Uncertainty analysis of the productivity of cattle populations in tropical drylands.

This article presents an uncertainty analysis of the productivity of cattle herds in traditional farming systems of West and Central African drylands. The study focused on productivity rates in animal numbers (RN) and meat weights (RW) estimated from a herd growth model, which were compared with FAOSTAT-based estimates. The uncertainty analysis contained the following two steps: uncertainty propagation and a global sensitivity analysis. The analysis was based on a state-of-the-art of the current knowledge and a set of available data on the herd performances. The calculations used Monte Carlo simulations to estimate the 95% confidence intervals (CI) of RN and RW and the standardized regression coefficients method to estimate the contribution of the input variables to the outputs variances. The mean rate RN was estimated to 0.127 animal/animal-year with a 95% CI of (0.091, 0.163) and the mean rate RW to 11.7 kg/animal-year with a 95% CI of (8.8, 14.7), corresponding to relative variation around the mean of about ± 29% and ± 25%, respectively. The input variables that contributed most to the variance of RN (almost 76% of the output variance) were the calving rate, the adult female mortality rate and the female proportion in the population (determined by the pattern of the male offtake in the herds). The input variables that contributed most to the variance of RW were the same as those for RN plus the adult live weights. The CI ranges that were estimated in this article indicate that productivity rates based on literature data or expert estimations of the herd performances should be considered with caution. Research efforts based on gold-standard herd monitoring protocols accounting for temporal and spatial variations should be undertaken in future to decrease the knowledge gaps on the input variables that contribute most to these ranges.

A simple model for simulating immunity rate dynamics in a tropical free-range poultry population after avian influenza vaccination.

In developing countries, vaccination against highly pathogenic avian influenza subtype H5N1 (HPAI) in free-range poultry flocks is usually implemented as periodic campaigns and newborn chicks are generally not vaccinated by farmers between vaccination passes. The demographic population turnover leads to a continuous decrease in the population immunity rate (PIR) over time. We present a simple Leslie matrix model for estimating population turnover and PIR dynamics in a hypothetical small-size vaccinated free-range poultry population. Four different vaccination scenarios were identified assuming necessary procedures to achieve immunity. The results indicate that high levels of population immunity are difficult to sustain. Assuming an animal immunity response of 80% after vaccination and a constant population size, PIR 4 months after vaccination was 30% in all the scenarios. Predictions averaged over time showed mean PIR between 36% and 48%, which is below the population immunity thresholds for eradication approximated from R0 estimates.

Evaluation of crude annual parturition rate estimates in a small-holder African ruminant farming system.

Many parameters have been proposed for evaluating livestock reproduction performances in tropical farming systems. In tropical free-ranged and small-holder systems, where reproduction cycles cannot be individually observed without expensive field surveys, one of these parameters is the average number of parturitions (h) expected by reproductive female, if the female spends the whole year in the herd. A frequent approach for estimating h is to use the ratio hc = m/T, so-called 'crude annual parturition rate', where m is the observed number of parturitions and T is the total time of presence of the reproductive females in the herd during the year. The bias encountered when h is estimated by hc was evaluated in this paper. Six methods of estimation were used, where T was the exact observed time of presence (hc1) or approximated by monthly, quarterly, half-yearly and yearly averages or final size of the reproductive herd size (hc2 to hc6). Data came from long-term follow-up of cattle and small-ruminant herds (with data recorded at animal level) in extensive agro-pastoral systems in Senegal. In general, h was correctly estimated by hc1. Nevertheless hc1 was sensitive to competing risks (e.g. deaths, sales and slaughtering of reproductive females) and was seriously biased when intensive withdrawals of females occurred before or during the parturition peak. Reliability of crude rates progressively decreased from hc2 to hc6, corresponding to the degradation of information used for approximating T. This decrease was much lower for cattle (for which all methods had acceptable reliability) than for small ruminants. Among the compared methods, the lower reliability was observed for hc6 that we do not recommend for small ruminants. Methods hc5 and hc6 are currently used in rapid cross-sectional retrospective surveys based on the recall of the farmers on the demographic events which occurred in the herd over the last past 12 months. The study has showed that such surveys and estimates hc5 and hc6 can generate seriously biased results. More globally, annual parturition rates can be highly variable depending on the 12-month periods considered. Annual parturition rates estimated on short-term data, even with precise herd follow-up surveys, must be considered cautiously.

Contagious bovine pleuropneumonia vaccines, historic highlights, present situation and hopes.

Contagious bovine pleuropneumonia (CBPP) is a contagious infection of cattle caused by a mycoplasma, M. mycoides subsp. mycoides SC (MmmSC). It induces lesions of pleuropneumonia in acute cases and the formation of pulmonary "sequestra" in chronic cases. The disease is prevalent mostly in Africa, where it is responsible for high losses, but it has also been sporadically present in Southern Europe until 1999. Vaccination is now prohibited in most countries except in Africa. An empirical "inoculation" procedure was developed as early as 1852 in Europe but it may have been used even earlier in Africa. The inoculation of pleural fluid was performed at the tip of the tail in Europe and on the bridge of the nose in Africa. It conferred good protection but induced a high number of fatal cases. Various inactivated preparations have been tested in the past with inconclusive results leading sometime to some protection and some other time to a sensitisation of the immunised animals. Such preparations have never been used in the field. Attenuated MmmSC strains have been developed in the 1950s and used extensively in the field both in Africa and Australia. The best known vaccine strains are KH3J, T1/44 and T1sr. Vaccination campaigns have succeeded in reducing considerably the CBPP prevalence in these two continents but eradication was achieved in Australia only by switching to strict measures of animal movement control and a stamping-out policy. The search for new CBPP vaccines has become a major issue for African countries that are facing an increase in outbreaks. The rationale for this search is based on a better understanding of the mycoplasma virulence mechanisms that could lead to a targeted attenuation of MmmSC strains. It is also based on a better understanding of the bovine immune response that may be driven to a pathogenic inflammatory response or conversely to a better balanced response leading to protection.

Graphical approaches to support the analysis of linear-multilevel models of lamb pre-weaning growth in Kolda (Senegal).

Linear-multilevel models (LMM) are mixed-effects models in which several levels of grouping may be specified (village, herd, animal, ellipsis). This study highlighted the usefulness of graphical methods in their analysis through: (1) the choice of the fixed and random effects and their structure, (2) the assessment of goodness-of-fit and (3) distributional assumptions for random effects and residuals. An LMM was developed to study the effect of ewe deworming with morantel on lamb pre-weaning growth in a field experiment involving 182 lambs in 45 herds and 10 villages in Kolda, Senegal. Growth was described as a quadratic polynomial of age. Other covariates were sex, litter-size and treatment. The choice of fixed and random effects relied on three graphs: (1) a trellis display of mean live-weight vs. age, to select main effects and interactions (fixed effects); (2) a trellis display of individual growth curves, to decide which growth-curve terms should be included as random effects and (3) a scatter plot of parameters of lamb-specific regressions (live-weight vs. quadratic polynomial of age) to choose the random-effects covariance structure.Age, litter-size, agexlitter-size, litter-sizextreatment and agexlitter-sizextreatment were selected graphically as fixed effects and were significant (p<0.05) in subsequent statistical models. The selection of random-effect structures was guided by graphical assessment and comparison of the Akaike's information criterion for different models. The final random-effects selected included no random effect at the village level but intercept, age and squared-age at the herd and lamb levels. The structure of the random-effects variance-covariance matrices were blocked-diagonal at the herd level and unstructured at the lamb level. An order-1 autoregressive structure was retained to account for serial correlations of residuals. Smaller residual variance at 90 days than at younger ages was modeled with a dummy variable taking a value of 1 at 90 days and 0 elsewhere.Ewe-deworming with morantel during the rainy season lead to higher lamb live-weights (probably related to a better ewe-nutrition and -health status). A positive correlation was demonstrated between early weight and growth rate at the population level (with important lamb and herd-level random deviations). The persistence of this correlation at older ages should be checked to determine whether early weights are good predictors of mature weights and ewe-reproductive lifetime performance.

A steady-state approach of benefit-cost analysis with a periodic leslie-matrix model. Presentation and application to the evaluation of a sheep-diseases preventive scheme in Kolda, Senegal.

A seasonal population-dynamics matrix model (periodic Leslie-matrix model) was developed to model short production cycles and high seasonal variations occurring in demographic rates and offtake patterns for small ruminants. The year was split into 24- and 15-day phases. Population-size changes were modelled by the recurrence equation x(j+1)=B(j)x(j), where j was the 15-day phase, x an age-class population size vector and B a fecundity-, mortality-, offtake- and intake-rate matrix. Given an initial vector x(1), annual dynamics were described by x(25)=B(24)...B(1)x(1)=Ax(1), where A was the annual projection matrix.A steady-state hypothesis was used to estimate offtake gains and financial returns from a trial of pasteurellosis vaccination and anthelminthic drench in traditionally managed sheep flocks in Senegal, from July 1987 to June 1988. Nineteen villages and 76 herds were involved in the experiment. Villages were randomly allocated to one of the four treatment combinations in a factorial design, and subsequent demographic rates and net offtake patterns were measured. In the trial, vaccination had a negative effect on offtakes among females. No vaccination effect was observed for males. A positive effect of deworming was found for both sexes. From the trial data, our model calculated that the overall ratio of offtakes (i.e. number of animals) for dewormed over undrenched sheep was 1.2 (95% confidence interval: 1.1, 1.4). The deworming financial benefit-cost ratio was 3.7 (1.9, 5.4).