Corrigendum Evaluation of strategies for tracking climatic variation in semi-arid grazing systems$

In a previous paper (Illius et al., 1998), we analysed the potential of tracking strategies to increase output under climatic variability. Regrettably, an error in the processing of data on the annual variability of sales led us to the wrong conclusions about the usefulness of tracking policies in smoothing inter-annual variation. This note presents corrections. Our conclusions about the general inability of tracking policies to increase sales are una€ected and remain as before. We had previously stated that, although tracking policies had no advantage over ®xed policies in terms of mean annual sales, they did have a considerable advantage in terms of reduced interannual variability of sales, which would be of economic bene®t to the commercial livestock sector. Table 1 presents a complete and correct set of results for sales and the CV of sales for each of the policies and for each of the options for female sales and re-stocking. It can be seen that the variability of sales under tracking policies is not, in fact, less than that under more ®xed policies. The main e€ect on the sales performance of the various strategies is due to the option allowing the sale of breeding females in order to meet the stocking rate target. Without female sales, the target stocking rate was often exceeded, leading to lower sales, and tracking policies had a slight advantage over more rigid policies. But large increases in output could be achieved by allowing female sales, thereby achieving a mean stocking rate nearer to the optimal rate. Then, ®xed and preemptive sales policies had the lowest inter-annual variability in sales. Policies designed to track climatic variation had minimal advantage in terms of sales and yet had about twice the inter-annual variability. A CV of 140% implies zero sales in nearly one year in four. In retrospect, inter-annual variability in sales is an obvious consequence of policies that aim to track climatic variation by varying stocking rate.