Introduction

The nutritive value of pasture plants is influenced by the concentrations of nutrients in the feed, the amount consumed, the digestibility of eaten components and the efficiency with which absorbed nutrients are utilized. While a lot is known about the nutrient content and laboratory estimates of digestibility of pasture plants, there is little information on the factors affecting voluntary consumption or the performance of grazing livestock. It is clear that nutrient intake is the most important factor influencing the performance of grazing animals and that the amount of pasture on offer can impose limitations on this. However even when there is no restriction on the availability of feed, herbage intake may be markedly influenced by the digestibility and metabolisability of the diet (1).

From an animal's point of view, the ideal pasture plant should be available in abundant quantities, readily consumed, highly digestible, and contain adequate nutrients for efficient digestion and metabolism. Ideal pastures are seldom found in practice. In the Mediterranean environment of south-western and southern Australia, production from grazing animals is constrained by the seasonal pattern of pasture growth. During the late autumn/winter the green feed is of good quality but there is insufficient for optimal animal production and in summer/autumn livestock production may be limited by the quality and/or quantity of dry feed on offer (2, 3). In spring, when there is normally sufficient good quality pasture available, problems may still occur through deficiencies of specific nutrients (eg minerals) or intake of feed may be influenced by stocking rate. However, these are of minor importance compared to the limitations in other seasons of the year.

Variations in the amount of pasture on offer and its quality over the year result in marked variations in wool growth rate and fibre diameter (2, 3) (Figure 1). Wool growth rates can vary from 8-12 g/sheep/day in winter to 16-20 g/day in spring and only 8 g/day in summer and autumn. Obviously, the length of the growing season and rates of pasture growth achieved will influence the extent of the variation in wool growth rate and fibre diameter. This variation in wool growth reduces the value of the fleece and emphasizes the importance of a more even plain of nutrition throughout the year.

Stocking intensity and grazing management can also influence the nutritive value of pasture species through altering the characteristics of the feed on offer and the extent to which animals are able to select the various components. It is therefore necessary to consider livestock production, not only from the point of view of individual animals, but also as production per unit area or per amount of pasture produced.

This paper considers the major factors determining nutritive value of pastures for sheep in the medium and high rainfall regions of south-western Australia. Attention is given to the requirements for pasture species which we should aim to develop in order to overcome the limitations that exist.

Figure 1. Seasonal pattern of wool growth rate and fibre diameter. Purser and Southery (1984)

Pasture growth rate

During the late autumn/winter/spring the amount of feed on offer has a major effect on feed intake. This is determined by the rates of pasture growth and the rates of removal through intake by sheep, through trampling and through natural decay. Many factors interact to determine pasture growth rates (4). These include the seed reserves of each pasture species, the proportions of different species in the sward after germination, soil nutrient status (including residual nutrients and those applied in fertilizer), soil type, structure and temperature, the amount and distribution of rainfall, the amount of light and stocking rate. There are marked differences in the total amount of pasture grown and in the distribution of that growth between locations (Figure 2). Even at the same location, the duration of the growing season and the total amount of dry matter produced varies greatly from year to year.

Figure 2a. Amount of pasture produced at Avondale and Vasse in 1985.

Figure 2b. Amount of pasture produced at Avondale and Vasse and Mt Barker
in 1986. Curtis and Albertson (Unpublished data)

However, it is clear that in general the main limitations to the nutritive value of pastures in the medium to high rainfall zones of south-western Australia are early growth rates and the quality and quantity of senescent feed available in summer/autumn. This paper considers these constraints to the nutritive value of pastures for sheep in these regions. Approaches to overcoming these major limitations are discussed and attention is given to selection between and within pasture species to aid in reducing the level of constraints.

Early pasture growth in late autumn/winter

During the late autumn/winter period, intake of nutrients by sheep is limited by the amount rather than the quality of feed on offer. This can be illustrated by measurements made with ewes grazing Phalaris aquatica and Trifolium subterraneum dominant winter pastures during late pregnancy (5). With only 500 kg DM/ha on offer, the intakes of pasture and digestible energy by ewes at high stocking rates was less than requirements for maintenance (Table 1). The ewes at the higher stocking rate were not consuming enough energy for maintenance and foetal growth. This situation would be exacerbated after parturition with increased energy requirements for lactation. At the lower stocking rate, ewes were consuming enough energy for maintenance and foetal growth. However, in lactation, requirements would increase to over 18 MJ ME/day and whether they could consume sufficient feed would be dependent upon pasture growth rate. Situations of low feed availability during autumn/winter are common in south-western Australia and are considered to be the major factor limiting carrying capacity.

Table 1. Intake of Phalaris aquatica/Trifolium subterranean pasture by ewes in late pregnancy in relation to their energy requirements (5)

The early feed deficit during autumn and winter can be overcome in a number of ways:

• by supplementary feeding;
• by incorporating pasture species which exhibit rapid early growth; by incorporating pasture species whose growth habit makes them more accessible;
• through the use of perennial species in the higher rainfall zones to achieve some growth in the summer/autumn;
• through use of fertilizers to promote early growth.

Grasses such as Wimmera rye grass and large seeded species, such as oats or barley, exhibit superior early growth rates and are more accessible when compared to clovers. While this may not always be the case when individual plants are compared, greater densities of grasses contribute to greater early production by these species. Feeding value (as distinct from nutritive value) refers to a production response by animals when feed availability is not limiting. Estimates of feeding value during winter, indicates that annual rye grass compares favourably with clover (2) (Figure 3). This together with the greater growth rate and accessibility indicates the superiority of grasses over clovers in nutritive value early in the growing season. Obviously, other factors need to be considered when selecting grass species to be incorporated into the pasture mix. For example, the occurrence of annual ryegrass toxicity may limit the use of this species in some areas.

Figure 3. Feeding values of various pasture plants relative to the feeding value of clover for winter, spring and summer. Calculated from data of Biddiscombe et al. (1976). Source: Purser (1980)

Preliminary information on the South Coast indicates that the perennial grass species which are currently available do not produce as well as annual species early in the season (Cransberg, unpubl. data). Information on the relative quality and feeding value of annual and perennial grasses early in the season is not available at this time. Even if perennials are of higher quality, which is unlikely, this would not offset the effects of lower production on nutritive value.

Early production of pastures can be further enhanced by application of fertilizers. While considerable controversy exists over responses in winter growth due to current applications of superphosphate, as opposed to responses due to residual value of previous applications, there is little doubt that grasses will respond to strategic additions of nitrogen (6, 7). Applications of nitrogen in the autumn have increased sheep liveweights through the autumn/winter and increased annual wool production (8). However, continued use of this strategy on the same pastures would lead to grass dominance and decreased livestock production. Thus, a balance needs to be struck between increased early pasture growth, production over the whole season and the quality of feed left for the summer/autumn. Strategic use of fertilizer on appropriate pastures provides a useful method of increasing the amount of early feed on offer, particularly in years when there is an extended period of warm weather and favourable rains during autumn.

In terms of liveweight gain, benefits of early pasture growth have been shown in some grazing trials (9), but these benefits may not always be as great as expected. Compensatory growth of animals during spring, which is accompanied by higher feed intakes, greatly diminishes differences in liveweight and to a lesser extent fleece weight. However, less variation in fibre diameter along the staple can be expected when the autumn/winter nutrition of the animal is improved and this would increase the value of the fleece. While autumn lambing has many disadvantages, it is still the preferred option in many areas. Early pasture growth would also reduce the nutritional stress on ewes and lambs during this period, increase production and perhaps reduce the incidence of tender wool.

Pasture species differences in spring

The relative feeding value of some grasses compares favourably with that of clover during spring (2) (Figure 3). However, there is evidence to suggest the intake of different subterranean clover varieties varies at this time (2). It appears that high contents of some isoflavones may affect the acceptability of some cultivars. While anti-nutritional factors may result in a detrimental decrease in intake during the growing season, this may be of advantage due to improved persistence of legumes and the additional availability of dry feed in summer when the feeding value of clovers is superior to that of grasses.

Pasture quantity and quality in summer/autumn

When the amount of feed on offer is not limiting clover is likely to have a higher relative feeding value than annual grasses and herbs (2) (Figure 3). In the higher rainfall zones, summer active perennial species can provide more digestible energy and essential nutrients per unit weight than senescent or dead annual herbage. In this context, it is relevant to consider the role of perennial plants and the factors which contribute to

variability in the nutritive value of senesced annual plants.

Summer active perennial plants

Where there is sufficient summer rainfall, summer active grasses, such as Kikuyu (if persistence is not a problem) and those which respond to summer rain, such as Brumby (a long-term perennial ryegrass) and Concord (a short term perennial ryegrass), have potential to increase the nutrient intake of sheep during the dry period. Small amounts of green feed provided by such species will alleviate the nutrient deficit faced by sheep in two ways. They will provide more digestible energy and nutrients than equivalent amounts of dead annual pasture and are likely to stimulate the intake of the annual feed through the supplementation effects of the critical essential nutrients they contain. The major problems with these pasture species are their establishment and persistence. Clearly, persistence will only be achieved in the Mediterranean environment through the development and adoption of appropriate grazing management practices. In the past, summer dormancy has been seen as critical to the persistence of perennial grasses in Mediterranean environments. This is because growth with limited rain in the summer can deplete plant reserves. Summer dormant perennials have a role in situations where false breaks deplete the seed reserves of annual species.

Perennial legumes, such as lucerne (Medicago sativa), tagasaste (Cytisus proliferus) and perennial lupins (Lupinus spp.), also offer the potential to improve the quality of feed available through the summer/autumn. Lucerne will only persist under sheep with rotational grazing of sufficiently low grazing pressures. Its value as a supplement to dry feeds has been clearly demonstrated in pen feeding studies (10, 11, 12). However, responses in grazing sheep have not always been as promising (13).

There has been a recent resurgence in interest in perennial shrubs. One of the reasons for this is their potential value as a high quality feed resource over the dry period when grown on appropriate soil types. It is likely that when standing tagasaste is grazed, the spatial distribution of edible parts will limit the amount consumed by sheep. An alternative method of feeding by lopping the shrubs may reduce this problem (14), but specialized machinery will be necessary when the shrubs have matured. Finally, the nutritive value of tagasaste is limited by the concentration of minerals in the edible components (15).

Because of the large area of salt affected land in south Western Australia, there is increasing interest in salt tolerant species. The saltbushes (Atriplex spp.) are also perennial shrubs, without the tall growth habit of tagasaste, and offer a potential feed resource through summer/autumn, producing up to 1000 kg/digestible dry matter per hectare (16). However, limited information is available on the nutritive value, persistence under grazing or establishment of these species at this time.

Senescent annual pasture

The systems which interact to determine the intake and digestion of plants by ruminants are complex. The characteristics of the plants themselves are important as:

• they provide the energy substrates for the rumen organisms and subsequently the animals tissues;
• they provide essential nutrients;
• their physical and chemical composition influences the rate and
• extent of digestion and determines the quantity the digestive tract can handle;
• their palatability to the animal is important.

Obviously no one measurement can adequately describe all of these characteristics or indicate the plants nutritive value. The system becomes even more complex and difficult to describe when only senescent or dead, as compared to green, plant material is considered. Yet in a Mediterranean environment this kind of pasture may be utilized for sheep production for about six months of the year. Because of the importance of senescent and dead annual pasture it's nutritive value is discussed in relation to various nutritional indices.

Cell Contents and Cell Wall Constituents

The energy substrates in forages can be conveniently divided into readily digestible components [cell contents or neutral detergent solubles (NDS = 100 - neutral detergent fibre, NDF)] and slowly digestible components [cell wall constituents or neutral detergent fibre]. In vitro organic matter digestibility (IVOMD) of plants is largely determined by the proportions of NDS and NDF. In addition, general relationships can be developed between IVOMD or in vivo digestibility and forage intake; low digestibility is indicative of low intake. While many studies have confirmed the existence of a correlation between intake and digestibility, deviations from this general trend have also been reported (17).

Within the agricultural regions of Western Australia it has been shown that digestibility of wheat stubbles decreases as the rainfall or growing season increases (18). It is likely a similar situation exists for grasses. With cereal straws there are strong correlations between the proportion of NDS and IVOMD of stem tissues (19). There are also clear differences between plant tissues as regards digestibility (leaf blade > leaf sheath > stem). Within a tissue, most of the variation in digestibility can be attributed to differences in the proportion of NDS present. It is also important that the NDF of leaf blades is more digestible than that of sheaths or stems in both cereal straws and pasture grasses. It would seem these factors may also explain the differences in digestibility of dry feeds in the low and high rainfall areas of south-western Australia.

While there are relationships between NDS and IVOMD of the tissues of ryegrass plants, these are not as strong as those for cereal straws (Pearce, pers. com.). This difference appears to be associated with the very low NDS levels in dead grasses, and possibly because of greater variation in the digestibility of the cell wall constituents. A limited amount of evidence indicates that there may be reasonable relationships between the level of non-cell wall material and IVOMD of dry clover residues (20) (Figure 4).

Figure 4. Relationship between in vitro digestibility and the proportion of non-cell-wall material in 42 genotypes of subterranean clover (20)

It would seem that, within a given environment (rainfall zone), mature pasture plants which contain more NDS and which have more digestible cell walls are what we should be looking for to improve the nutritive value of dry pasture. However, progress from selection within a plant species or from selecting alternative species will only be made with a more complete understanding of the effects of environmental factors in determining the growth pattern and eventual quality of senescent herbage (21, 22). Also, the rate of decline of quality of dry residues with high NDS and more digestible cell walls needs to be known under a range of conditions to assess their desirability. It may be that summer rainfall events will diminish the benefits by contributing to high rates of quality decline. If this is so benefits would only be gained through conservation of such feeds.

Considerable variability exists in the digestibility of dry mature subterranean clover (20, 21, 23, 24),which can not always be explained by the physiological stage of development of the plants. While this suggests there is scope for plant breeders to improve quality within particular groups, this will only be achieved if the reasons for the differences (more NDS or more digestible NDF) are defined. Without such information plant breeders would not know what to select for.

Essential Nutrients

Limitations in essential nutrients, nitrogen and minerals, can also be expected to constrain the intake and/or digestion of dry herbage when availability is not a limitation. At the onset of the dry period, the content of essential nutrients in different pasture species and within a pasture species can be quite different and can decline at different rates. Higher contents of these elements in dry annual legume compared to grass clearly contribute to the superior feeding value of these materials in summer.

Variations can also occur within a pasture species. The contents of nitrogen and phosphorus in five cultivars of subterranean clover harvested at Mount Derrimut Field Station, Victoria, between September and January were greater in later maturing cultivars (21) (Table 2). Nitrogen content did not fall below 10 g/kg DM, the level below which the supply of this nutrient is known to limit intake. However, phosphorus contents below 1.2 g/kg DM, which would be expected to limit intake and digestion of forage, were found in the early maturing cultivars. This information is only from one location, but it does indicate that specific pasture plants can extend the supply of essential nutrients into the period when pasture quality is lowest.

While laboratory analyses for concentrations of essential nutrients in forages is becoming more practical with improved analytical procedures, such information alone can be misleading. The primary reason for this is that they give little insight into the availability of the nutrients. For example, the amount of readily available nitrogen (as a proportion of total N) declines more rapidly in early than late maturing varieties of subterranean clover during senescence (21) (Table 3). In addition, the rate of disappearance (release for use by rumen organisms) of the remaining insoluble nitrogen is also slower in early maturing types. Estimating the availability of essential nutrients is time consuming and impractical on a large scale. However, in evaluating new pasture species, it is important to quantify differences in nutritive value at the onset of and during the dry period.

Table 2. Nitrogen and phosphorus contents (g/kg DM) of five cultivars of sub-clover between September and January (Source: McLaren and Doyle, unpublished data, 21).

Table 3. Availability of nitrogen in the reticulo-rumen from three cultivars of sub-clover between September and January (Source: McLaren and Doyle, unpublished data, 21).

Physical Characteristics

Some physical characteristics of forages such as bulk, the amount per unit volume, and tensile strength of the leaves and stems also relate to the amount consumed. They are associated with the amount of digesta retained in the reticulo-rumen and the rate of digestion in and rate of passage through the rumen. The limitation imposed by physical factors to the rate at which ingested forage is broken down to particles which can pass the rumen is a major factor regulating food intake. For example, the intake of legumes is higher than that of grasses of equivalent digestibility (25). This is due to shorter retention times of feed in the rumen with legumes. The higher relative feeding value of clover compared to grasses (Figure 3) can be partly attributed to this difference.

It is not possible to measure all of the physical characteristics of pasture plants by a single laboratory procedure. However, measurements such as grinding energy or shear strength can give useful indications of how resistant to physical breakdown forages may be. General relationships can be developed between such measurements and feed intake, but these relationships are often subject to large errors.

There are large differences in the grinding energy of mature material from different plant species. For example, a mature subterranean clover residue had a grinding energy of 84 J/g DM compared to values of 140 to 200 J/g DM for wheat straws and 100 to 120 J/g DM for rice straws (Doyle, unpubl. data). The errors involved in predicting intake from these types of assessment (IVOMD, NDF, essential nutrients, physical characteristics) mean that at times it will be necessary to actually assess intakes by grazing animals.

Intake of Mature Legume Pastures

Differences in production by sheep grazing different pasture legumes over the summer/autumn have led to the initiation of investigations into the quality and amount of feed consumed. For example, wool production by sheep grazing subterranean clover or medic pastures has been found to vary significantly (26) (Table 4). A significant part of the differences in wool production occurred over the December to May period. While this can be partly attributed to higher dry matter and seed production on the M. polymorpha varieties this was not the case when M. truncatula is compared to T. subterraneum. Obviously, the burr from the medics is more available than that from subterranean clover. Preliminary studies indicate that this translates into higher intakes of nutrients by sheep grazing medics (Casson et al unpubl. data).

Table 4. Wool production from Medicago polymorpha cvs. Circle valley and Serena, Medicago truncatula cv. Paraggio and Trifolium subterraneum cv. Dalkeith (26).

Conclusions

From the animal's point of view, in south-western Australia the supply of nutrients is limited by the quantity of feed on offer following the onset of rains in late autumn/winter and by both the quantity and quality of feed available in the summer/autumn. The first of these limitations can be overcome by using pasture species with superior early growth potential which will persist in the system and by improving conditions for early growth through the strategic use of fertilizers. While this approach is attractive to continuous pasture systems, problems would be created where crop/pasture rotations are practised.

During the summer/autumn, potential exists to improve the supply and quality of feed by use of perennial pasture species and shrubs where rainfall is favourable. This has additional benefits related to improved water use in many areas providing the plants can be established and then managed in a way which will maintain a stable sward. Again such an approach may not be relevant to areas which will be cropped.

There is also scope to provide higher quality feed through using alternative pasture species or through altered management of existing pastures. To fully realize the potential that exists in this area a better understanding is required of the genetic and environmental factors during growth which interact to determine quality of senescent and dead plants. It is acknowledged that a danger exists in taking advantage of any gains made as summer rainfall events in this environment lead to rapid deterioration in the quality of dry pasture.

The appropriate strategy to improve the quantity and quality of feed will depend on the growing conditions of the particular location and broad generalizations within the medium and high rainfall zones are unlikely to be meaningful.

References

1. Hodgson, J. 1982. In "Nutritonal Limits to Animal Production from Pastures", pp. 153-66, editor J.B. Hooker. (C.A.B., Farnham Royal, U.K.).

2. Purser, D.B. 1980. In "World Animal Science B1: Grazing Animals", pp. 159-80, editor F.H.W. Morley. (Elsevier, Amsterdam).

3. Purser, D.B. and Southey, I.N. 1984. In "Proc. Seminar on Wool Production in Western Australia", pp. 99-111, editors S.K. Baker, D.G. Masters and I.H. Williams. (Aust. Soc. Anim. Prod., W.A. Branch).

4. Rossiter, R.C. 1966. Adv. Agron. 18: 1-56.

5. Dove, H., Foot, J.Z. and Freer, M. 1989. In "Proc. International Grassland Congress" (in press).

6. Cox, W.J. 1973. Plant Research division 1973 Trial Report W.A. Department of Agriculture.

7. Bettenay, R.A. 1984. Report on results of experiments carried out by Dairying Division Officers 1961-65, W.A. Department of Agriculture.

8. Davies, H. Lloyd, Greenwood, E.A.N. and Watson, E.R. 1966. Proc. Aust. Soc. Anim. Prod. 6: 222-28.

9. Smith, R.C.G., Biddiscombe, E.F. and Stern, W.R. 1973. Aust. J. Exp. Agric. Anim. Husb. 13: 549-55.

10. Minson, D.J. and Milford, R. 1967. Aust. J. Exp. Agric. Anim. Husb. 7: 546-51.

11. Ndlovu, L.R. and Buchanan-Smith, J.G. 1985. Can. J. Anim. Sci. 65: 693-703.

12. Roberts, G.B., Kenney, P.A. and Reeve, J.L. 1979. Aust. J. Exp. Agric. Anim. Husb. 19: 156-60.

13. Kenney, P.A., Reeve, J.L., Baxter, R.W. and Cumming, I.A. 1980. Aust. J. Exp. Agric. Anim. Husb. 20: 15-19.

14. Fortune, J.A. (1989). Fifth Australian Agronomy Conference (in press).

15. Hemsley, J.A. and Downes, P.A. 1986. In "Proc. Workshop on Tagasaste", pp. 3-5. (CALM, W.A.).

16. Malcolm, C.V., Clarke, A.J., D'Autuono, M.F. and Swaan, T.C. 1988. Agric. Ecocystems Environ. 21: 265-279.

17. Minson, D.J. 1982. In "Nutritioanl Limits to Animal Production from Pastures", pp. 167-82, editor J.B. Hacker. (C.A.B., Farnham Royal, U.K.).

18. Purser, D.B. 1982. In "The Utilization of Fibrous Agricultural Residues as Animal Feeds", pp. 27-41, editor P.T. Doyle. (School of Agriculture and Forestry, University of Melbourne, Parkville, victoria).

19. Pearce, G.R. 1984. In "Ruminant Physiology: Concepts and Consequences", pp 117-125, editors S.K. Baker, J.M. Gawthorne, J.B. Mackintosh and D.B. Purser. (School of Agriculture (Animal Science), University of Western Australia).

20. Taylor, G.B., Rossiter, R.C., Klein, L. and Collins, W.J. 1987a. In "Temperate Pastures, Their Production, Use and Management", pp. 432-434, editors J.L. Wheeler, C.J. Pearson and G.E. Robards. (Australian Wool Corporation, CSIRO).

21. Doyle, P.T. 1985. In "Proc. Seminar on Pastures $$ ... Your Most Valuable Crop", pp. 9-18, editors A.C. Dunlop and C.W. Thorn. (Aust. Soc. Anim. Prod., W.A. Branch).

22. Pearce, G.R., Lee, J.A., Simpson, R.J. and Doyle, P.T. 1987. In "Plant breeding and the Nutritive Value of Crop Residues", pp. 195-229, editors J.D. Reed, B.S. Capper and P.J.H. Neate. (International Livestock Centre for Africa, Addis Ababa, Ethiopia).

23. Mulholland, J.G. and Dear, B.S. 1987. In "Herbivore Nutrition Research", pp. 21-22, editor M. Rose.(Occasional Publ. Aust. Soc. Anim. Prod.).

24. Taylor, G.B., Rossiter, R.C. Klein, L. and Collins, W.J. 1987b. In "Proc. 3rd National workshop National Subterranean Clover Improvement Programme", pp. 89-92, editors B.S. Dear and W.J. Collins. (National Subterranean Clover Improvement Programme).

25. Thornton, R.F. and Minson, D.J. 1973. Aust. J. Agric. Res. 24: 889-98.

26. Flecker, S.E., Thorn, C.W., Rowe, J.B. and Casson, P. 1987. In "Proc. 3rd National Workshop National Subterranean Clover Improvement Programme", pp. 93-98, editors B.S. Dear and W.J. Collins. (National Subterranean Clover Improvement Programme).

• Disclaimer
• Privacy Statement
• Terms & Conditions

• The Regional Institute
• Help/Feedback
• phone: 61 2 4369 6006
• email: admin@regional.org.au
• Sitemap