Abstract:
Proc. Aust. Soc. Anim. Prod. (1972) 9: 314 THE NUTRITIONAL VALUE OF DIETS FOR WOOL GROWTH K. A. FERGUSON* Summary For sheep in energy balance at different liveweights, wool growth is proportional to the digestible organic matter intake. The non-protein portion of this intake has a relatively constant effect on wool growth, consistent with its effect on microbial synthesis in the rumen. The protein portion has a variable influence on wool growth, consistent with its degree of breakdown in the rumen and amino acid composition. I. INTRODUCTION The relative value of different diets for maintenance, growth and milk production of ruminants depends on the capacity of the diet to supply energy, but the qualities that determine the values of different diets for wool growth have never been defined, although it is well known that wool growth is closely related to feed intake (Marston 1948; Ferguson, Carter and Hardy 1949). Thus there has been no theoretical basis for evaluating hand-fed diets or pastures for wool growth. Ferguson ( 1959) showed that wool growth is insensitive to the dietary crude protein percentage. However, the infusion of protein, cystine or methionine, into the abomasum stimulates wool growth (Reis and Schinckel 196 1, 1963, 1964), and it is likely that additional protein supplied in the diets used by Ferguson did not reach the abomasum (Hogan and Weston 19687). The protection of dietary protein from microbial fermentation in the rumen also increases wool growth (Ferguson, Hemsley and Reis 1967; Reis and Tunks 1969) and increases the protein digested in the small intestine (Hemsley, Hogan and Weston 1970). As well as being influenced by the supply of amino acids, wool growth is also affected by the proportions of different amino acids absorbed from the small intestine (Reis and Schinckel 1963; Reis and Colebrook unpublished). The major sources of the amino acids absorbed are microbial protein synthesized in the rumen, and feed protein which escapes fermentation in the rumen. The non-protein fraction (DNPOM) of the total digestible organic matter (DOM) intake can only contribute to the first of these sources, whereas the protein fraction (DCP) can contribute to both. * Ian Clunies ROSS Animal Research Laboratory, CSIRO Division of Animal Physiology, P.O. Box 239, Blacktown, N.S.W., 2148. TABLE 1 Composition and intakes of experimental diets In this paper the dependence of wool growth on these two fractions of digestible organic matter intake is examined for normal diets, and for diets treated with formalin to protect the protein against fermentation in the rumen. Such a partition of digestible nutrients allows examination of the effects on the wool growth response of dietary protein percentage, protein source and the effects of protection of the protein against fermentation in the rumen. II. MATERIALS AND METHODS Adult medium-wool Merino ewes (experiments 1-3) and wethers (experiments 4-6) were maintained in indoor pens and fed once daily. Wool growth was measured every 2 or 4 weeks on 10 x 10 cm sample areas delineated by tattoo lines on the midsides. The ratio of total to sample wool growth was measured over a 12 month period for the ewes in experiments l-3. For the wethers in experiments 4-6 the ratio was estimated from the equation Wt/Ws = 6 Bs where Wt is the total wool growth rate, Ws the rate on the sample area and B is the live weight at the time the 10 x 10 cm patch was delineated. Wool samples were cleaned by successive washes with neutral detergent (Non-Idet P40, Shell Chemicals), water and absolute ethanol. The relation between growth and nutrient intake was estimated from the data obtained on diets of different composition and fed at different intakes. Since the wool growth response to a particular intake depends on the energy balance (Ferguson 1962)) comparisons were restricted to intakes and live weights where energy balance judged by liveweight change was approximately zero, the liveweights maintained ranging from 24 to 64 kg on a fleece-free basis. The higher intakes in experiments 3-6 were ad lib. for most sheep. The diets and intakes used for the different sets of experimental data are given in Table 1. The sheep were fed for long periods to attain constancy of liveweight, but only the data for the end of the period during which digestibility was measured are given. In experiments l-3, published values were used for the digestion coefficients of organic matter and crude protein (Schneider 1947) which were applied to chemical analyses of aliquots of the diets fed. In experiments 4-6 the digestion coefficients were measured during the experimental period on at least two animals on each diet at each level of feeding. The effect of feeding level on digestibility was small, and pooled estimates of the digestion coefficients were used for each diet. III. RESULTS Figures 1 and 2 indicate that, with one exception (diet F38T), increasing feed intake caused a proportional and linear increase in wool growth for formaldehydetreated and untreated diets under conditions of approximate energy balance, compared with the curvilinear relationship found under conditions where liveweight is changing differentially in response to different intakes (Ferguson, Carter and Hardy 1949; Ferguson 1962). There was little difference in the response to the 316 (a) Wool growth (b) Diets (a) The relation between wool growth and DOM intake 317 untreated diets of different protein content. For the treated diets, the conversion of DOM to wool increased from 1.7 per cent to 3.2 per cent with increasing protein concentration. (b) The partial regression of wool growth o'n intakes of DNPOM and DCP It may be expected that the linear and proportional dependence of wool growth on DOM intake will extend to the two separate fractions of DOM intake if the ratio of these two fractions in the diet does not change. However, it is not possible to measure the separate effects of DNPOM and DCP intakes on wool growth unless the ratio does change. Analysis of such data by partial regression techniques is complicated by the high inverse correlation between the intakes of DNPOM and DCP unless feed intakes of different diets are chosen to avoid this, as in experiments 4 and 5. However, it is possible to calculate the partial regression coefficients for constant intakes of diets of different ratios of DNPOM and DCP (Experiments 1-3) if wool growth is proportional to the intakes of DCP and DNPOM; The partial regression coefficients of equation ( 1) k, and k,, can be estimated by calculating the regression of W/DCPI on DNPOMI/DCPI. If wool 318 growth is not separately proportional to the intakes of DCP and DNPOM, the regression of W/DCPI on DNPOMI/DCPI will not be linear. Figure 3 shows the regression for the combined data of the formaldehyde-treated diets in experiments 4 and 5. There is no significant departure from linearity, and this was also true for the other experiments where at least 3 points on the curve were available (Experiments 1 and 2). Thus it may be concluded that wool growth is linearly proportional to both fractions of digestible organic matter intake where the increasing intake of DCP consists largely of protein of the same composition as in both series of diets. TABLE 2 Partial regression coefficients of wool growth on intakes of DCP and DNPOM The values of k, and k,, for each experiment are shown in Table 2. k,, is similar for the formaldehyde-treated and untreated diets, but k, is considerably increased by formaldehyde treatment. IV. DISCUSSION The relative constancy of the effect of DNPOM intake on wool growth is consistent with the evidence that this fraction of DOM intake is almost completely fermented in the rumen and thus generates a fairly constant yield of microbial protein. The effect of DCP intake on wool growth is, however, markedly increased by protecting the protein against fermentation. In other experiments it has been found that protein from different oil seed meals vary in their effects on wool growth (Colebrook et al. 1968), possibly due to different degrees of protection brought about during manufacture of the meals. The effect on wool growth of formaldehyde treatment of these meals also varied (Ferguson, Hemsley and Colebrook unpublished), and this may in part reflect differences in essential amino acid composition of the meals. The results with the series of formaldehyde-treated diets shows that the wool growth value of a diet increases with increasing DCP content at least up to 319 about 60 per cent of the DOM, i.e., well beyond the level attainable with extracted oil seed meals, and well beyond the level necessary for maximum growth rate or milk production. The nutritional efficiency of wool growth under equilibrium conditions does not decline appreciably with increasing intake. Thus if it is profitable to feed a supplement in terms of the wool growth response, it will be most profitable to feed the supplement ad lib. so as to maximize the return on non-feed costs. The sheep used in the present experiments were not selected for high wool growth rates. Individual wool growth efficiencies on the formaldehyde-treated diet F38 ranged from 1.5 to 4.5 g per 100 g DOM intake. V. REFERENCES Ferguson, K. A. (1959). Nature, London. 184: 907. Ferguson, K. A. (1962). Australian Journal of Biological Sciences. 15: 720. Ferguson, K. A., Carter, H. B. and Hardy, l&I. H. (1949). Australian Journal of Scientific Ferguson, K. A., Hemsley, J. A. and Reis, P. J. (1967). Australian Journal of Science. 30: 215. Hemsley, J. A., Hogan, J. P. and Weston, R. H. ( 1970). Procedings of Eleventh International Hogan, J. P. and Weston, R. H. (1967). Australian Journal of Agricultural Research. 18: 973. Marston, H. R. (1948). Australian Journal of Scientific Research. Bl: 362. Reis, P. 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