Direct isotopic evidence of biogenic methane production and efflux from beneath a temperate glacier.

The base of glaciers and ice sheets provide environments suitable for the production of methane. High pressure conditions beneath the impermeable 'cap' of overlying ice promote entrapment of methane reserves that can be released to the atmosphere during ice thinning and meltwater evacuation. However, contemporary glaciers and ice sheets are rarely accounted for as methane contributors through field measurements. Here, we present direct field-based evidence of methane production and release from beneath the Icelandic glacier Sólheimajökull, where geothermal activity creates sub-oxic conditions suited to methane production and preservation along the meltwater flow path. Methane production at the glacier bed (48 tonnes per day, or 39 mM CH4 m-2 day-1), and evasion to the atmosphere from the proglacial stream (41 tonnes per day, or 32 M CH4 m-2 day-1) indicates considerable production and release to the atmosphere during the summer melt season. Isotopic signatures (-60.2‰ to -7.6‰ for δ13CCH4 and -324.3‰ to +161.1‰ for DCH4), support a biogenic signature within waters emerging from the subglacial environment. Temperate glacial methane production and release may thus be a significant and hitherto unresolved contributor of a potent greenhouse gas to the atmosphere.

Insights for the assessment of the economic impact of endemic diseases: specific adaptation of economic frameworks using the case of bovine viral diarrhoea.

Generic frameworks for the economic analysis of farm animal disease are now well established. The paper, therefore, uses bovine viral diarrhoea (BVD) as an example to explore how these frameworks need to be adapted to fit the characteristics of a particular disease and the specific objectives of the analysis. In the case of BVD, given the relative strength of tests available to correctly identify virus-positive animals, thus enabling them to be culled, the emphasis has been on cost-benefit analysis of regional and national certification/eradication schemes. Such analyses in turn raise interesting questions about farmer uptake and maintenance of certification schemes and the equity and cost-effective implementation of these schemes. The complex epidemiology of BVD virus infections and the long-term, widespread and often occult nature of BVD effects make economic analysis of the disease and its control particularly challenging. However, this has resulted in a wider whole-farm perspective that captures the influence of multiple decisions, not just those directly associated with disease prevention and control. There is a need to include management of reproduction, risk and enterprise mix in the research on farmer decision-making, as all these factors impinge on, and are affected by, the spread of BVD.

Les cadres généraux de l'analyse économique des maladies affectant les animaux d'élevage sont désormais bien établis. Les auteurs utilisent l'exemple de la diarrhée virale bovine pour définir les adaptations à apporter à ces cadres afin d'intégrer les caractéristiques d'une maladie donnée et les objectifs spécifiques de l'analyse. Dans le cas de la diarrhée virale bovine, compte tenu de la robustesse des tests disponibles pour détecter les animaux infectés (qui seront abattus), l'accent est mis sur l'analyse coûts-bénéfices des dispositifs régionaux et nationaux de certification sanitaire et d'éradication. Ces analyses soulèvent des questions intéressantes quant à l'engagement et à la persévérance des éleveurs à l'égard des dispositifs de certification et à la mise en oeuvre équitable et rentable de ces dispositifs. La complexité de l'infection due au virus de la diarrhée virale bovine et le caractère durable, répandu et souvent inapparent de ses effets rendent particulièrement difficiles les analyses économiques de cette maladie et de son contrôle. Ces analyses ont toutefois permis de mieux appréhender la situation dans la perspective d'une exploitation, en tenant compte des effets de décisions multiples qui ne se limitent pas à celles directement destinées à prévenir et à contrôler la maladie. La gestion de la reproduction, la gestion des risques et les choix de diversification doivent impérativement être intégrés dans la recherche sur les processus décisionnaires des éleveurs, car tous ces aspects affectent et sont affectés par la propagation de la diarrhée virale bovine.

Hoy en día ya existen modelos genéricos sobradamente contrastados para analizar en clave económica las enfermedades de los animales de granja. Partiendo de esta realidad, los autores utilizan el ejemplo de la diarrea viral bovina (DVB) para determinar el modo de adaptar esos modelos genéricos para que encajen con las características de una enfermedad en particular y con los objetivos específicos de un determinado análisis. En el caso de la DVB, teniendo en cuenta la relativa solidez de los ensayos existentes para identificar correctamente a los animales infectados (para su posterior sacrificio), los autores se centraron en analizar la relación costo-beneficio que presentan algunos dispositivos regionales y nacionales de certificación sanitaria o erradicación. Estos análisis, a su vez, abren interesantes interrogantes sobre el nivel de adhesión y perseverancia de los productores respecto de los programas de certificación y sobre el grado de equidad y rentabilidad con que se aplican esos dispositivos. La compleja epidemiología de las infecciones por el virus de la DVB y el carácter duradero, extendido y a menudo oculto de sus efectos dificultan especialmente el análisis en clave económica de la enfermedad y de las medidas para combatirla. Sin embargo, estos análisis han permitido aprehender desde una perspectiva más amplia la situación de la explotación en su conjunto, teniendo en cuenta la influencia de múltiples decisiones, y no solo de aquellas directamente relacionadas con la prevención y el control de la enfermedad. En toda investigación sobre el proceso decisorio de los productores es necesario tener en cuenta la gestión de la reproducción, la gestión de los riesgos y el tipo de actividades de la explotación, pues todos estos factores influyen en la propagación de la DVB y son influidos por ella.

An approach to holistically assess (dairy) farm eco-efficiency by combining Life Cycle Analysis with Data Envelopment Analysis models and methodologies.

Eco-efficiency is a useful guide to dairy farm sustainability analysis aimed at increasing output (physical or value added) and minimizing environmental impacts (EIs). Widely used partial eco-efficiency ratios (EIs per some functional unit, e.g. kg milk) can be problematic because (i) substitution possibilities between EIs are ignored, (ii) multiple ratios can complicate decision making and (iii) EIs are not usually associated with just the functional unit in the ratio's denominator. The objective of this study was to demonstrate a 'global' eco-efficiency modelling framework dealing with issues (i) to (iii) by combining Life Cycle Analysis (LCA) data and the multiple-input, multiple-output production efficiency method Data Envelopment Analysis (DEA). With DEA each dairy farm's outputs and LCA-derived EIs are aggregated into a single, relative, bounded, dimensionless eco-efficiency score, thus overcoming issues (i) to (iii). A novelty of this study is that a model providing a number of additional desirable properties was employed, known as the Range Adjusted Measure (RAM) of inefficiency. These properties altogether make RAM advantageous over other DEA models and are as follows. First, RAM is able to simultaneously minimize EIs and maximize outputs. Second, it indicates which EIs and/or outputs contribute the most to a farm's eco-inefficiency. Third it can be used to rank farms in terms of eco-efficiency scores. Thus, non-parametric rank tests can be employed to test for significant differences in terms of eco-efficiency score ranks between different farm groups. An additional DEA methodology was employed to 'correct' the farms' eco-efficiency scores for inefficiencies attributed to managerial factors. By removing managerial inefficiencies it was possible to detect differences in eco-efficiency between farms solely attributed to uncontrollable factors such as region. Such analysis is lacking in previous dairy studies combining LCA with DEA. RAM and the 'corrective' methodology were demonstrated with LCA data from French specialized dairy farms grouped by region (West France, Continental France) and feeding strategy (regardless of region). Mean eco-efficiency score ranks were significantly higher for farms with 30% maize in the total forage area before correcting for managerial inefficiencies. Mean eco-efficiency score ranks were higher for West than Continental farms, but significantly higher only after correcting for managerial inefficiencies. These results helped identify the eco-efficiency potential of each region and feeding strategy and could therefore aid advisors and policy makers at farm or region/sector level. The proposed framework helped better measure and understand (dairy) farm eco-efficiency, both within and between different farm groups.

An analysis of cattle farmers' perceptions of drivers and barriers to on-farm control of Escherichia coli O157.

Structural equation modelling and survey data were used to test determinants' influence on farmers' intentions towards Escherichia coli O157 on-farm control. Results suggest that farmers more likely to show willingness to spend money/time or vaccinate to control Escherichia coli O157 are those: who think farmers are most responsible for control; whose income depends more on opening farms to the public; with stronger disease control attitudes; affected by outbreaks; with better knowledge and more informed; with stronger perceptions of biosecurity measures' practicality; using a health plan; who think farmers are the main beneficiaries of control; and whose farms are dairy rather than beef. The findings might suggest that farmers may implement on-farm controls for E. coli O157 if they identify a clear hazard and if there is greater knowledge of the safety and efficacy of the proposed controls.

Adaptation by macrophytes to inorganic carbon down a river with naturally variable concentrations of CO2.

The productivity and ecological distribution of freshwater plants can be controlled by the availability of inorganic carbon in water despite the existence of different mechanisms to ameliorate this, such as the ability to use bicarbonate. Here we took advantage of a short, natural gradient of CO2 concentration, against a background of very high and relatively constant concentration of bicarbonate, in a spring-fed river, to study the effect of variable concentration of CO2 on the ability of freshwater plants to use bicarbonate. Plants close to the source, where the concentration of CO2 was up to 24 times air equilibrium, were dominated by Berula erecta. pH-drift results and discrimination against (13)C were consistent with this and the other species being restricted to CO2 and unable to use the high concentration of bicarbonate. There was some indication from stable (13)C data that B. erecta may have had access to atmospheric CO2 at low water levels. In contrast, species downstream, where concentrations of CO2 were only about 5 times air-equilibrium were almost exclusively able to use bicarbonate, based on pH-drift results. Discrimination against (13)C was also consistent with bicarbonate being the main source of inorganic carbon for photosynthesis in these species. There was, therefore, a transect downstream from the source of increasing ability to use bicarbonate that closely matched the decreasing concentration of CO2. This was produced largely by altered species composition, but partly by phenotypic changes in individual species.

Environmental efficiency of alternative dairy systems: a productive efficiency approach.

Agriculture across the globe needs to produce "more with less." Productivity should be increased in a sustainable manner so that the environment is not further degraded, management practices are both socially acceptable and economically favorable, and future generations are not disadvantaged. The objective of this paper was to compare the environmental efficiency of 2 divergent strains of Holstein-Friesian cows across 2 contrasting dairy management systems (grazing and nongrazing) over multiple years and so expose any genetic × environment (G × E) interaction. The models were an extension of the traditional efficiency analysis to account for undesirable outputs (pollutants), and estimate efficiency measures that allow for the asymmetric treatment of desirable outputs (i.e., milk production) and undesirable outputs. Two types of models were estimated, one considering production inputs (land, nitrogen fertilizers, feed, and cows) and the other not, thus allowing the assessment of the effect of inputs by comparing efficiency values and rankings between models. Each model type had 2 versions, one including 2 types of pollutants (greenhouse gas emissions, nitrogen surplus) and the other 3 (greenhouse gas emissions, nitrogen surplus, and phosphorus surplus). Significant differences were found between efficiency scores among the systems. Results indicated no G × E interaction; however, even though the select genetic merit herd consuming a diet with a higher proportion of concentrated feeds was most efficient in the majority of models, cows of the same genetic merit on higher forage diets could be just as efficient. Efficiency scores for the low forage groups were less variable from year to year, which reflected the uniformity of purchased concentrate feeds. The results also indicate that inputs play an important role in the measurement of environmental efficiency of dairy systems and that animal health variables (incidence of udder health disorders and body condition score) have a significant effect on the environmental efficiency of each dairy system. We conclude that traditional narrow measures of performance may not always distinguish dairy farming systems best fitted to future requirements.

Stable isotope switching (SIS): a new stable isotope probing (SIP) approach to determine carbon flow in the soil food web and dynamics in organic matter pools.

RATIONALE: Recent advances in stable isotope probing (SIP) have allowed direct linkage of microbial population structure and function. This paper details a new development of SIP, Stable Isotope Switching (SIS), which allows the simultaneous assessment of carbon (C) uptake, turnover and decay, and the elucidation of soil food webs within complex soils or sedimentary matrices. METHODS: SIS utilises a stable isotope labelling approach whereby the (13)C-labelled substrate is switched part way through the incubation to a natural abundance substrate. A (13)CH(4) SIS study of landfill cover soils from Odcombe (Somerset, UK) was conducted. Carbon assimilation and dissimilation processes were monitored through bulk elemental analysis isotope ratio mass spectrometry and compound-specific gas chromatography/combustion/isotope ratio mass spectrometry, targeting a wide range of biomolecular components including: lipids, proteins and carbohydrates. RESULTS: Carbon assimilation by primary consumers (methanotrophs) and sequential assimilation into secondary (Gram-negative and -positive bacteria) and tertiary consumers (Eukaryotes) was observed. Up to 45% of the bacterial membrane lipid C was determined to be directly derived from CH(4) and at the conclusion of the experiment ca. 50% of the bulk soil C derived directly from CH(4) was retained within the soil. CONCLUSIONS: This is the first estimate of soil organic carbon derived from CH(4) and it is comparable with levels observed in lakes that have high levels of benthic methanogenesis. SIS opens the way for a new generation of SIP studies aimed at elucidating total C dynamics (incorporation, turnover and decay) at the molecular level in a wide range of complex environmental and biological matrices.

The effect of improving cow productivity, fertility, and longevity on the global warming potential of dairy systems.

This study compared the environmental impact of a range of dairy production systems in terms of their global warming potential (GWP, expressed as carbon dioxide equivalents, CO(2)-eq.) and associated land use, and explored the efficacy of reducing said impact. Models were developed using the unique data generated from a long-term genetic line × feeding system experiment. Holstein-Friesian cows were selected to represent the UK average for milk fat plus protein production (control line) or were selected for increased milk fat plus protein production (select line). In addition, cows received a low forage diet (50% forage) with no grazing or were on a high forage (75% forage) diet with summer grazing. A Markov chain approach was used to describe the herd structure and help estimate the GWP per year and land required per cow for the 4 alternative systems and the herd average using a partial life cycle assessment. The CO(2)-eq. emissions were expressed per kilogram of energy-corrected milk (ECM) and per hectare of land use, as well as land required per kilogram of ECM. The effects of a phenotypic and genetic standard deviation unit improvement on herd feed utilization efficiency, ECM yield, calving interval length, and incidence of involuntary culling were assessed. The low forage (nongrazing) feeding system with select cows produced the lowest CO(2)-eq. emissions of 1.1 kg/kg of ECM and land use of 0.65 m(2)/kg of ECM but the highest CO(2)-eq. emissions of 16.1t/ha of the production systems studied. Within the herd, an improvement of 1 standard deviation in feed utilization efficiency was the only trait of those studied that would significantly reduce the reliance of the farming system on bought-in synthetic fertilizer and concentrate feed, as well as reduce the average CO(2)-eq. emissions and land use of the herd (both by about 6.5%, of which about 4% would be achievable through selective breeding). Within production systems, reductions in CO(2)-eq. emissions per kilogram of ECM and CO(2)-eq. emissions per hectare were also achievable by an improvement in feed utilization. This study allowed development of models that harness the biological trait variation in the animal to improve the environmental impact of the farming system. Genetic selection for efficient feed use for milk production according to feeding system can bring about reductions in system nutrient requirements, CO(2)-eq. emissions, and land use per unit product.

Testing the use of septum-capped vials for 13C-isotope abundance analysis of carbon dioxide.

Studying ecosystem processes in the context of carbon cycling and climate change has never been more important. Stable carbon isotope studies of gas exchange within terrestrial ecosystems are commonly undertaken to determine sources and rates of carbon cycling. To this end, septum-capped vials ('Exetainers') are often used to store samples of CO(2) prior to mass spectrometric analysis. To evaluate the performance of such vials for preserving the isotopic integrity (delta(13)C) and concentration of stored CO(2) we performed a rigorous suite of tests. Septum-capped vials were filled with standard gases of varying CO(2) concentrations (approximately 700 to 4000 ppm), delta(13)C values (approx. -26.5 to +1.8 per thousand(V-PDB)) and pressures (33 and 67% above ambient), and analysed after a storage period of between 7 and 28 days. The vials performed well, with the vast majority of both isotope and CO(2) concentration results falling within the analytical uncertainty of chamber standard gas values. Although the study supports the use of septum-capped vials for storing samples prior to mass spectrometric analysis, it does highlight the need to ensure that sampling chamber construction is robust (air-tight).

An integrated approach to assessing the viability of eradicating BVD in Scottish beef suckler herds.

The viability of eradicating bovine viral diarrhoea (BVD) in Scottish suckler herds is dependent on the continued compliance with eradication schemes. At the farm level, the costs of BVD have been identified in previous studies and show a substantial financial imperative to avoid infection. At a regional level the incentives of BVD eradication to individuals are unclear, for example the requirement for vaccination strategies despite achieving disease-free status. Ensuring farmer compliance with an eradication scheme is therefore difficult. Experience of eradicating BVD from beef-dominated areas is limited and theoretical models have tended to focus on the dairy sector. Here we present a stochastic epidemiological model of a typical beef suckler herd to explore the interaction of a farm with a regional pool of replacements, utilising information from a BVD virus seroprevalence survey of Scottish beef suckler herds. Our epidemiological model is then used to assess the relative costs to individuals assuming different regional endemic prevalences, which are used to represent the likelihood of BVD re-introduction. We explore the relative cost of BVD, taken as likelihood and consequence, at an endemic steady state in contrast to previous models that have assumed the introduction or control of BVD in an epidemic state (e.g. a closed and mostly susceptible population). Where endemic, BVD is unlikely to affect all farms evenly and will cost most farmers very little due to herd immunity or self-clearance of the virus. Compliance is likely to be boosted by pump-priming to initiate and complete eradication schemes with cost-sharing.

Modelling the effects of previous infection and re-infection on the costs of bovine viral diarrhoea outbreaks in beef herds.

A previously published model was re-employed to examine the potential impact of different epidemiological circumstances on output losses due to bovine viral diarrhoea virus (BVDV) infection in typical British hill cow-calf enterprises. The average annuity equivalent of unchecked losses from 100 simulated 10-year disease scenarios ranged from almost pound0/cow to approximately pound40/cow. Significant differences were found under certain circumstances, depending on the initial disease status of the herd, the initial source of virus, the probability and source of further infection, the probability of virus transmission within the herd and herd size. For naïve herds, losses depended only on the risk of incursion. In most other circumstances, the losses could be mitigated if the annual risk of incursion was <0.3 and risk of within herd transmission was extremely low. Greater understanding of the interaction between these risk factors and management actions are required so that total costs of BVDV infection can be minimised under different circumstances.

Use of a benefit function to assess the relative investment potential of alternative farm animal disease prevention strategies.

Using the example of bovine viral diarrhoea (BVD) in Scottish suckler (cow-calf) beef herds, this paper demonstrated a method to establish the maximum average net benefit of disease control under specific epidemiological and farm business circumstances. Data were generated for the method using a stochastic epidemiological model set to estimate the mean and variance of control costs and output losses from BVD for 50-cow or 120-cow herds, either free of BVD at the outset or of unknown BVD status. Control of disease was by increased investment in a variety of ('biosecurity') measures aimed at reducing the probability of virus entering the closed herd in any 1 year of a 10-year period of simulated exposure to risk from BVD virus introduction either with or without vaccination. Herds free of BVD at the outset enjoyed much greater maximum average net benefits than herds of unknown BVD status. Best allocations of hypothetical incentives to encourage farmers to establish their freedom from BVD were therefore outlined. Vaccination and biosecurity were generally found to be complementary rather than substitutes for one another. The advantages of the maximum net benefit measure over the more usual average total cost of endemic disease were demonstrated and discussed. The maximum net benefit method focuses on the relationship between costs and benefits, which often exhibits diminishing marginal returns meaning that profit maximisation and disease minimisation are incompatible. The method can also allow for constraints on and competition for limited farm resources. It was argued that these attributes are important to persuade farmers to invest in animal health.

A model of the relationship between the epidemiology of Johne's disease and the environment in suckler-beef herds.

A non-predictive, dynamic and stochastic herd-level simulation model of an outbreak of Johne's in a suckler-beef herd is reported. Importantly, the model incorporates, with a simple method, the environment as the primary source of infection, reflecting the consensual understanding of the disease. The model also takes into account the density of the infectious agent in the environment. A sensitivity analysis suggests that the model is highly and equally sensitive to certain parameters (probability of infection in the presence of one unit of bacterial density, infectious area and bacterial shedding rate). Mathematical reasons for this similarity in sensitivity are presented. Compared to many other diseases, data for Johne's are scarce. Therefore models of Johne's outbreaks including this one cannot be predictive or easily validated. The qualitative results: (a) demonstrate the modelled effect of inclusion of infection via the environment; (b) suggest management factors that could be tested by experimentation or observation. Estimates for the rate of transmission, arising from the model output, are similar to published empirical estimates. The results of future empirical research should aid scientific understanding of the disease, help validate this model and might bring economic benefits through improved management.

Modelling BVD at herd level compared with individual animal level.

We assess two closely related stochastic models of BVD in a beef herd for their utility. One is an individual (animal) based model and the other a herd based model.

Assessing economic and social pressure for the control of bovine viral diarrhoea virus.

The objective of this paper is to present a preliminary assessment of variation in the economic impact of bovine viral diarrhoea virus (BVDV) at dairy farm level between a sample of nations within the EU and hence assess differences in pressure to respond to this disease that may be impeding progress in control and hence restricting collective benefits from healthier livestock. We used a questionnaire to obtain national average values of key epidemiological and economic parameters for a typical dairy farm from BVDV experts in the countries concerned. These parameters were converted into assessments of economic impact using a computer simulation model. Uncontrolled output losses for a BVDV-naïve herd with virus introduced in year 1 of a 10-year epidemic represented 22, 7, 8, 5, 8 and 20% of the BVDV-free annuity for the UK, Northern Portugal, Holland, Norway, Italy and Germany, respectively. Differences between countries will be widened by differences in the risk of acquiring BVDV. These will be much reduced in countries, such as Norway that have a national BVDV eradication programme. Farmers in such countries can therefore justify spending much less on maintaining BVDV-free status than BVDV-free farms in other countries. This result illustrates the paradox that in countries where BVDV prevalence is high, farmers have least to gain from unilateral BVDV eradication because of the high cost of maintaining freedom from the disease. We discuss this issue in the light of increasing recognition at international level of the importance of BVDV control.

Optimal risk management versus willingness to pay for BVDV control options.

Previous work has shown that the least-cost BVD-control option for cow-calf herds in Scotland was not necessarily the risk-minimising option. Thus, assessing BVDV prevention measures must account for risk reduction as well as decision-makers' attitude towards risks. We therefore describe a method to do this using a hypothetical example. Data for this analysis were generated using a simulation model of BVDV transmission in a typical Scottish cow-calf herd over a 10-year period. Herd infection/re-infection was adjusted to reflect the expected risk of infection/re-infection and the use of either biosecurity or vaccination strategies at various levels of effectiveness. The level of risk-free financial return that maximises farmers' utility of wealth was significantly affected by the assumed effectiveness of the control strategy. More importantly, it was observed that utility maximisation of wealth as a business objective is not an optimal solution in terms of animal welfare when dealing with a BVDV outbreak.

Financial incentive to control paratuberculosis (Johne's disease) on dairy farms in the United Kingdom.

This paper estimates the financial incentive to control paratuberculosis on dairy farms by establishing the level of expenditure that would minimise the total cost of the disease (output losses plus control expenditure). Given the late onset of the clinical signs and the lack of treatments, control was focused on minimising the financial impact of paratuberculosis by adjusting the dairy cow replacement policy. The optimum replacement policies for disease-free herds and infected herds were compared by using dynamic programming. At the standard settings, the disease justified adjusting the culling policy; under constant bioeconomic assumptions, it reduced the expected annuity from milk production under the optimal replacement policy by about 10 per cent (27 pounds sterling per cow annually), a considerably lower figure than for other major endemic diseases that affect dairy cows in the uk. The effect was even less at lower milk prices, suggesting that there is at present little incentive for dairy farmers to put more resources into controlling the disease. However, the incentive could be increased if more information were available about how best to manage the disease under specific farm circumstances. Any effect that paratuberculosis may have on the future demand for milk and hence on milk prices would also be an important consideration.

Modelling and costing BVD outbreaks in beef herds.

Results from an epidemiological model of an outbreak of Bovine Viral Diarrhoea (BVD) within a Scottish beef suckler herd are presented. These results concurred with field observations and encouraged us to fulfill the objective of providing an estimate of losses due to BVD to assist decision makers. Using the output from the model, estimates were made of losses associated with the outbreak. Without taking into account any financial premiums associated with disease-free status, the estimate of pound 37 (58) mean loss per cow per annum suggests that health schemes and vaccination should be of immediate financial interest to farmers and veterinary advisors.

A linear programming approach to estimate the economic impact of bovine viral diarrhoea (BVD) at the whole-farm level in Scotland.

We combined epidemiological and economic concepts and modelling techniques, to integrate animal health into whole-farm business management. This allowed us to assess the relative contribution that disease prevention could make to whole-farm income and to the variability in farm income (risk). It also allowed us to assess disease losses in the context of a farm business rather than as a disease outbreak in isolation. A linear program ("MOTAD") establishes the combination of decision maker's activities that minimise risk for a given level of income within farm-business constraints. The MOTAD model was applied to farm-management decision making in Scottish cow-calf herds and was linked to an epidemiological model of bovine viral diarrhoea (BVD). When BVD was considered in isolation (i.e. without taking into account risk), the minimum expected total cost of BVD (sum of output losses plus expenditure on prevention) was similar whether the herd was susceptible to BVD or of unknown BVD-status at the outset. However, the expected total cost of BVD fell in response to increasing expenditure on prevention in 'susceptible' herds. This relationship was not apparent in herds of unknown BVD-status. As a consequence of this difference, 'susceptible' herds were better able to use investment in BVD biosecurity as a means to increase farm income at minimum risk than herds of unknown BVD-status. 'Susceptible' herds therefore were able to achieve high income targets with less-intensive production than herds of unknown BVD-status. This suggested that maintaining a cow-calf herd free of BVD contributes to farm income and risk management indirectly through its effect on the management of the whole farm. It follows that measurement of the economic impact of BVD requires a whole-farm perspective that includes a consideration of risk. Because farmers generally are considered to be risk adverse, this means that the least-cost disease-control option might not always be the preferred option.

Dynamic programming to investigate financial impacts of mastitis control decisions in milk production systems.

An adaptive stochastic dynamic programming model was used to solve the optimum replacement decision problem for the dairy cow under a range of alternative mastitis control procedures. The model predicted that reducing milk yield losses and somatic cell count penalties by, using milking machine test, post-milking teat disinfection and dry cow therapy added approximately pounds sterling 4, pounds sterling 10 and pounds sterling 13 respectively to an original annuity equivalent net present value for the replacement heifer of pounds sterling 286. Assuming that these procedures also reduced involuntary culling due to mastitis by 50% added pounds sterling 8.90 to the annuity. This latter figure indicated that an important part of the benefit of mastitis control procedures might come from a reduction in the culling risk of persistent clinical cases. We concluded that the strength of the dynamic programming model in this context was that it provided an integrated evaluation of the various impacts of each alternative mastitis procedure in the long term, which is essential for correct economic evaluation of mastitis.