Simulated livestock dynamics--effects of pastoral offtake practices and drift on cattle wealth.

Pastoral cattle herds were simulated to demonstrate a species-independent package (DADAS) for the analysis and simulation of livestock dynamics. A cow culling age of 11 years was expected to maximise offtake rate and to stabilise the cattle population. This threshold was derived by the deterministic module of the package used. A closer look at six offtake patterns using the stochastic module of the simulation program revealed that those who cull cows at an age limit would fare less well than those who cull cows only when sterile. Within each offtake pattern the cumulative effects of chance fluctuations in sex-ratio of calves, mortality, sterility, age at first calving and calving interval resulted in cattle wealth indicators varying widely between replicate herds. The example shows that livestock dynamics simulation on personal computers is feasible for planning, teaching and research purposes.

Asymmetrical responses to automatic selection for body size in Drosophila melanogaster.

An apparatus has been made for the automatic selection of Drosophila for body size, operating on the principle of a fractionating sieve. The measurements of individual flies by this method were approximately normally distributed and the repeatability of measurements on successive days was 0.5. A two-way selection experiment for this character was carried out with two replicates for ten generations. The realised heritability for the measured score was 0.14±0.02 for high score and 0.20±0.02 when it was for low score. The correlated response in body weight was asymmetrical, the change downwards being much greater than that upwards. There was a clear divergence in activity measurements between the lines selected in the two directions but no clear trends in fertility. Examination of the selected lines after eleven generations showed that the relationship between score and body weight was clearly different in the lines selected in the two directions and was non-linear in both.It is suggested that the response in activity observed as a consequence of selection for score is partly due to the direct response for activity and partly to a correlated response because of a negative genetic correlation between body size and activity. The observed non-linear relationship between score and body weight observed within generations may be a direct cause of the asymmetry of direct and correlated responses which may also have a parallel in other situations.