A pasture production model for use in a whole farm simulator

Modular simulation models of crop and animal enterprises can be used in concert to simulate a particular whole farm operation and thus provide management information for individual farmers and farm advisers. This paper reports the development of a pasture module for such a whole farm simulator designed to study irrigation management in New Zealand. The model (CANPAS) describes the production of perennial ryegrass/white clover pastures under either dryland or irrigated conditions. Seasonal growth studies and the principles of ecological physiology form the theoretical base of CANPAS. Whenever possible, parts of other models were also used. With time steps of one day, CANPAS predicts the net above-ground yield of live and dead herbage, herbage digestibility, leaf area index and soil water supply. Harvested yields are also predicted for simulated grazing or cutting managements, assuming part of the above-ground yield of live and dead herbage is left as stubble. Field measurements of harvested pasture yields from the Canterbury Plains of New Zealand were used to validate the model. Validation procedure included the use of a simple set of statistical tests. The final version (CANPAS III) gave an overall r² of 0·83, but poorer performance was found under some test managements. Discussion covers the general aspects of this kind of modelling as well as the specific details of the pasture module.     

A growth-irrigation scheduling model for wastewater use in forest production

Applying wastewater and sludge to land for remediation has been recommended by the Environmental Protection Agency (EPA) as a method to recycle nutrient and organic matter and conserve water resources. The level of sewage treatment can range from simple primary treatment using a lagoon to tertiary treatment using a standard wastewater treatment plant. Small communities are selecting primary treatment and land application as the most cost-effective way of treating municipal wastewater.Wastewater was used to irrigate an Eucalyptus camaldulensis plantation in Ojinaga, Chihuahua, Mexico. The overall objective of the research was to develop a daily growth-irrigation scheduling model (GISM) for Eucalyptus tree plantations based on the trees’ water needs with the source of irrigation water being wastewater from a lagoon sewage treatment system. A second objective was to check this model against measured growth data to determine the limitations of using a simple irrigation-scheduling model to manage the irrigation system to maximize tree growth and wood production.The GISM calculated the evapotranspiration (Et) from the volume balance soil water model and a tree biomass sub-model, based on a water use efficiency (WUE) (biomass/Et) that partitions biomass determined from Et into the component parts of leaves and stems plus branches. The water balance portion of the model computes the Et for grass growing between the trees until a closed canopy system is reached. Weather data and a soil water stress function were used to calculate Et based on calculated reference Et and crop coefficients (K_cs) for both the trees and grass scaled to nonstressed Et.The GISM model accurately modeled height and diameter growth, although, it slightly overestimated the height growth of Eucalyptus for the high irrigation treatment in the second and the third years. The GISM model was successful in predicting height and diameter growth within a 95% confidence level of the measured height and diameter of the trees under all irrigation treatments.Based on the modeled and measured data analysis, the GISM model can be a useful tool to predict tree growth and schedule irrigations for Eucalyptus tree plantations, understand the trees response to environmental and water stress, and to provide better analyses for future research efforts. The climate-driving variables (temperature and rainfall) needed by the model are readily available for any location in the world from the National Climatic Data Center (NCDC).     

Dynamics of plant and animal production of a subterranean clover pasture grazed by sheep: Part 2—structure and validation of the pasture growth model

A process-based model of the growth of a grazed subterranean clover pasture is described. The model predicts daily changes in the quantity and quality of seed, and green and dry vegetative biomass. Many of the functions describing processes were empirically derived. The model was calibrated against field data and then shown to give close agreement in predicting biomass of three other grazed pastures, two of which were in a different location.     

Rotational grazing in a continuously growing pasture: A simple model

The effects of rotational and continuous grazing on long-term steady-state productivity of a continuously growing pasture were examined in a simple mathematical model representing both plant growth and herbivore consumption as functions of green plant biomass. The results showed that moderate rotation, with few subdivisions and short cycles, had only minor effects on productivity, compared with continuous grazing. Intensive rotation (many subdivisions and/or long cycles) resulted in a substantial decrease in long-term productivity in the absence of an ungrazeable plant residual or when the stocking rate was moderate and the initial condition (biomass) of the pasture high. When the stocking rate was high and the initial condition of the pasture poor, but an ungrazeable residual was present, intensive rotation substantially increased the long-term productivity compared with continuous and moderate rotational grazing.     

An analysis of the growth rates of pasture and animal production

An analysis of changes in the liveweight of grazing animals is presented. It identifies six determinants of animal liveweight change and provides a framework within which observations made in desparate experiments can be ordered and studied. It emphasises the dynamic nature of the grazing system, and its application is illustrated using a data sub-set collected during a grazing trial in the humid sub-tropics. Some implications of the analysis for traditional agronomic research are discussed.     

Development of a simple potato growth model for use in crop-pest management

A simple whole-plant level potato growth model was developed that accumulates and partitions dry matter into four state variables—leaves, stems, roots and tubers. Daily growth is computed from a function of the total solar radiation, the proportion of total radiation intercepted by the crop, temperature and soil water status. Dry matter is partitioned using modified Michaelis-Menten equations. At initialization, the seed piece size, plant and row spacing, and the values for the dry matter partitioning parameters are required. After initialization, the inputs required are the daily minimum and maximum temperatures and the daily soil water potential. Daily site-specific, seasonal radiation is presently estimated from a sine function. Model parameterization and output plotted against field data are presented for two cultivars, each planted a total of three times in two seasons. The model is intended for use in analyzing the dynamics of yield under variable pest pressures and environmental conditions. Model sensitivity, behavior and use in relation to modeling yield loss due to pests are discussed.     

A dairy herd cash flow model

A model that estimates milk production, revenue from milk sales and concentrate requirements for a diary herd is described. The model is designed for a programmable calculator for use by staff of the Agricultural Development and Advisory Service in their consultancy work on farms.     

A model for sericulture and milk production

With a view to increasing the annual output of edible nutrients (milk), employment generation and obtaining better economic returns per unit area under sericulture, a model was designed through the application of the integrated systems approach. By the application of this model, it is possible to produce 23·23 GJ edible energy and 266·33 kg edible protein from milk; 2975 man-days employment and Rs.30·11 profit per man-day labour input from 1 ha in a year. In contrast, the traditional system of rice cultivation in Southern India, with double cropping intensity, could generate 50·67 GJ energy and 244·00 kg protein from food grains, 288 man-days employment and only Rs.8·34 profit per man-day labour input from 1 ha in a year.     

A timothy-beef production model for Atlantic Canada

A five-part timothy-beef deterministic simulation model was prepared to integrate information on various production processes and hence to improve the basis for management advice given by extension workers.The five parts or sub-models are: the growth of the timothy, harvesting the crop, storing it, converting the timothy and other feeds to beef and, lastly, the feeding of cattle and the disposal of manure. Costs and returns are estimated.The model structure and principal biological relationships are described in some detail. Sample outputs for three different cow-calf-feeder regimes are shown and compared.The uses, limitations and potential of the model are discussed.     

A model for evaluating lamb production systems

A mathematical model for evaluating and comparing lamb production systems is constructed. The model is designed for a microcomputer for use by the Meat and Livestock Commission in its consultancy work.     

Deferred grazing of Mediterranean annual pasture for increased winter sheep production

Deferred grazing of Mediterranean annual pasture involves the removal of sheep while the pasture re-establishes following the break of season, to allow a more rapid development of leaf area—and hence growth rate—of pasture during the ensuing winter months. Reported animal production responses to this practice have been variable, therefore a published mathematical model of a deferred grazing system was used to explore some of the reasons underlying this variation.Computer simulation was used to predict the effects of stocking rate, length of deferment and initial plant density. The results suggest that the response to deferment is markedly dependent on all three factors. The combinations of stocking rate and length of deferment maximising the response to deferred grazing were found to vary widely, depending on intial plant density and the economic weighting given to the value of liveweight produced. It was concluded that future experimentation needs to critically examine the interaction between these three factors if profitable response situations are to be defined more accurately.     

Dynamics of plant and animal production of a subterranean clover pasture grazed by sheep: Part 1—Field measurements for model calibration

A grazing system with Merino sheep and subterranean clover pasture was studied in a 550 mm rainfall, mediterranean climate in Western Australia.Changes over twelve months in seed, the quantities of green and dry herbage, soil moisture, animal intake and liveweight, wool growth and body composition were measured. Six paddocks, representing two soil types, were grazed continuously at 8·75 sheep per hectare. The system was also simulated and the actual results were compared with those from the simulation model.From a seed pool in March of 300 kg ha^?1, 80% of which was soft and non-dormant, 4000 clover seedlings per square metre became established; subsequent drought reduced this to 1450 plants per square metre. From measurements of soil moisture it was shown that this population survived at available moisture levels as low as 0·5 mm in the main root zone in gravelly sandy loam. Pasture growth rate reached a spring peak of 102 kg ha^?1 day^?1 and total growth (estimated from pasture grazed for 26 weeks) was 6700 kg ha^?1 for 500 mm of rainfall between germination and maximum biomass. At maturity, burr and seed made up 57% of the plant residues on offer, with a seed pool of 1160 kg ha^?1. During the summer this biomass decreased at 5 kg ha^?1 day^?1 without grazing and 19 kg ha^?1 day^?1 under grazing.The liveweight losses and gains of the sheep were atypical, no liveweight gain until 1200 kg ha^?1 of gree herbage was available—about treble the expected amount. Measurements of food intake indicate a gross inefficiency in energy utilisation during the winter and a low intake of energy in the spring.Total green and dry plant residues showed general agreement between actual and simulated results for most of the growing season. However, the field data highlighted error in the pasture sub-model which were corrected and are reported elsewhere.     

Finding niches for whole-farm design models – contradictio in terminis?

Whole-farm design models quantitatively analyze the effects of a variety of potential changes at the farm system level. Science-driven technical information is confronted with value-driven objectives of farmers or other social groupings under explicit assumptions with respect to exogenous variables that are important drivers of agricultural systems (e.g., market conditions). Hence, farm design is an outcome of objective specification and the potential of a system. In recent publications, whole-farm design modelling has been proposed to enhance (farm) innovation processes. A number of operational modelling tools now offers the opportunity to assess the true potential of whole-farm design modelling to enhance innovation. In this paper, we demonstrate that it is not trivial to find niches for the application of goal-based farm models. Model outcomes appeared not to match questions of farm managers monitoring and learning from their own and other farmers’ practices. However, our research indicates that whole-farm design modelling possesses the capabilities to make a valuable contribution to reframing. Reframing is the phenomenon that people feel an urge to discuss and reconsider current objectives and perspectives on a problem. Reframing might take place in a situation (i) of mutually felt dependency between stakeholders, (ii) in which there is sufficient pressure and urgency for stakeholders to explore new problem definitions and make progress. Furthermore, our research suggests that the way the researcher enters a likely niche to introduce a model and/or his or her position in this niche may have significant implications for the potential of models to enhance an innovation process. Therefore, we hypothesize that the chances of capitalizing on modelling expertise are likely to be higher when researchers with such expertise are a logical and more or less permanent component of ongoing trajectories than when these researchers come from outside to purposefully search for a niche.     

A model for evaluating grassland management decisions on dairy farms in the UK

The paper describes the structure, use and validation of a farm advisory model designed to evaluate grassland management on dairy farms. Using the model, it is possible to assess the implications of altering the timing and frequency of conservation cuts, of changing the pattern of fertiliser application, of altering the level of concentrate feeding and of changing grazing management. Comparisons of predicted with recorded grass yields, conservation areas and grazing efficiencies have shown that the model, in general, simulates farm and experimental observations reasonably well. However, for individual farms, cut yields and areas, the prediction errors can be large.     

Assessing the options for upland livestock systems under CAP reform: Developing and applying a livestock systems model within whole-farm systems analysis

This paper presents a scenario-based analysis of the impacts of Common Agricultural Policy (CAP) reform for upland agriculture using a Welsh case-study. Specifically the paper examines the impacts of the introduction of the single-farm payment (SFP), the modulation of direct payments under Pillar I of the CAP and the increase in agri-environment payments under Pillar II. Three enterprises are examined, upland sheep rearing with lamb finishing, spring- and autumn-calving suckler-cattle with calf rearing. These enterprises are modelled under conditions in 2002/3, 2004/5 and for the reformed CAP in 2005/6. To support this analysis a livestock system model (LSM) was implemented. The model assesses alternative management regimen using a flexible state-transition approach. This simplifies the realisation and parameterisation of potentially complex management regimen. The model tracks fodder requirements to achieve targets based on defined diets. The LSM underpins whole-farm analyses of stocking-rates, labour and other resource requirements and net-farm income. From the case study the paper concludes that the impacts of the introduction of the CAP reform on the financial performance of the systems are small but negative (a net reduction of around 5% in support). The larger reduction in direct payments (15–18%) is partially offset by agri-environment measures. The paper concludes that while SFP encourages a more market-oriented outlook, the adaptive capacity within systems as they stand is very limited. There are a range of possible adaptation strategies, but for the uplands the extensification of cattle systems by reducing stock numbers and cutting back on labour seems most probable.     

A model to estimate the performance,revenues and costs of dairy cows under different production and price situations

A model was constructed and validated to determine the course of performance, revenues and costs of dairy cows with different levels of milk production and of number of days open. For each month in lactation the revenues from milk production, which are dependent on the fat and protein contents, were determined. The feed costs were calculated from consumption of roughage and concentrate, which were estimated from the energy requirements. Furthermore, the course of the carcass value, calf revenues and the probability of, and the financial loss associated with, involuntary disposal were considered. Seasonal variation in production and prices was included in the model.Parameters of, and prices in, the model were chosen to represent the Black and White cows in The Netherlands at the normalized price level of 1981–1982.In the future the model will be used in studies on replacement policies in dairy cattle.     

Primary production of grazed annual natural pasture and of grazed wheat in a semi-arid region of Israel

The effect of grazing on the above-ground primary production and water use of a fertilised annual pasture and of fertilised sown wheat was studied. Similar studies were made of the effect of cutting on sown wheat. Two simple models—one exponential, the other logistic, both based on the dry weight of the above-ground biomass—were used to describe the primary production in all treatments.The production of undisturbed pasture and undisturbed wheat was 6480 and 7080 kg DM/ha, respectively. Grazing had no significant effect on the total production of the pasture but reduced the DM yield of wheat by up to 32%. Cutting of the wheat reduced the yield by 56%. Water use was similar in all treatments of pasture and wheat and was not directly related to production. The predicted production by both models was similar to observation in the early part of the season but differed from observation in the later part. Possible reasons for the discrepancies are discussed.     

A model to find and test decision rules for turnout date and grazing area allocation for a dairy cow system in spring

Use of grazing is decreasing in dairy cow systems in many European countries. However, pasture has a lower production cost than most conserved forage. Furthermore, agronomic decision rules are available to provide dairy cows with sufficient quality and quantity of dry matter. The decline in use of grazing is partly due to organisational difficulties in planning the grazing period. Land use at the farm level requires that this plan must be made in September, about 6 months before the turnout date. This area planning depends on the decision rule used for the turnout date. So we propose to search for a combination of area planning and turnout decision rules that allows a dairy herd to be fed in the spring by means of grazing without any risk of an interruption in the feeding. For this we propose to use a simulation model combining a grass growth model and a model for scheduling the farmer's decisions. A test of a turnout date rule showed that it is necessary to use an area allocation decision rule that depends on the turnout date rule and not the converse. We therefore proposed another turnout date rule based on the available herbage per ha and we calculated the area to allocate on the basis of this turnout rule.     

A body composition model for predicting beef animal growth

Two beef animal growth computer simulations were compared as to the effects of altering feed energy density. Steers of varying initial degrees of fatness were evaluated. The BEEF-NC114 model used NRC regression equations to predict growth while the BEEF-S156 model used physiological age and body components. Otherwise, the models were essentially identical. On average, the predictions of both models were reasonably close to each other. However, the BEEF-S156 model was more sensitive to extreme conditions of high or low nutrition.     

Smallholder dairy production and markets: A comparison of production systems in Zambia,Kenya and Sri Lanka

Three smallholder dairy production systems in Zambia, Sri Lanka and Kenya are analysed and compared. The focus is on the relationships between the animal production system, the farm household system, and the institutional environment. Attention is given to the valuation of marketed and non-marketed products and the intangible benefits of livestock in insurance, financing and status display. The comprehensive and comparative analysis of the production systems shows the direct relationship between type and intensity of dairy production and the presence or absence of markets for milk and other products, services and employment. The generally unobserved income components resulting from products other than milk and from the intangible benefits prove to be a substantial proportion of the total income in all three systems. The comprehensive perspective on the dairy production system results in policy suggestions that include institutional linkages.