74 MISSOURI AGR. EXP. STA. RESEARCH BULLETIN 49 



The error from competition is greater when different varieties 

 are compared than when different strains of the same variety are 

 compared, and the extent of error is roughly in proportion to the de- 

 gree of difference in type of the varieties tested. Competition was 

 not found to be correlated closely enough with earliness of heading, 

 earliness of maturity, height, or grain-straw ratio in these experi- 

 ments to permit its control by grouping varieties in respect to these 

 characters. The factor found most closely correlated with competitive 

 value was yield, but the correlation even in this case was not close 

 enough to permit of effective control by grouping varieties. Moreover, 

 it would be impossible in practice to group varieties with regard to 

 yield, since the relative yield of varieties varies so widely with the 

 season. The variety expected to yield poorly is not ordinarily included 

 in the variety test. 



When different strains of the same variety are grown, the error 

 from competition, in some cases at least, may be slight enough to 

 justify the use of single-row plots. However, competition in such 

 cases is not wholly absent, and may occasionally be quite marked. The 

 importance of competition as a source of error in tests of pure line 

 selections of the same variety merits detailed investigation. If it is 

 found that the effects of competition between pure lines is slight it 

 may be practicable to use single-row plots, or at any rate to use 3-row 

 plots without discarding border rows. The latter method will reduce 

 the error from competition materially, without necessitating the loss 

 of any of the experimental area. When the same total area is used, 

 however, single row plots are somewhat more reliable than 3-row 

 plots, because more replications can be used. The best size of plot for 

 ordinary variety testing, as indicated by this investigation, is probably 

 the 3-row plot with border rows discarded. The length of the plot 

 as harvested is assumed to be 16 feet, but the same considerations will 

 apply for any other convenient length. The number of replications 

 will vary with the heterogeneity of the field and the degree of precision 

 required (and, to some extent, with the season and the variety), 



Check plots have been used in preliminary variety tests mainly 

 for the following purposes: 



(1) For the adjustment of the yields of the test plots, and 



(2) To provide a measure of plot variability for the field used, 

 and thus to determine the degree of precision of the experimental re- 

 sults, or the number of replications which would be required for a 

 given degree of precision. 



In both cases the behavior of the check variety is the basis for 

 conclusions regarding the tested varieties. This involves the as- 

 sumption that different varieties of the same crop respond similarly 



