Though soybean matures in 3 to 4 months after seeding. The critical weed free period to obtain maximized yield is about the first 40 days. Pre emergent herbicide can cover that window. It is unlikely that much of the herbicide is residual particularly under a tropical condition but the waiting period is used to give a safety factor. In terms of residual potential issues I would be more concerned about GMO soybean where systemic materials are applied very close to harvest dates and in fact substantial glyphosate residuals are generalized in these soybeans compared to the residual that conventional genetics treated with pendimethalin as a pre emergence. An organic soybean can be competitive in yield without the GMO or the pre  emergence but the use of additional cultivation employment of rotation are needed. Many farmers judge themselves based on the way fields are free of weeds this can mean a larger than needed effort is applied to satisfy the tidiness fact. Waiting the specified period would allow the producer to be strict adherence to complying with legal requirements and give the conventional farmer a sense of tranquility of maintaining standards under those requirements.  

Dear  Angumuthu, 

1. According of  research  (http://www.ncbi.nlm.nih.gov/pubmed/26993984)  residue   of pendimethalin is  below detectable limits in soybeans.  So this is not sufficient. 

2, In official label of   BASF STOMP  (pendinmethalin)  there  are no special waiting period for Soybeans regarding of  this herbicide. even for potato is  just 60 day...Please check again or send reference about 110 Day waiting period for cross checking

Dear Sir, Thanks, Kindly find the compiled document and check the page no.23 in attached file

Thank you, dear  Angamuthu,

I think there  should be  only two reasons:

a)   The   effect  of Emulsifiable concentrate vs Capsule suspensions.  The last one is less dangerous  according of your reference,  But to say truth less proved...

  2. The Just  some mechanical mistakes, look at:

Pendimetalin,  30  %  EC - Emulsifiable concentrate

Soybean 

Waiting period:  110 DAY

Dosage -  0,7 - 1,0 l/Ha, maximum - 2,5 - 3,3 L/Ha

However, 

Pendimetalin,  38,7  %  CS - Capsule suspensions

Soybean 

Waiting period:  40 DAY

dosage - 0,58- 0,677  l/Ha, maximum - 1,5 - 1,75 L/Ha

Best  regards,

Pendimetalin is generally effectively used as a pre emergence weed control. As such the waiting period of 40 days should not be of any concern for a soybean crop which requires 3 to 4 months to reach full maturity. While this pre emergent strategy can give effective control during the early crop development stages under tropical condition and when soybean stand is compromised late season weed development can be an issue which can affect soybean seed quality creating an environment favoring seed deterioration particularly under humid conditions. When herbicide resistant soybeans are available the ability of spraying over top at the 30 to 40 day critical weed competition period gives a control that is generally more conclusive and less open to late season weed competition issues. The low cost and ease of this approach has made this the predominant system used for North American farmers of conventionally produced beans capturing over 90% of that market. Any of these synthetic chemical practices would not be permitted in a certified organic soybean cropping system.