52 A^riciilniral Research and Productivity 



crop in a life cycle is produced from planted cane. Subsequent 

 crops in the life cycles are regrowths of the same plants after cut- 

 ting, and are referred to as ''ratoon crops/' Depending on climate 

 and other factors, yields decline with each ratoon crop. It even- 

 tually becomes profitable to plow the fields and replant the cane.'^ 



Consequently, simple comparison of yields by variety, uncor- 

 rected for the age differences in the new and old varieties, may be 

 misleading. The actual yields in South Africa of the stage 2 and 3 

 varieties introduced during the 1930s were 43 percent higher 

 than the yield levels of the native stage 1 variety, Uba. This calcu- 

 lation was based on data for the 1935-39 period, when both types 

 of varieties were grown. The correction for the age distribution of 

 the canes reduces this to a 27 percent advantage for the stage 2-3 

 varieties. A comparable set of variety-yield-ratoon data for the 

 period 1954-57 allowed a comparison to be made of the advantage 

 of stage 4 varieties over stage 3 varieties. This was calculated to be 

 28 percent." 



Variety data reported for other countries are not as detailed as 

 those for South Africa, although data on variety com.position of 

 production are generally available. An analysis of yield changes in 

 several countries with regard to the adoption periods of stage 3 

 and 4 varieties is shown in table 3.7. The estimated equation is 



DY = A + b^DV + b2DF+ b^DR 

 where 



Y Yield in tons per hectare 



V A measure of varietal composition based on area planted 

 F Fertilizers application 



R Rainfall 



D First difference operator, for example: Dy = y^ ~ y,_ . 



The variety changes from year to year may be regarded as a 

 kind of "turnover" measure. It is essentially the sum of the 



10. In South Africa, the index of relative yields by ratoon crops is. 

 Age: Plant 12 3 4 5 



109.8 108.5 100.5 91.0 86.7 82.3 



The average life of cane is five to six years. 



11. All computations were made on the basis of sucrose yield, not cane yields. 

 In fact, much of the advantage of the stage 3 varieties over the stage 1 varieties 

 was in sucrose content. 



