Drought, stocking rates and soil loss.

Abstract

Proc. Aust. Soc. Anim. Prod. Vol. 19 DROUGHT, STOCKING RATES AND SOIL LOSS F. H. W. MORLEYA and G. DAN.IEL B *University of Melbourne, Clinical Centre, Werribee, Vic. 3030. *` Comoo' Tumbarumba, N.S.W. 2653. , SUMMARY Destocking of pastures when plant cover has been reduced to a defined minimum was examined in relation to stocking rates. Perennial and annual pastures in 7 locations were considered. On average about 1 DSE per ha per 100 mm annual rainfall will lead to destocking on about 1 month in 12, if 1 t/ha of plant dry matter gives the signal to destock. Keywords: pastures, soils, stocking, erosion, destocking. INTRODUCTION Heavy grazing causes soil erosion. Losses of soil and nutrients result from reduction of both surface cover and of the root systems which protect and bind the soil, and also from the trampling and formation of tracks, especially near watering points. If grazing systems are to be sustainable strategies must be developed to prevent such losses. Estimation of optimum stocking rates (OS) has almost invariably focussed on gross margins per hectare as the objective. Unfortunately such optima may well fail to allow for hidden costs from losses of soil and nutrients. These costs must be included in budgets of management systems, whether short or long term. This paper develops a method of estimating optimum stocking rates of pastures in south east Australia by analyses of 100 years of monthly rainfalls. This method is then used to estimate OSs at 7 locations in Victoria and N.S.W. The optimum stocking rate was defined here as that at which plant cover is reduced to a defined level at which destocking of pastures is required, by the rules of the system, for a defined number of months in 12. Daniel (1987) reviewed the requirements of plant cover needed to keep losses within acceptable limits. At least 1 t/ha of plant surface dry matter seems to be necessary for annual pastures, and about 0.5 t/ha for dense, well established perennials. These parameters must vary with topography, soil type and intensity of rainfall. White et al. (1980) discussed relationships between soil erosion and stocking rates for improved perennial pastures in Victoria, and Whalley and Lodge (1987) for semi-arid pastures. Most discussions on grazing management have paid little more than lip service to methods of management to prevent erosion. MATERIALS AND METHODS Long-term rainfall data were analysed by a spreadsheet model. The critical level (C) for removal of stock was either 1 t/ha of plant residue, as might be appropriate for annual pastures, or 0.5 t/ha for well established perennial pastures. These values were reduced for the cooler months, when rainfall intensity would be lower. The stocking rates at which the weight of plant residues fell below C values were estimated iteratively for each situation, using the following functions. where PC is pasture available (t/ha) at end of current month, Pp is pasture available (t/ha) at end of previous month, S is stocking rate (wethers/ha), D is number of days in month, DK is percentage pasture decay per month (%Pc), G is potential pasture growth (kg/ha.day), Dg is digestibility of pasture on offer, Rt is rainfall total for month (mm), RL is minimum (` ' rain necessary for growth, E is lag ) evapotranspiration ratio (t H,O/t DM). Monthly rainfall records for about 100 years were obtained in Victoria and New South Wales from (years in parentheses) Balmoral (102), Bendigo (loo), Geelong (91), Holbrook (104), Sale (82), Trangie (104) and Tumbarumba (94). The analysis was based on ` consensus' estimates from models and results such as those of White et al. (1980, 1983) and Bowman (1989). Pasture growth was predicted as a function of rainfall from ` evapotranspiration' ratios estimated from a pasture growth study in Argentina (Morley 1977). The estimated requirements ranged from 500 to 1000 t of soil water per tonne of pasture dry matter grown. 323 Proc. Aust. Sot. Anim. Prod. Vol. 19 A ` ' level of rainfall was assumed to have been required to initiate pasture growth. This made lag negligible any responses to occasional showers. The potential growth of pastures (G) at each location followed the models of growth in White et al. (1983) Examples of E, RL, G, and DK for twelve 12 of the year at Bendigo, an important pastoral region in North-Central Victoria, are given in Table 1. Table 1. Average monthly evapotranspiration ratios (E, kg/t), rainfall lags (RL, mm), pasture growths (G, kg/ha.day) and decay rates @K,%t/ha.m) for 100 years at Bendigo Validation of model The estimates of tonnes per ha of dry material on the surface were compared with those from 1965-67 from near Bendigo, as estimated by White et aZ.(1983) and Bowman (1989). The mean value for this study was 1.5 t/ha compared with 1.7 t/ha from Bowman( 1989). The correlation between our estimates and Bowman' field data was 0.78. Since this statistic must approach the likely repeatability s of measurements of pasture availability, the model can be accepted with confidence as a fair approximation for sites similar to that of Bowman. Variations in soil type, topography, and water relations must however be considered before the model can be taken as a trustworthy guide for management. RESULTS AND DISCUSSION The results are summarised in Table 2. for 2 frequencies (averaged for 1 or 2 months per year), and 2 critical levels (0.5 or 1 t/ha) of pasture dry matter. The stocking rates were dry sheep equivalents (DSE) per ha at 28 February, but this value would vary with the month. Table 2. Predicted stocking rates at which pastures were below critical levels (C) of 1.0 or 0.5 t/ha for 1 or 2 months per year (m/y) and average rainfall statistics for the locations S, Stocking rate predicted for 1 .O t/ha on 1 month average/year There is no clear association in this study between S and features of the rainfall, other than the total rainfall. However the predicted frequency of destocking is highly sensitive to the critical levels of pastures. This is not surprising. It follows that any measure to reduce these critical levels could be important to management. Thus the establishment of highly persistent perennial grasses, the design of subdivisions, soil amelioration, and the siting of watering points, should all be examined for their implications for site vulnerability. Perennial pastures may not always produce more plant matter than annuals but, if they persist under heavy grazing, might confer greater soil protection, and hence higher carrying capacity, for given levels 324 Proc. Aust. Sot. Anim. Prod. Vol. 19 of risk. The roots of perennial pastures might well stabilise the soil. Hence annual pastures might require higher levels of protective cover than the perennials. A residue level of 1.0 t/ha for annuals, and 0.5 t/ha for perennials, could be appropriate, but facts are needed. The predictions estimated here were made fi-om models of annual and perennial pastures. They are no more than simulations. Pursuing this argument, the results of Table 2 suggest that for a given level of destocking, perennial grasses would carry, on average,about 45% more stock. The profitability of such management systems needs to be examined carefully. From a soil conservation viewpoint perennials might confer useful increases in productivity through higher stocking rates, at a given levels of soil and nutrient losses. Persistence under heavy grazing must be a critical requirement for higher stocking rates. The OS estimated here for annual pastures are substantially lower than the 2 to 3 DSE/ha per 100 mm rain when estimated from maximization of gross margins per ha (Morley, in preparation), except at Trangie. Prolonged heavy stocking has caused losses of perennial species at Trangie (Biddiscombe 1953; Campbell et al. 1973; Whalley and Lodge 1987). Similar consequences seem likely in more temperate climates (Morley 1966; White et al. 1980). Therefore the benefits of management might be reduced, or negated, if perennial species are lost because of heavy stocking. If destocking at or below critical levels of pasture enhances the persistence of perennials,and hence protection of the soil, it could be highly profitable. Costs and benefits must clearly be estimated in the appropriate circumstances. Some preliminary estimates suggest that if the cost of maintenance feeding is about $A2.00 per head per month, destocking could be a worthwhile procedure at stocking rates slightly lower than those at which gross margins per ha are maximum. The interactions of OS with rainfall amounts, intensities, and distribution, on different soils with different topographies, and for different pastures communities, using appropriate management strategies, deserve high priority in research. The long term survival and ecology of perennial pastures, and their economic performance, also require further investigation. CAMPBELL,R. J., SAVILLE,D. G. and ROBARDS,G. E. (1973). Aust. J. Exp. Agric. Anim. Husb. 13: 238-44. BIDDISCOMBE, E. F. (1953). Aust. J. Agric. Res. 4: l-28. DANIEL, G. (1987). M.Vet.S. Thesis, University of Melbourne. Applications in Agricultural Modelling' (Eds. A. J. de Boer and C. W. Rose.) ,, MORLEY, F. H. W. (1977). Zn ` p. 320 (Griffith University: Brisbane.) MORLEY, F. H. W. (1966). Proc. Aust. Sot. Anim. Prod. 6: 127-36. WHALLEY, R. D. B. and LODGE, G. M. (1987) In ` Temperate Pastures' (Ed. J. L. Wheeler.) p. 553. (Aust. Wool CorpKSIRO: Melbourne.) WHITE, D. H., BOWMAN, P. J., MORLEY, F. H. W., McMANUS, W. R. and FILAN, S. J. (1983). Agric. Systems 10: 149-89. WHITE, D. H., McCONCHIE, B. J., CURNOW, B. C., and TERNOUTH, A. H. (1980). Aust. J. Exp. Agric. Anim. REFERENCES Husb. 20: 296-307. 325