Abstract:
253 Wool, isotopes and diet selection in grazing sheep P.J. Murray , G.B. Witt , and E.J. Moll 1 2 2 l Department o f Animal Production, The University of Queensland, Gatton College, Lawes QLD 4345 *Department of Natural Systems Management, The University of Queensland Gatton College, Lawes QLD 4345 The ratio of 13C:l2C (dl3C %o; number of 13C atoms Per 1000 atoms of carbon) has been used to estimate the proportion of C3 (most woody species, forbs and temperate grasses) and C4 (tropical and subtropical grasses) species eaten by grazing animals in dietary and metabolism studies (Jones et al. 1979, Tyrrell et al. 1984). The photosynthetic pathway of a plant is primarily responsible for the dl3C of its tissue, and this ratio is not significantly altered when the plant matter is converted to animal tissue. Thus, an animal on a pure C, diet would have tissue dl3C values in the order of -13%0 (per ml, relative to the PDB standard) while animals consuming only C3 plants would be approximately -27%0 (Minson et al. 1975). Wool is a major carbon sink in sheep (Reis 1979). Given that wool grows continuously and that skin is metabolically very active, the carbon in wool fibres should represent a permanent ongoing record of the isotopic composition of a sheep's diet. This was tested experimentally. Our results have implications for animal nutrition studies. We also suggest that animal tissue which is laid down chronologically (such as wool) can be used to monitor the availability of grasses in sub-tropical and tropical environments (see also Witt et al. 1997). Two sheep, individually penned, were fed 2 diets in a switch over design. Diet 1 (C, diet) was a mixture of lucerne chaff and oat straw (2:5). Both ofthese are C3 plants. Diet 2 (C, diet) was lucerne chaff (C,) and rhodes grass (C,) mixed at 2:5. Both sheep were fed their respective diets for 6 weeks with a 3-day change over at the end of the 4th week. At the end of the 6th week, diets were switched between the sheep and the experimental procedure outlined above was repeated. Wool was clipped every 3 days from a midside patch, scoured and analysed for its stable isotope ratio (reproducibility better than =t 0.1%0). Feed intake was recorded and samples of the diet were analysed to determine their 613C values. Figure 1 shows the relationship between the 613C of clipped wool and the changing isotope composition of the diets. Our results suggest that wool is a rapid assimilator of dietary carbon. After the major change over, at week 6, most carbon in the wool is derived from the new diet within a 2-week period. The transition to the new isotopic equilibrium in the wool resembles an exponential decay function (see 613C of wool in Figure 1 between days 50 and 75). Clearly the 613C of wool does not reach isotopic equilibrium with either diets 1 or 2. Wool is approximately 3%0 depleted in 13C relative to the C, diet while being 3%0 enriched relative to from the C3 diet. We attribute this to 2 main factors: 1 The C4 feed mixture contained rhodes grass chaff (C,) which was low in digestibility relative to the lucerne (C,). For this reason the proportion of carbon derived from rhodes grass would be much lower than the amount contained in the bulk feed. 2 There are minor variations in the isotopic composition of compounds within a plant which may have been preferentially taken up by the sheep. We conclude that animal tissue which is laid down chronologically (such as wool) can be used to monitor the relative intakes of sub-tropical and tropical grasses. References Jones, R.J., Ludlow, M.M., Troughton, J.H., and Blunt C.G (1979). Estimation of the proportion of C3 and C4 plant species in the diet of animals from the ratio of natural 1% and 13C isotopes in the faeces. Journal ofAgricultural Science (Cambridge) 92,9 l-l 00. Minson, D.J., Ludlow, M.M. and Troughton, J.H. (1975). Differences in the natural carbon isotope ratios of milk and hair corn cattle grazing tropical and temperate pastures. Nature 256, 602. Tyrrell, H.F., Pelletier, G, Chevalier, R., Hillaire-Marcel, C. and Gagnon, M. (1984). Use of carbon 13 as a tracer in metabolism studies. Canadian Journal of Animal Science 64,127-l 29. Reis, P. J. (1979). Effects of amino acids on the growth and properties of wool. In: The PhysioZogicaZ and Environmental Limitations to Wool Growth (Eds. P.J. Reis and J.L. Black). University ofNew England Publishing Unit: Armidale. Wilt, GB., Moll, E.J., Beeton, R.J.S., and Murray, P.J. (1997). Isotopes, wool and rangeland monitoring: let the sheep do the sampling. Environmental Management. (m press). Recent Advances in Animal Nutnlion in Australia 1997 University of New England, Armidale NSW2357, Australia