Treatment             1:1           1:2            1:3          1:4


The relative comparisons are identical for the two equivalents.

4.3 Biological Indices of Advantage or Dominance

The most important index of biological advantage is the relative yield total (RYT) introduced by de Wit and van den Bergh (1965) or land equivalent radio (LER) reviewed by Willey (1979). The index is based on relating the yield of each crop in an intercrop treatment mixture to the yield of that crop grown as a sole crop. If the two crop yields in the intercrop, mixture are MA, MB, and the yields of the crops grown as sole crops are SA, SB, then the combined index is

                                  L = MA + MB = LA + LB SA                                             SB


The most frequently used value index is that of financial return. Other value indices include protein and dry matter. The main criticism made specifically of financial indices is that prices fluctuate and hence the ratio of KI to K2 may vary considerably. A partial answer to this criticism is to employ several price ratios. Thus the results for the four treatments discussed earlier in this section might be presented for five price ratios as follows:

Price Ratio for Maize/Cowpea


1:5


4943 6918
4722 7450


3569 5961
4722 2890


3111
5642 4722 1490


4027 6280
4722 4470


"85
6599 4 7 22 5960


While some comparison patterns, such as (2 vs. 1) or (2 vs. 3), remain consistent for this range of price ratios others, such as (I vs. 3) or (2 vs. 4), do not.

One other form of single measurement comparison which is exactly equivalent to the financial value index is the crop equivalent. In calculating a crop equivalent, yield of one crop is "converted" into yield equivalent of the other crop by using the ratio of prices of the two crops. The exact equivalence of crop equivalent yield to financial index is immediately obvious algebraically but may be perceived clearly also by considering the four treatments for a I : 3 price ratio. This ratio implies that a unit yield of cowpea is worth 3 units of maize. We can therefore calculate yields as maize equivalents or cowpea equivalents as follows:


Cowpea equivalent


458 + 2653/3 = 1342 319 + 5323/3 = 2093
    4721/3 = 1574 1490         = 1490


Treatment


Maize equivalent


2653 + 3(458) = 4027 5323 + 3(319) = 6280 4722         = 4722
    3(1490) = 4470