Grain Fill Stages

As the calendar flips to August and crop development is heading into the final stretch, I often consider this period as one of the more important times for final yield determination. Through most of the spring and summer months, corn and soybean crops build the factory to collect sunlight, water, and nutrients which are ultimately transformed into stored energy. July is when storage units (seed) are manufactured which begins the final phase of grain fill and ultimate yield potential. 

Recognizing and understanding the different stages of grain fill can be confusing at times, yet it can be very important for many management decisions that are made in the final stages of crop development.  Understanding the stages and using appropriate management can: 1) impact the extent of damage from insects, 2) may determine the end result of stresses such as water shortages, heat or frost injury and 3) can guide you to know when the crop may reach full maturity.

Planting dates vary from region to region depending on springtime growing conditions; therefore, recognizing and understanding final crop and grain developmental stages may prove to be critically important when analyzing the potential for meeting full grain maturity.  There is nothing we can do to speed up the grain fill process, but we can better understand the impacts of the environment on development and yield as we move into the final stages of crop development.

Corn development during grain fill

R1 Stage or Silking. R1 begins when silks are visible outside the husks.
R2 Stage or Blister (10 to 14 days after silking). R2 kernels are white on the outside and resemble a blister in shape (Figure 1).
R3 Stage or Milk (18 to 22 days after silking). The R3 kernel displays yellow color on the outside, and the inner fluid is milky white due to accumulating starch.
R4 Stage or Dough (24 to 28 days after silking). Starch continues to accumulate in the endosperm causing the milky inner fluid to thicken to a pasty consistency. Just prior to R5, kernels along the length of the ear begin to dent or dry on top.
R5 Stage or Dent (35 to 42 days after silking). At R5 all or nearly all kernels are dented or denting. The kernels are drying down beginning at the top where a small hard white layer of starch is forming. This starch layer appears shortly after denting (Figure 2).
R6 Stage or Physiological Maturity (55 to 65 days after silking). All kernels on the ear have attained their maximum dry weight or maximum dry matter accumulation. The hard starch layer has advanced completely to the cob and a black or brown abscission layer has formed.

R2 grain development, blister stage. Figure 1. R2 grain development, blister stage.
R5 grain development, dent stage, ¼ milk line. Figure 2. R5 grain development, dent stage, ¼ milk line.

Soybean reproductive and grain fill developmental stages

R1 or Beginning flowering. Plants have at least one flower on any node.
R2 or Full flowering. There is an open flower at one of the two uppermost nodes.
R3 or Beginning pod. Pods are 3/16 inch (5 mm) at one of the four uppermost nodes.
R4 or Full pod. Pods are 3/4 inch (2 cm) at one of the four uppermost nodes.
R5 or Beginning seed.  Seed is 1/8 inch long (3 mm) in the pod at one of the four uppermost nodes on the main stem (Figure 3).
R6 or Full seed.  Pod containing a green seed that fills the pod capacity at one of the four uppermost nodes on the main stem.
R7 or Beginning maturity.  One normal pod on the main stem has reached its mature pod color.
R8 or Full maturity.  95% of the pods have reached their full mature color.

R5 seed development, beginning seed. Figure 3. R5 seed development, beginning seed.


Ritchie, S.W., Hanway, J. J., Bensen, G.O. 1993. How a corn plant develops. Special Report No. 48. Iowa State University.

Licht, M. 2014. Soybean growth and development. PM 1945. Iowa State University. 

A visual guide to soybean growth stages. 2017. COOLBEAN. Nutrient and Pest Management Program. University of Wisconsin-Madison.

Clampitti, I.A. (Kansas State University), Elmore, R.W. (University of Nebraska-Lincoln), Lauer, J. (University of Wisconsin). 2016. Corn growth and development. (Based on information from How a Corn Plant Develops, Special Report No. 48, 1986 and Corn Growth and Development, PMR 1009, 2011, Iowa State University. Kansas State University Agricultural Experiment Station and Cooperative Extension Service.

Web sites verified 5/3/22.

Todd Vagts

Legal Statement
from location to location and from year to year, as local growing, soil and weather conditions may vary. Growers should evaluate data from multiple locations and years whenever possible and should consider the impacts of these conditions on the grower’s fields.

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