Many studies and observations have supported the concept that uneven field emergence causes some reduction in final yield. There can be many causes of uneven field emergence, including moisture distribution, temperature, tillage, debris, pathogens, insects and seed germination quality. The problem is worst when the late emerging plants are adjacent to earlier emerging plants because of competition for root space, mineral absorption and light. Late emerging plants also push out silks later perhaps partly because reduced root volume fails to withdraw enough water to keep the silk cells from expanding at the normal rate. Late emergence of silks can come after most pollen from nearby plants has expired. At minimum, this can result in ear tips not filling but also can result in barren plants.
It is not always clear why germinations are not uniform in some seedlots. It appears to involve some aspect of membrane repair from aging or other deterioration of cell components. This supported by the observation that more uneven germination occurs after the seed is exposed to 7 days of 50°F before warmed to 70°F compared to the same seed sample placed only at 70°F. Given that each individual seed can have a slightly different physiological status, the slow emergers perhaps take a longer time to gain enough physiological strength to push the shoot above the soil surface.
The complexity of seed quality status, and difficulty of precisely measuring it, plus the multitude of field interactions does not make it easy to predict the problems. Two reports that verify the reduction of yields from uneven emergence are:
Also, this Corn Journal blog of March 10, 2016 (http://www.cornjournal.com/corn-journal/uneven-corn-seedling-emergence) relates my personal experience with monitoring late emerging plants that not only impressed me with the importance of annotating late emerging plants in germination tests but also set me off on exploring a different understanding for occurrence of stalk rot (it was not the late emerging plants).
About Corn Journal
The purpose of this blog is to share perspectives of the biology of corn, its seed and diseases in a mix of technical and not so technical terms with all who are interested in this major crop. With more technical references to any of the topics easily available on the web with a search of key words, the blog will rarely cite references but will attempt to be accurate. Comments are welcome but will be screened before publishing. Comments and questions directed to the author by emails are encouraged.