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MISSOURI AGR. EXP. STA. RESEARCH BULLETIN 49 



ments, if the experimenter determines the extent of the error and draws 

 his conclusions accordingly. But a preliminary variety test in which 

 error from competition is not controlled may be very nearly worthless 

 as an indication of the relative value of varieties for field conditions, 

 because actually the relative values of the varieties tested may frequent- 

 ly differ by 50 or 100 per cent from the values determined 

 the test, without the slightest indication in the experimental results. 



in 



Illustrations of Effects of Competition. The error from competi- 

 tion may be illustrated by numerous examples from each of the eight 

 tests here reported. An extreme case is the effect of competition on 

 the relative yield of wheat and rye. Two varieties of rye, common 

 rye and Rosen rye, were included in the wheat variety test, for com- 

 parison with wheat. The average yields of Rosen rye and of the va- 

 rieties of wheat adjoining it on either side, in interior rows and com- 

 peting border rows of the four series, were as follows : 



The disturbance of the true comparative value of the varieties 

 by competition may be determined by comparing their relative yields 

 in interior rows and in border rows. Thus Niagara wheat in 1920 

 yielded 67 per cent as much as Rosen rye in plots protected from com- 

 petition, but only 33 per cent as much in rows not protected from 

 competition. Similarly the yield of Velvet Chaff No. 2 wheat was re- 

 duced from 68 per cent to 35 per cent by competition with Rosen rye. 

 In the following season the reduction in yield of the two varieties of 

 wheat adjoining Rosen rye (Red Hussar and Poole) was not so 

 great, but was still decidedly significant. This clear case of compe- 



