Mixed models and multilevel data structures in agriculture

Multilevel data structures can occur in many areas of agricultural research for instance on-farm trials, where there can be information at the village, farm and plot or animal level. Analysis of variance except in balanced or nested designs - has been difficult to apply to data with a multilevel structure. Mixed modelling is becoming a standard approach for analysing these types of data. The mixed model facilities are now available in some of the more powerful statistical packages such as Genstat and SAS. The purpose of this guide book is to review the general concepts of mixed models. The document illustrates by example how to recognise the structure in the data and how to fit and interpret a mixed model analysis. The reader is expected to be familiar with simple analysis of variance methods. Three examples are used in the discussion. Example one, 'fodder production trial', is a fairly traditional agricultural experiment, and is included to show how mixed modelling links to more traditional analysis. Example two, 'concentrate feeding trial', is an on-farm trial with a slightly 'messy' hierarchical structure. It is used to show how the ideas of example one can be extended to other situations, and to demonstrate the benefits of mixed modelling. Example three, 'sheep breeding trial', is a more specialised example of breeding trial. It discusses the implications of formulating models in different ways

Towards increased use of trypanotolerance: Current research and future directions. Proceedings of a workshop

This workshop was conceived in recognition of the fact that, over the years, a great deal of research has been conducted on trypanotolerance. With the advent of new and powerful means to identify markers and genes controlling traits of interest, and with the developments in animal breeding bringing greater speed in dissemination of desirable genotypes, there is increasing interest in trypanotolerance as a way forward in trypanosomiasis control. It was felt that the time had come for a further look at trypanotolerance research and the prospects for utilization of the trait to improve productivity of livestock under trypanosomiasis challenge, with the overall objective of assisting planning of future ILRAD/ILCA collaborative activities in the area. The first part of the report presents and discusses the research on the biological, social and economic aspects of trypanotolerance. The salient points from the discussions were then used as a focus for subsequent roundtable discussions in the different areas (conservation, preservation, enhancement and propagation). In a final session, the workshop agreed a number of recommendations

Enhancement of capacity in applied biometry in East and southern Africa

Biometric skills in developing countries are inadequate to support the changing priorities of agricultural research with the consequence that the quality of research aimed at increasing food security and alleviating poverty is often deficient because it lacks good statistical design" (CTA, 1997). It was on this basis that the workshop was planned, realising that not only do national scientists and statisticians (biometricians) need training in applied biometrics but they also need to be exposed to the range of problems and complexity of agricultural study design necessary for the developing world. Few courses in applied biometrics exist in universities in East and southern Africa. Training in biometrics provided hitherto by overseas institutions has been beneficial, but lecturers do not often have a full appreciation, or practical hands-on experience, of the special features of agricultural research and development in Africa. The subjects of experimental design, statistical analysis, interpretation and presentation of results are still recognised to be among the top priorities for training by national agricultural research system (NARS) scientists participating in collaborative networks co-ordinated by the International Livestock Research Institute (ILRI) and the International Centre for Research in Agroforestry (ICRAF). There is thus a major need to establish strong training programmes in the region itself with links to existing programmes abroad

Using the economic surplus model to measure potential returns to international livestock research. The case of trypanosomosis vaccine research

This study illustrates how productivity impacts resulting from livestock research can be measured using a herd simulation model, how the results of this model can be extended spatially using geographic information systems (GIS) to determine the potential increase in livestock production that would result from adoption of a new technology, and how an economic surplus model can be used to value the estimated productivity impacts. The particular problem examined is trypanosomosis in cattle in Africa, and the potential research product is a multi-component vaccine. The results suggest that the potential benefits of trypanosomiasis control, in terms of meat and milk productivity alone are worth over US$ 700 million per year in Africa. The methodology developed in this study can be used to measure the benefits of alleviating constraints to livestock production and the potential returns to research and development approaches addressing those constraints. The results of this study will assist in research priority setting and have highlighted the need for further research aimed at better understanding who the beneficiaries of the vaccine will be, and how it will reach them

Herschel*-ATLAS: correlations between dust and gas in local submm-selected galaxies

We present an analysis of CO molecular gas tracers in a sample of 500 μ m-selected Herschel -ATLAS galaxies at z < 0 . 05 ( cz < 14990 km s − 1 ). Using 22 − 500 μ m photom- etry from WISE , IRAS and Herschel , with H i data from the literature, we investigate correlations between warm and cold dust, and tracers of the gas in different phases. The correlation between global CO(3–2) line fluxes and FIR–submm fl uxes weakens with increasing IR wavelength ( λ & 60 μ m), as a result of colder dust being less strongly associated with dense gas. Conversely, CO(2–1) and H i line fluxes both ap- pear to be better correlated with longer wavelengths, suggesting that cold dust is more strongly associated with diffuse atomic and molecular gas phases, co nsistent with it being at least partially heated by radiation from old stellar populations . The increased scatter at long wavelengths implies that sub-millimetre fluxes are a po orer tracer of SFR. Fluxes at 22 and 60 μ m are also better correlated with diffuse gas tracers than dense CO(3–2), probably due to very-small-grain emission in the diffu se interstellar medium, which is not correlated with SFR. The FIR/CO luminosity ratio a nd the dust mass/CO luminosity ratio both decrease with increasing luminosit y, as a result of either correlations between mass and metallicity (changing CO/H 2 ) or between CO luminosity and excitation [changing CO(3–2)/CO(1–0)].Web of Scienc

Herschel-ATLAS: the surprising diversity of dust-selected galaxies in the local submillimetre Universe

Galaxie