Factors influencing the growth and development of meat animals

2.1 General

‘As an animal grows up two things happen: (i) it increases in weight until mature size is reached; this we call Growth and (ii) it changes in its body conformation and shapes, and its various functions and faculties come into full being; this we call Development’ (Hammond, 1940). The curve relating live weight to age has an S-shape and is similar in sheep, cattle and pigs (Brody, 1927). There is a short initial phase when live weight increases little with increasing age: this is followed by a phase of explosive growth; then finally, there is a phase when the rate of growth is very low.

When animals are developing, according to Hammond, a principal wave of growth begins at the head and spreads down the trunk: secondary waves start at the extremities of the limbs and pass upwards: all these waves meet at the junction of the loin and the last rib, this being the last region to develop.

The sequence of development of various muscles in the body reflects their relative importance in serving the animal’s needs. Thus, the early development of the muscles of the distal limbs confers the mobility required to forage for food; and the development of the jaw muscles promotes effective mastication of the food secured (Berg and Butterfield, 1975).

With the onset of sexual maturity, further differential muscular development occurs, whereby, in the male, the muscles of the neck and thorax grow relatively fast. These assist in fighting for dominance.

In most species of animals, although the female matures earlier, the male is larger and heavier than the female in adult life; and since the different parts of the tissues of the body grow at different rates, the difference in size between the sexes results in a difference in development of body proportions. Castration in either sex tends to reduce sex differences in growth rate and body conformation (Hammond, 1932a). Subjective assessment of the maturity of beef carcasses can be made from the colour of the cartilage at the tips of the dorsal spine of the sacral, lumbar and thoracic vertebrae (Boggs et al., 1998). The accuracy of the prediction can be increased by objective evaluation of the colour by image processing (Hatem et al., 2003).

Other as yet unidentified influences cause differences in the relative rates of growth of the individual members of the musculature. The pattern is both inherited and extraneously modified.

The establishment of different breeds of sheep, cattle and pigs is partly attributable to artificial selection practised by man under domestication, but the types of pre-existent animals from which such selection could be made have been determined by numerous, long-term extraneous influences, which continue – however much obscured by human intervention. These influences have caused overall alterations in the physiology of the animals concerned, involving the expression, suppression or alteration of physical and chemical characteristics. It must be presumed that such changes have been caused by mutations in the genes in response to the micro- or macro-environment and that they have been subsequently perpetuated by the genes.* In decreasing order of fundamentality, the factors influencing the growth and development of meat animals can be considered in four categories: genetic, physiological, nutritional and manipulation by exogenous agencies.

2.2 Genetic aspects

Genetic influences on the growth of animals are detectable early in embryonic life. Thus Gregory and Castle (1931) found that there were already differences in the rate of cell division between the embryos of large and small races of rabbits 48 h after fertilization. The birth weight of cattle and sheep, but not that of pigs, is influenced to an important extent by the nature of the respective embryos (Table 2.1). More recent data have also emphasized the high heritability of body composition traits in comparison with those of reproductive efficiency and meat quality characteristics (Table 2.2; Sellier, 1994).

Among the parameters affected at commercial level is the degree of fatness at comparable carcass weights or animal age. In Tables 2.3 and 2.4 respectively, some relative data for breeds of sheep and cattle are given. The leanness of the carcasses from crosses with the large continental breeds is evident.

At birth the pig is by far the most immature physiologically of the three domestic species. Differences in the physiological age at birth mainly depend on how great a part of the total growing period is spent in the uterus. The birth weight is influenced by the age, size and nutritional state of the mother, by sex, by the length of the gestation period (5, 9 and 4 months in sheep, cattle and pigs respectively) and by the numbers of young born (Pállson, 1955). An interesting aspect of this latter influence is the finding that embryos next to the top and bottom of each horn of the uterus develop more rapidly than those in intermediate positions (McLaren and Michie, 1960; Widdowson, 1971). The supply of nutrients to these embryos is particularly good since the pressure of blood is high at the top through the proximity of the abdominal aorta and at the bottom through the proximity of the iliac artery. Environmental and genetic factors are closely interrelated: favourable environmental conditions are necessary for the full expression of the individual’s genetic capacity. Irrespective of the birth weight, however, the rate of weight increase in young pigs is largely determined by the establishment of a suckling order: those piglets feeding from the anterior mammary glands grow fastest, probably because the quantity of milk increases in proceeding from the posterior to the anterior glands of the series on each side of the sow (Barber et al., 1955).

In general, the birth weights of the offspring from young mothers are lower than those from mature females and the birth weights of the offspring from large individuals are greater than those from small mothers.

Certain major growth features in cattle are known to be due to recessive genes. One of these is dwarfism (Baker et al., 1951), where the gene concerned (Merat, 1990) primarily affects longitudinal bone growth and vertebral development in the lumbar region, and males rather than females (Bovard and Hazel, 1963). Another is doppelender development (McKellar, 1960; Boccard, 1981), the gene concerned being mh (Hanset and Michaux, 1985). Neither has so far proved controllable. The...