You've heard many times that if corn plants emerge late, they become weeds, not producing ears and instead robbing needed moisture and nutrients from plants which are trying to produce ears and contribute to yield. Bob Nielsen at Purdue University has demonstrated the value of uniform stands in tests conducted several years ago.
Shaun Casteel, Purdue soybean specialist, says that while the situation is somewhat different in soybeans, it's still important to have soybeans emerge in a uniform manner. Otherwise, some soybeans may not produce seed, depending upon how far behind other plants they actually emerge. However, late-emerging soybean plants, if they don't emerge too late, can still have some value to the crop, even if they don't directly contribute to yield.
In a recent study Casteel and others determined that soybeans that emerge when most plants are at the VC, or cotyledon, stage, will only produce about 20% of the yield in that area of the field. Bye merging late they are at a disadvantage. Their study also determined that plants which emerge when the majority of plants are at the V2 or V 4 stage will not produce seed that contributes to yield. Staging soybean plants is described in detail in the Purdue University Corn & Soybean Field Guide, now available in the 2012 edition.
However, if plants emerge when the majority of plants already up are at V2 or smaller, then although the emerging plants won't contribute by producing seed, they will help the canopy close sooner. Canopy closure can aid soybeans in several ways, including maintaining good weed control.
Soybeans that emerge so late that their neighbors are at V4 won't even help on canopy closure. They won't necessarily detract from yield, but they will turn into spindly plants that don't produce seed.
This is just a preview of what Casteel discusses in his new column, Soybean Success, coming in the May issue of Indiana Prairie Farmer. Look for it in the Crops section of the magazine.