Bone and muscle growth in sheep : with particular reference to the effects of nutrition and intestinal parasitism

Two studies were undertaken examining change in bone and muscle under different growth regimes. In the first study, ontogenetic growth of bone and muscle was examined over the range of liveweight (LW) 5 kg to 67 kg. A second group of animals were subjected to a 56 day period of LW stasis at 20 kg LW through controlled undernutrition, followed by recovery growth over the range 20 kg to 67 kg LW. The second study employed a similar design but substituted infection with intestinal parasites for simple undernutrition. A novel approach was developed for description of change in geometric form in bone and muscle. Measurement of linear dimensions, volume and weights of chemical components allowed construction of indices highlighting the impact of changes in form and composition on tissue weight. This provided a more mechanistic and objective description of growth and change in these tissues than was previously available. From the study of ontogenetic growth it was shown that both bone and muscle exhibit significant changes in shape over the range of growth examined (10 to 45 kg LW). In bone, volume⅓ showed relatively lower growth than most linear dimensions, while density increased. However, the former phenomena had a greater effect on bone weight than the latter leading to a reduction in weight relative to external linear dimensions (mean change = -17%). Muscles studied demonstrated negligible change in density but did show substantial changes in shape. For two muscles of the thigh, M. semitendinosus and M. rectus femoris, weight increased relative to length (mean change = +18%) while the muscle of the vertebral column, M. longissimus dorsi, showed the opposite effect (change = -16%). For three of the four bone-muscle pairs studied, the net effect of these changes was to increase gravimetric muscle: bone ratio. In consideration with data from the literature it was concluded that changes in tissue shape make a substantial contribution to increases in the gravimetric muscle: bone ratio of the body during ontogenetic growth. During restricted growth, muscle displayed marked changes in shape. While absolute volumes decreased (mean change = -17%), consideration of change relative to length revealed larger effects (volume: length³, mean change = -33%). In contrast, bone largely showed changes in composition. Pronounced increases in bone fat were explained on the basis of fat replacing protein and ash within bone. This was supported by the data of Trial Two which showed that in animals subject to similar changes in BW, there was a 30% reduction in width of the femur cortex while external bone dimensions increased. The data highlight the inadequacy of weight as a measure of change in tissues under such a growth regime. Certainly, bone was shown to make a dynamic response to undernutrition through changes in composition and the spatial distribution of material. Change in bone weight failed to provide any indication that these effects had occurred. Recovery growth was characterized by significant changes in relative growth favouring restoration of normal body and tissue form and composition. In some cases this led to "classical" compensatory growth ie. greater than normal growth in absolute terms, but recovery was predominantly a function of changes in relative growth. Despite severe depletion of body fat (chemical), growth of fat was slow during early recovery. However, relative growth of fat was high suggesting that rate of fat accumulation may be related to the size of fat depots. A second study examined the effect of intestinal parasitism on bone and muscle growth. Young animals (c.25 kg) were subject to an 83 day period of infection (4000 Trichostrongylus colubriformis larvae/animal/day), followed by recovery growth (INF group), while a control (CTRL) group underwent continuous growth. A further group (PW) were individually paired with infected animals on the basis of change in LW. Despite documented effects of parasitism on protein and mineral metabolism, no specific effects were observed in bone ie. bone exhibited a response to undernutrition that was similar in INF and PW groups. In contrast muscle was depleted to a greater degree in parasitized animals (average weight change -36% and -24% for INF and PW groups, respectively). However, effects attributable to infection were generally small compared to those attributed to simple restriction of growth. This general restriction of growth could be attributed to reductions in voluntary feed intake and in the efficiency with which nutrients were utilized. There was some evidence that the deleterious effects of parasitism on muscle (but not of undernutrition) were still apparent in animals slaughtered at 45 kg L W. Overall, these studies demonstrated the inadequacy of tissue weight as a sole measure of change and the significant contribution that can be made to the understanding of change through additional consideration of geometric form and chemical composition of tissues. Applications for such an approach to other studies of growth are discussed.