview relating to non-additive genetic variation, i.e. heterosis and inbreeding effects as well as additive genetic variation, i.e. between and within breed genetic variation and covariation (heritabilities and genetic correlation).
Estimates of heritability for female fertility traits in tropical cattle are low to moderate and there is more variation than for temperate cattle breeds, which suggests selection as a method of improving fertility in tropical cattle Selection for fertility has been demonstrated to be effective. Also, selection of bulls on scrotal circumference as a fertility indicator, appears to be a partial solution for genetically improving both male and female fertility. Selection for testosterone concentration is expensive and can be justified under special circumstances.
Since fertility and growth are considered to be two primary traits in a breeding strategy, relationships between the two are also discussed. From various studies under tropical and sub-tropical conditions it is concluded that there is no incompatibility between cow fertility and post-weaning growth.
Keywords: Fertility, beef, cattle, heritabilities, tropical environments, correlation, growth
Fertility of beef cattle in the tropics is characteristically low. The problem is largely environmental, mainly as a result of low quality nutrition during the dry winter months and limited intake during the hot and humid summer months. Fertility is a complex trait made up of several component traits and is greatly affected by differences or changes in the environment, especially nutritional environment. Fertility might be considered as two traits: inherent fertility and expressed fertility (Brinks, undated). Inherent fertility refers to the genetic potential for reproductive performance and is not directly measurable. Genes that affect overall physiological and endocrine functions may control inherent fertility and account for the generally favourable genetic relationships of measures of early reproductive fitness with growth, milk and overall productivity. On the other hand, expressed fertility can be measured by age at puberty, quality and quantity of spermatozoa, conception rate, etc. Expressed fertility is dependent upon the external environment and the environment (additional stress) created by the animals potential for growth. size or milk production. Under nutritional stress environments the relationship of productivity with expressed fertility may be antagonistic even though the relationship between interest fertility and productivity may be favourable. Thus, the observed genetic improvement in fertility traits may be less than expected even though the expected genetic progress is being made in inherent fertility.
One might think of improved inherent fertility as a protective mechanism. Under low stress conditions there may be little difference in expressed fertility between herds of moderate versus superior inherent fertility. However, under stress conditions (tropics and sub-tropics) herds with superior inherent fertility may have acceptable expressed fertility compared to those with lower potential. Herds with superior inherent fertility offer more flexibility in a given environment to increase growth and/or milk without sacrificing expressed fertility.
Prospects for genetic improvements of fertility have generally been considered to be limited because of the low estimates of heritability in Bos taurus cattle in temperate areas (Dearborn et al., 1973). However, heritability estimates from tropical and sub-tropical environments have reported moderate heritabilities of 0.36 for number of calves per lifetime (Davis, in press); 0.61 for age at puberty (MacNeil et al., 1984) and 0.25-0.34 for calving rate (Deese & Koger, 1967; Seebeck, 1973). Furthermore, there is greater phenotypic variability due to lower population means in these herds. Thus, the total amount of genetic variation may be high which emphasize that progress through selection is possible. Hetzel & Mackinnon (1989) have demonstrated that selection for fertility can be effective. Also, the repeatability of cow fidelity is moderate (Seifert et al., 1980) and continued culling of cows which fail to calve will increase herd fertility, both phenotypically and genetically (Seifert k Rudder, 1975).
Reported heritability estimates for bull fertility ranged from 0.0-0.17 and was highest in purebred Bos indicus cattle (Mackinnon et al., 1990; Meyer et al., l990), Swanepoel et al. (1986) concluded that scrotal circumference is a more reliable parameter for sexual development in B. inchcus than in B. taurus breeds. Heritability for scrotal circumference was higher than heritability estimates for bull fertility averaging 0.3l (Davis, in press) and as high as 0.68 (Coulter & Foote, 1979). All the studies indicate that scrotal circumference is a moderate to highly heritable trait (approximately 0.50) and that selection should be very effective in changing scrotal circumference. For testosterone concentration heritability estimates were also high, averaging 0.52 (Mackinnon et al., 1991). Thus, greater genetic variation in male reproduction traits is exposed by using measures more closely related to physiological processes associated with reproduction. However, selection for testosterone concentration is expensive and only justifiable under special circumstances. The use of scrotal circumference has the advantage of being able to be measured at a young age, seven months with a reasonable degree of accuracy (Swanepoel A. Hoogenboezem, 1994) and is highly correlated to sperm production (Swanepoel et. al., 1992a).
Possibilities for genetic improvement of cow fertility is more limited than fat bull fertility because:
Heritability estimates far female traits are low to moderate, whereas those for male traits are moderate to high
Selection for cow fertility can only be practiced in females, usually at lower selection intensities.
A further limitation is that in bulls one can select for scrotal circumference relatively early in life, whereas in the female only later in life because, to date, no reliable measure of fertility prior to mating exist in heifers
These limitations would be largely overcome, if male fertility traits are genetically correlated co female fertility However, culling for reproductive failure will improve herd fertility (Seifert & k Rudder, 1975).
Genetic correlations of
scrotal circumference with pregnancy rate of r = 0.62, with age at first breeding of
r = -0.55 (favourable), with age at first calving of r = -0.66 (favorable) and with calving interval of r = -0.42 (favorable at 365 days) have been reported by Toele & Robison (1985). These very strong relationships together with a correlation of 0.98 between scrotal circumference and age at puberty in heifers (Lunstra, undated) indicate that these two traits are essentially the same. Mackinnon et al. (1990) reported genetic correlations averaging 0.16, between scrotal circumference and components of cow fertility.
From these results it is clear that cow and bull fertility are favourable genetically correlated and that cow fertility can therefore genetically be improved by indirect selection on bull fertility. Generally, selection for increased testicle size would lead to improvement in cow reproduction, particularly calving rate, age at puberty and calving interval. Furthermore, the fact that the correlations (Toele & k Robison, 199S) are equally reliable when scrotal circumference was measured at 7 months or 12 months, again stressed the benefit of early selection in male animals. This is substantiated. by Swanepoel et al. (1992b) with their conclusion that male traits such as scrotal circumference can be used to speed up genetic progress in cow fertility.
The genetic and phenotypic correlations of scrotal circumference with measures of growth reported in the literature are generally favourable.
The genetic correlation between scrotal circumference and birth mass of 0.10 (Knights g g., 1984) appears to be relatively low whereas the correlation between scrotal circumference and yearling mass of 0.68 (Knights et al., 1984) is relatively high. This suggests that larger scrotal circumference (earlier puberty) and faster growth rate is compatible in young bulls (Swanepoel & Heyns, 1987). This is further substantiated by Meyer et al. (1991). Selection for increased scrotal circumference should result in increased growth from birth to yearling ages while keeping birth mass relatively constant. These relationships suggest a favourable growth curve, i.e. reaching a higher percent of mature mass at earlier ages while maintaining or increasing early growth rate and possibly holding mature mass under control.
Swanepoel et al. (1992b) suggested that there is no genetic antagonism between increased cow fertility and post-weaning growth of their calves. Meyer et al. (1991) reported a favourable genetic correlation between reproduction and growth traits in beef cattle, and Wolfe et al. (1990) concluded that selecting for weaning mass, final mass and muscling score had no detrimental effect on the age of puberty in beef heifers. MacNeil (1988) also reported that male progeny with relatively high growth rates were, produced by cows which tended to be more fertile,
From extensive reported research in the literature the following major conclusion can be drawn:
Heterosis for age at puberty in heifers is large (30-40 days earlier) and an increase in scrotal circumference of 13-17% in males can be expected. The effects of heterosis on fertility is twice as great in Bos indicus x Bos taurus breeds compared to pure Bos taurus crosses.
Beffa (1988) reported that inbreeding of dam displayed a marked depressive effect on calving rate, especially dams with more than 7% inbreeding. The effect of inbreeding on embryonic survival appears to operate in two phases, inbreeding of the mating affected early embryo survival, while inbreeding of the dam has a ]larger effect on later survival. Inbreeding of calf also has a detrimental effect on calf survival from birth to weaning. Calf birth weights were also depressed by inbreeding of the calf. Lubout (1987) reported that with Pedi cattle birth mass decreased as the level of inbreeding increased up to 18%.
A final conclusion from most of the reports in the literature regarding the effect of inbreeding on reproduction is that it is generally detrimental even in the presence of concurrent strict selection and therefore causes serious questions about its usefulness in beef cattle breeding.
Reproductive efficiency, obtained by cost effective means, is the most important factor affecting profitability in beef cattle breeding.
It would seem apparent that the breeding objective would be to improve inherent fertility to a high level.
Cow fertility can be improved in the tropics and sub-tropics by direct selection within a herd.
Age at puberty in both males and females appears to be a good indicator of inherent fertility and also of expressed reproductive efficiency, especially in the early productive years.
Scrotal circumference is an excellent measure of age at puberty in young bulls. Scrotal circumference is easy to measure, highly heritable and is favorably related to semen quantity and quality
Cow fertility can be improved by indirect selection in bulls. Scrotal circumference may be of particular interest to increase calving rate and to decrease age at first breeding.
Larger scrotal circumference (earlier puberty) and faster growth rate is compatible in young bulls.
Favourable association exist between lifetime cow fertility and growth traits of their progeny. Hence, joint selection for fertility and growth should improve the genetic potential for both.
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