The botanical and chemical composition of the diet selected by steers grazing Townsville stylo grass pastures during a period of liveweight gain.

Abstract

THE BOTANICAL AND CHEMICAL COMPOSITION OF THE DIET SELECTED BY STEERS GRAZING TOWNSVILLE STYLOGRASS PASTURES DURING A PERIOD OF LIVEWEIGHT GAIN R.A. HUNTER; B.D. S Summary Throughout a period of liveweight gain steers grazing a Townsville style/native grass pasture fertilized with superphosphate selected a diet with a higher nitrogen content and gained weight at a greater rate than steers grazing an unfertilized pasture of native grass alone. Townsville stylo made an appreciable contribution to the diet for approximately 3 months during which the cell wall content of the diet of the steers on the Townsville style/native grass pasture was lower than that of the steers on the native grass pasture. I. INTRODUCTION and M.J. BREEN* It has been shown by Edye, Ritson, Haydock and Griffiths-Davies (1971) that in northern Queensland. large increases in animal production are possible if native pastures are improved by the introduction of Townsville stylo and the use of superphosphate fertilizers. Animals grazing such pastures usually gain weight at a faster rate and for a longer period than animals grazing native pastures (Winks 1973). It has been assumed that the increased production reflects greater nutrient intake associated with higher voluntary intake and/or the selection of a diet of higher nutritive value. In this paper the botanical and chemical composition of the diet ' selected is examined. II. MATERIALS AND -METHODS (a) Period and location The experiment was conduct&d between January and July 1975 at the C.S.I.R.O. Pasture Research Station 50 km south of Townsville. (b) Pastures Two 4.8 ha paddocks, one containing native grasses and the other containing a mixture of Townsville sty10 (Stylosanthes humilis) and native grasses were selected. The yield of both pastures was such that quantity of feed on offer should not have limited animal performance. The native grass pasture had never been fertilized and consisted mainly of Heteropogon contortus and Bothriochloa decipiens. It had been mown to a height of about 10 cm following seasonal rains in early January. The Townsville style/grass pasture had received 125 kg/ha of superphosphate in each of the years 1967, 1968 and 1973. In addition to Townsville stylo, the predominant species were Heteropogon contortus and Digitaria ciliaris. Dead plant residues were burnt in early January. (c) Animals Twelve Droughtmaster steers, initially 15 months of age, were divided into 3 groups of 4 animals. Following rain at the beginning of the season, * --CSIRO Division of Animal Physiology, Davies Laboratory, Townsville, 4810. 457 one group grazed the perennial grass pasture, another the Townsville style/grass pasture while the remaining animals were oesophageally fistulated (OF). The OF animals alternated between the 2 pastures. (d) Live weight Live weight after an overnight fast was determined at 14 day intervals. (e) Sampling of pasture via oesophageal fistulae The diet selected by cattle in each pasture was estimated on alternate weeks, i.e. each pasture was sampled fortnightly. On 2 successive days midway between the fortnightly liveweight measurements, OF steers were fasted overnight and the next morning allowed to graze for approximately 30 minutes before the plugs were removed and unperforated plastic bags fitted. The animals were again allowed to graze until more than 300 g of extrusa were collected. During sampling, a flexible polyurethane foam plug was placed in the oesophagus posterior to the fistula. After collection the excess liquid in the extrusa was removed by light squeezing through nylon gauze. The solid fraction remaining was freeze-dried or dried in an oven at 70�C, ground through a 1 mm screen and stored for analysis. (f) Chemical analysis ' Dry matter (DM),- organic matter <OM), and nitrogen (N) content were determined as described by Siebert and Kennedy (1972) and cell wall content by the method of Van Soest and Wine (1967). (g) Botanical analysis Botanical composition of the extrusa was determined by the method of Harker, Tore11 and Van Dyne (1964). The botanical composition of the pasture on a dry matter basis was estimated monthly by the method of '.t Mannetje and Haydock (1963). III. RESULTS (a) Botanical composition Townsville stylo made an appreciable contribution to- the diet for approximately 3 months (Fig. 1). Although it accounted for 30 per cent of the dry matter of the pasture in early February, it was not selected to any extent until early March. From then until mid-May it contributed approximately 30 per cent of the diet. After that time even though 34 per cent of dry matter in the pasture was Townsville stylo, its contribution to the diet of the animals was minimal. From January through to early June the animals in both pastures . selected 'a diet which contained more than 70 per cent leaf. Thereafter, the proportion of leaf declined markedly to below 50 per cent by July. Similarly, prior to May green material accounted for more than 75 per cent of the diet. The amount of green herbage then constituted a decreasing proportion of the diet, falling to 25 per cent in early July. (b) Chemical composition Variations in cell wall content and nitrogen content of the two pastures are shown in Table 1. The cell wall content of the perennial grass diet changed from 68.0 to 81.4 g/100 g OM during the 5 months cf the experiment. The presence of Townsville stylo in the diet was associated with a lower cell wall content than when the diet comprised grass alone. During February and March, N content of the diet fluctuated because 458 of variations in pasture growth associated with rainfall. However, from the beginning of April, after which no effective rain fell, there was a rapid decline in nitrogen content. TABLE 1 * (c) Live weicrht Mean liveweight changes of the 2 groups of steers between February and July are shown in Fig. 1. The steers on the perennial grass pasture 459 gained weight until mid-May. On the pasture which included Townsville styls, the period of liveweight gain continued for an extra 3 weeks. From the commencement of the experiment until mid-May the daily liveweight gain, estimated from regression analysis, was 0.6 kg on the grass pasture and 0.8 kg on the Townsville style/grass pasture. The rates of gain were significantly different from each other (P<O.Ol). IV.DISCUSSION There were distinct differences in the botanical and chemical composition of the diet selected by steers grazing native grass and Townsville style/native grass pastures. The rate of liveweight gain of the steers on the 2 pastures was also different. From February to mid-May steers grazing the Townsville style/grass pasture gained 0.2 kg/head/day more than steers on the native grass pasture. For most of this period Townsville stylo made an appreciable contribution to the diet. The presence of the legume was associated with a higher nitrogen content and a lower cell wall content than when the diet consisted of grass only. The greater concentration of digestible cell contents in the Townsville style/grass diet probably contributed to the greater rate of liveweight gain on this pasture. Both pastures provided adequate nitrogen for growth during this period. Animals declined in live weight when they were unable to select a diet with a nitrogen content greater than approximately 0.9 g/100 g OM. Such a diet had a cell wall content greater than 75 g/100 g OM and consisted of at least 40 per cent stem, a very high proportion of which was dry stem. In contrast, in the first months of the experiment more than 80 per cent of the diet was green leaf. Although Townsville stylo made an appreciable contribution to the diet for only about 3 months of the year, the pasture in which it was present supported a greater rate of liveweight gain than the pasture of native grass alone. It is not certain if this was associated only with the presence of Townsville stylo as the pasture had been previously fertilized with superphosphate. This is a common practice. The additional 40 kg of liveweight gain by the steers on the Townsville stylo pasture is probably of marginal importance in fattening steers for market. However, in heife.rs, greater oestrus activity would be expected in animals of the higher live weight(Siebert and Field 1975). v. ACKNOWLEDGEMENTS The authors wish to thank Mrs. R. Steele for technical assistance and Mr. P.N. Jones for statistical advice. VI. REFERENCES EDYE, L.A. RITSON, J.B., HAYDOCK, K.P. and GRIFFITHS-DAVIES, J. (1971). Australian Journal of agricultural Research 22: 963. HARKER, K.W., TORELL, D.T. and VAN DYNE, G.M. (1964). Journal of Animal Science 22: 465. SIEBERT, B.D. and FIELD, J.B.F. (1975). Australian Journal of experimental Agriculture and Animal Husbandry 15: 12. SIEBERT, B.D. and KENNEDY, P.M. (1972)T=Australian Journal of agricultural Research 22: 35. 't MANNETJE, L. and HYDOCK, K.P. (1963). Journal of the British Grassland Society 18: 268. VAN SOEST, P.57 and WINE, R.J. (1967). Journal of the Association of official agricultural Chemists zg (1): 50. WINKS, L. (1973). Tropical Grasslands 2: 201. 460