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It is important to understand critical growth stages of corn and soybean because of their influence on determining final yield potential. Mitigating stress during critical periods can help to preserve the ability of plants to reach their maximum remaining yield potential.
Because corn does not have the ability to compensate for poor stands early in the season, establishing a uniform stand is the first step in optimizing yield potential. The second critical growth stage occurs during the rapid stage of corn vegetative growth at V6 to V8 (6 to 8 leaf collars), when kernel row length is determined. Stress-free corn plants during this time can help maximize the potential number of harvested kernels.1 The third critical growth stage occurs during the pollination process. Pollination is critical to converting potential kernel numbers into developing kernels.
Weather can greatly affect the success of pollination. Drought stress can desiccate silks and pollen grains, which may result in barren ears and/or short ears with unfilled tips (Figure 1).
The final critical growth stage is the grain fill or kernel development period. This stage begins at pollination and ends at kernel black layer formation. Stress during this stage can reduce kernel number, size, and weight of harvested kernels.1
Figure 1. Corn ears with barren tips.
During the grain fill stage, any stress on the photosynthesis process can reduce yield potential. Photosynthesis produces the energy (carbohydrates) that a corn plant needs to survive and produce grain. Drought, high temperatures, extended periods of cloudy weather, foliar diseases, hail damage, and nitrogen (N) deficiency can, individually or in combination, significantly reduce photosynthesis.2
After pollination, corn plants redirect carbohydrate movement to fill the developing kernels, which may reduce the health of the stalk, leaves, and roots.2 This process can physically weaken the plant, resulting in the plant being more susceptible to stalk and root diseases. Fields that are at the highest risk for stalk rot are those that have developed ears with high yield potential because of ideal conditions during vegetative growth, but have experienced severe stress during the grain fill stage.
The effects of plant stress can be intensified by sandy soils that have minimal water-holding capacity or plants that have a restricted root system due to compacted soils, nematode damage, or corn rootworm feeding.
Severe stress during the dough and dent stages of grain fill can lead to premature formation of kernel black layer. This can reduce yield potential due to decreased kernel size and weight.3 When the black layer forms, no additional nutrients can flow into the kernel and drydown begins.
Soybean reproductive stages begin at flowering (R1 to R2) and include pod development (R3 to R4), seed development (R5 to R6), and plant maturity (R7 to R8). The effects of frost, hail, moisture stress, insects, or diseases on yield potential can be estimated by determining the soybean growth stage when the event occurs. Stress, such as defoliation or root damage, that occurs during the early reproductive stages (R1 to R5.5) can affect growth rate and may have an impact on yield potential.
At full flower (R2), distinguished by an open flower at one of the two uppermost nodes on the main stem, the largest yield reducing stress is defoliation, which can occur from various sources including insect damage, disease, or hail. Fifty percent defoliation at this stage can reduce yield by 6%.4,5
Figure 2. Soybean at around R3 growth stage.
At the beginning pod (R3) growth stage, one of the four uppermost nodes has a pod that is 3/16 inch long. Stress during this growth stage may affect yield potential by decreasing total pod number, bean number per pod, or seed size. Typically, soybean plants can compensate, at least partially, for temporary stress. One reason for the ability to compensate is the long flowering period; however, the soybean plant loses this ability as it matures from R1 to R5.5. Typically, 60 to 75% of the flowers will abort and as many as 50% of the formed pods may abort. Stress during this reproductive stage may increase those abortion rates thus decreasing yield potential. Conversely, favorable conditions may increase pod number per plant and increase yield potential.
At the beginning of the full pod (R4) growth stage, one of the four uppermost nodes will have a pod that is 3/4 inch long. At first, rapid pod growth and seed development take place, followed by finalization of pod number. Pod dry weight increases from R4 to R5 (beginning seed). This stage marks the beginning of the critical period for determining seed yield potential. Stress during R4 to R6 can cause more reduction in yield potential than at any other growth stage.4 The most critical time during pod formation is late R4.5 to early seed fill at R5.5. Reductions in yield potential can occur from fewer pods. If needed, irrigation during this critical time may help reduce potential yield loss.
Sources: 1eXtension. 2008. Corn yield potential. eXtension. http://articles.extension.org/ 2Nielsen, R.L. 2011. Top leaf death or “dieback” in corn. Purdue University. http://www.agry.purdue.edu/. 3Nielsen, R.L. 2013. Effects of stress during grain filling in corn. Purdue University Extension. http://www.agry.purdue.edu/ 4McWilliams, D.A., Berglund, D.R., and Endres, G.J. 2015. Soybean growth and management quick guide. Publication number A-1174. North Dakota State University Extension. www.ag.ndsu.edu. 5Hodgson, E.W., McCornack, B.P., Tilmon, K., and Knodel, J.J. 2012. Management recommendations for soybean aphid (hemiptera: aphididae) in the United States. Journal of integrated pest management. 3(1). http://lib.dr.iastate.edu. Pedersen, P. 2004. Soybean growth and development. PM 1945. Department of Agronomy. Iowa State University Extension. Web sources verified 6/18/18. 180624135533