Influence of soil ecology on animal health and welfare Colin L Trengove Pro Ag Consulting POB 169, Burnside SA 5006 Introduction Ecology has been defined as the study of relations of animals and plants, particularly of animals and plant communities, to their surroundings, animate and inanimate. 1 It was initially developed to provide a mechanistic backbone to the science of natural history and so is entwined with evolution. 2 However, the ecological study of soil biota is only a recent phenomenon and the taxonomic classification of the very diverse soil microbial population is still emerging. Soil ecology is a major determinant of animal health and welfare through its influence on the quality and quantity of nutrition available to grazing livestock. The interaction between microbes and the soil is integral to mineralisation - the processes by which minerals are converted into an available form for uptake by plants and animals. The nature of the soil determines the resources or physical components of the environment available to soil-borne organisms to facilitate mineralisation. These resources include water, carbon, nitrogen and various other macro- and micronutrients. Liebig s Law of the Minimum originally developed by Carl Sprengel in 1828 states that plant growth is limited by the scarcest resource, not the total amount of nutrients or resources. 3 This understanding of resource availability lead to the development of the fertiliser industry to the extent that physical resources are often no longer the limiting factor to achieving productivity gains in plants and animals. Instead it is often the soil biota that is the constraint to plant growth. Access to soil nutrients Soils have specific nutritional requirements for optimum health similar to plants and animals. This is often referred to as soil chemistry and along with physical properties creates the home for various living organisms such as insects, worms, bacteria and fungi collectively known as the soil biology or microbial biomass. Without these organisms the soil would be dead and unproductive. Figure 1 shows key microbe groups involved in maintaining soil health. The soil microbial biomass comprises less than 5% of the organic matter in soil, but performs several critical functions in the soil plant environment. 4 It improves nutrient availability as a labile source of carbon (C), nitrogen (N), phosphorus (P) and sulphur (S); acts as sink for C, N, P and S; improves soil structure, moisture availability and drainage; forms symbiotic associations with roots; assists nutrient transformation and pesticide degradation; and acts as biological agents against plants pathogens. An example of the latter is the presence of an endophytic fungus in perennial ryegrass making it less susceptible to Argentine stem weevil attack. The microbial compositions of the soil influence which plant species tend to dominate. For example, a fungi:bacteria ratio of 0.75:1 suits grass dominance, while vines and fruit trees prefer a ratio of 5:1. Vesicular arbuscular mycorrhizal fungi referred to as VAM live in a symbiotic relationship with most plant types and can increase plant nutrient uptake, particularly phosphorus, as well as nitrogen, potassium, magnesium, zinc and other nutrients. 5 They can also improve plant drought tolerance. In addition, they produce a long-lived carboniferous protein called glomalin which binds soil particles to improve spoil structure creating micro-pores in which other beneficial soil microbes like to grow. 836
Figure 1: Illustration of the role of key microbial groups in the soil and an example of a soil test compared to the desired biomass. 5 A balanced approach to soil and plant management will promote long-term sustainability and the best outcome in terms pasture and animal health, welfare and profitability. The effort required to achieve soil balance varies widely with soil type and environmental conditions and an understanding of cation exchange in the soil colloid are critical to this process. 6 An audit of the biological, chemical and physical aspects of soil is desirable to assess the input required to improve productivity. The cost benefit of this input will depend on the pre-existing nature of the soil and it is often financially prudent to initially focus on those soils that need the least amendment. A soil test is used to assess the amount of plant available nutrients in the soil as well as identify limitations to production such as the biological activity and diversity, ph, soil structure, sodicity, salinity and low or excessive nutrient status. There has been great interest in the assessment of the microbial biomass for over 40 years, and the process has gained popularity with the development of relatively rapid and more reliable assessment techniques in recent years. 4 Further improvement in molecular biotechniques will encourage this assessment to become a routine tool in the management of the land and the environment. Soil amendments and ph Amendments that may be applied to address soil limitations include the application of humates and composts to ensure adequate carbon and nitrogen for the soil biomass, as well as lime, gypsum and dolomite to create the desirable ph for optimal microbial activity. Soil ph is an important parameter of soils and plant-soil interactions affecting:- microbial population and diversity; root development inhibited by extremely high or low ph; seedling germination restricted especially by low ph; nutrient availability and the predominant form of a specific nutrient; and rates of weathering of minerals and organic matter. 837
Generally, high (alkaline) or low (acidic) soil ph conditions reduce the availability of macro and trace elements. For example, phosphate, is absorbed to either iron or aluminium oxides at low ph or calcium carbonate or iron oxides at high ph, but like most nutrients, is most available when the ph is in the desired range (6.0 7.0) to suit optimum biological activity - as illustrated in Figure 2. Figure 2: The availability of nutrients at various ph (water), with the widest bar representing availability. 7 Minerals in plant health All higher plants require at least 16 elements 5. Carbon (C), hydrogen (H) and oxygen (O) from water and carbon dioxide contribute 90-95% of the dry matter of all plants. The remaining 5 10% of the plant consists of elements sourced from the soil through the plant root system. The 13 essential elements that come from the soil are six macro elements - nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg) and sulphur (S) and seven trace elements iron (Fe), manganese (Mn), zinc (Zn), copper (Cu), boron (B), chloride (Cl) and molybdenum (Mo). Legumes also require cobalt (Co) for rhizobia in their root nodules to fix atmospheric nitrogen. Minerals in animal health and welfare Minerals have three major roles in animals to:- provide structural material to bones and connective tissue; allow electrical impulses to be transmitted across nerves; and act as catalysts involved in the numerous physiological processes such as DNA replication, digestion, immune function, endocrine synthesis, neurological activity, energy storage and release, muscle contraction and numerous other functions. As a consequence, 838
mineral deficiencies can have devastating consequences on animal health and welfare. For example, a deficiency of magnesium and zinc can account for 300 physiological malfunctions due to their role in enzymatic processes. An example of interactions determining mineral availability to plants is shown in Figure 3. Figure 3: Mulder s chart displays a summary of the stimulatory and antagonistic interactions that influence plant mineral availability. 9 The availability of nutrients in appropriate quantities is a major factor determining the health of grazing livestock with at least 15 minerals recognised as being essential to animal. 10 These essential elements are similar to those for plants although plants have a higher requirement for K, Fe and Ca (legumes) and a lower requirement for Cu than animals. In addition, animals have specific requirements for sodium (Na), iodine (I), selenium (Se) and fluorine (F). As a consequence, in the presence of soil deficiency it is often more cost effective to apply nutrients as solid or foliar fertiliser so that both plants and animals can benefit, rather than direct to the animal. The critical nutrient levels for plant growth are sometimes lower than the minimum levels required for animals. For example, plants can be showing healthy vigorous growth, but have inadequate nutrients for grazing livestock. Conversely, high levels of certain nutrients in plants such as Cu, Mo, Se, Fe, S, Na, K, F, nitrite (NO 2) and nitrate (NO 3) can be harmful to animals without affecting plant growth. Mineral deficiencies and disease syndromes in livestock at pasture are often associated with the major elements - Ca, P, Mg, Na, S and trace elements - Co, Cu, I, Mn, Se and Zn. 10 These deficiencies are difficult to predict due to variations in seasonal conditions (e.g. inundation and drought) affecting feed availability and mineral composition, grazing intensity and selectivity, soil ingestion and physiological differences between pasture species and livestock breeds. In addition, differences in the chemical, physical and 839
biological properties of different soils will also influence the mineral status of the grazing animal. A strategy combining soil, pasture and animal sample analyses can provide a comprehensive tool for animal health and welfare investigations, but an understanding of the impact of nutrient interactions, prevailing seasonal conditions and grazing management is critical to interpretation of the test results gathered. Similarly, this approach is ideal for the development of management programs for optimum health and welfare in soil, plants and animals. The approach involves the use of both soil chemistry and biology tests, plant analyses as a means of monitoring nutrient availability and in conjunction with tests on blood, liver, milk, urine, faeces, grains, conserved feeds and water as appropriate. Conclusion Soil ecology has a fundamental impact on nutrients available to associated plants and grazing livestock. Molecular techniques to monitor the soil biomass and the direct effect on plant and animal production are being further developed for routine adoption. The farm manager and animal health consultant equipped with this information as well as their knowledge of season, pasture availability, grazing management and the class of stock is able to manage animal health and welfare with a high degree of confidence. References 1. Abercrombie M, Hickman CJ, and Johnson ML. A dictionary of biology. Penguin Books Australia Ltd, Ringwood, Victoria, Australia 1973; 93. 2. Morris SJ, and Blackwood CB. The ecology of soil biota and their function. In Soil microbiology, ecology and biochemistry. Paul EA editor 4 th ed. Elsevier, London NW1 7BY, UK, 2015:273-309. 3. Soil Foodweb Institute. Liebig s law of the minimum. http://www.soilfoodweb.com.au/index.php?option=com_content&view=article&id =173:2010may-liebigs-law-of-the-minimum&catid=45:2010-may&itemid=144 4. Dalal RC. Soil microbial biomass-what do the numbers really mean? Aust. J. Exp. Agric. 1998, 38(7):649-665. 5. Microbiology laboratories Australia. http://www.ciaaf.com.au/soil-microbiology-testing/tests/ 6. Trengove CL. Whole farm nutrition the role of soil in animal nutrition. In Sheep Medicine. Post graduate foundation in veterinary science, University of Sydney. 2004,355:277-285. 7. Apal agricultural laboratory soil test interpretation guide. www.apal.com.au 8. McLaren RG. and Cameron KC. Nutrient availability. In Soil Science. Oxford University Press, New Zealand. 1996:178-191. 9. Mulders chart. http://www.nutriag.com/docs/images/mulders-chart.jpg 10. Judson GJ, and McFarlane JD. Mineral disorders in grazing livestock and the usefulness of soil and plant analysis in the assessment of these disorders. Aust. J. Exp. Agric. 1998, 38:707-723. 840