Two variable inputs

Generally speaking, labour is not the only input which can be varied. Where there are two or more variable inputs then substitution of one for the other may be possible, and decisions must be taken on the appropriate combination of inputs, and method of production. This may be illustrated using the example from the previous chapter, of maize yield response to variations in weeding labour. It is now assumed that the amount of seed sown, per one hectare plot, is a second variable input. An increase in seed use which raises maize yield, thereby raises the average product per day of weeding labour but may reduce the amount of weeding necessary. More specifically there may be scope for substituting seed for weeding labour in producing a particular yield of maize. A possible response surface for these two variable inputs is represented by the data given in Table 5.1 and plotted in Figure 5.1.

The original response curve for weeding labour is now seen to relate to a fixed seed rate of 10 000 plants per hectare, as shown in the first row of Table 5.1 and by the line AB in Figure 5.1. Although some yield is obtained, even when no weeding labour is used (at least for seed rates above 15 000), the situation is different for seed. There can be no yield when no seed is used, so the response curves for seed pass through the origin.