Corn Reproductive Growth Stages

Corn growth stages can be used to help growers make timely management decisions. The following information outlines important developments that occur during the reproductive growth stages of corn.

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Figure 1

Figure 1. Corn grain filling and maturation occur in the last 50 to 60 days of its growth.

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R1 (Silking) - Corn reproductive growth begins when silks emerge outside the husks. This is the most critical stage in the development of a corn plant, occurring around 55 to 68 days after corn emergence. Successful pollination occurs when pollen grains from anthers on the tassels contact silks of each potential kernel on the ear, growing down the silks to fertilize the ovules and begin the kernel development process. Normally, all of the silks on a single ear emerge and are pollinated in about two to three days, but the process can occur up to 10 days after silk emergence. Heat stress and drought during this time can delay silk emergence and shorten the duration of pollen shed resulting in poor pollination. Good pollination occurs when silk emergence is in synchrony with pollen shed. Heat stress can desiccate silks making them not receptive to pollen grain germination. The corn plant needs good soil moisture several weeks before and after silking for good pollination and seed set. Good soil moisture is important for uptake of nutrients at this time. The largest potential corn yield loss can occur from moisture stress at silking and pollination (early R1 stage). Populations of corn rootworm adult beetles should be monitored at this time because they can cause poor pollination by clipping the silks.

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Figure 2

Figure 2. A normal ear in picture on the far left and abnormal ears with tip dieback due to kernel abortion and small ears due to poor kernel set (left). Zipper ear (center) and ears with incomplete kernel set (right). High temperatures, moisture stress, and nutrient deficiencies during pollination and early grain fill can cause these abnormalities.

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R2 (Blister) - This stage occurs about 10 to 14 days after silking when the kernels are white, small, watery, and shaped like a blister. The success of pollination can be determined on an early R2 ear by carefully removing the husks and gently shaking the ear upside down. Kernels with detached silks have successfully fertilized and those with attached silks have not. Good soil moisture is important at this time and irrigation should occur if needed. Nitrogen and phosphorus uptake is still rapid, but the plant has started moving these nutrients from the stalks and leaves to the grain. Severe stress can cause abortion of kernels at this stage.

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Figure 3

Figure 3. Nitrogen deficiency symptoms can show up on lower corn leaves. Grain yield can respond to nitrogen (N) applied as late as the R3 stage when it is deficient. Dry soil conditions can lead to poor uptake of N by the plant. Excessive irrigation can promote the loss of N. R4 to R5 corn often shows these symptoms on lower leaves as N is moved out of these leaves first to the grain. The degree of these symptoms does not always indicate that the plant had an insufficient N supply for maximum yield potential.

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R3 (Milk) - This stage occurs about 18 to 22 days after silking with kernels now yellow on the outside and milky fluid inside. Starch accumulation creates this milky fluid as cell expansion occurs and dry matter accumulates in the kernels which are now at about 80% moisture. This is a good time to inspect kernel set on ears and begin assessing yield potential. Incomplete kernel set can be caused by both unsuccessful pollination and kernel abortion. Stress at this time can still cause kernel abortion, but not as easily as in the R2 stage. Yield reduction will now primarily occur due to a decrease in the number of kernels that ultimately develop and to the final size and weight of the kernels.

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R4 (Dough) - This stage occurs about 24 to 28 days after silking when the kernel’s milky fluid is changing to a doughy or pasty consistency due to continued starch accumulation. Stress at this time can continue to affect eventual yield by reducing kernel weight. Kernel abortion is much less likely to occur by this time, but unfavorable environmental conditions or nutrient deficiencies can result in unfilled kernels and “chaffy�? ears (Figure 4). Long periods of gray, cloudy weather from R2 to the R4 stage can limit kernel development and result in shorter ears. Smoky conditions caused by wildfires that have been occurring over an extended period of time could have an affect on yield by limiting photosynthesis.

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Figure 4

Figure 4. Chaffy ears can have corn grain low in test weight.

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R5 (Dent) - This stage occurs about 35 to 42 days after silking when nearly all of the kernels are denting near their crowns. Kernel moisture at the beginning of the dent stage is about 50 to 55%. A distinct line appears near the dent end of the kernel that slowly moves down to the tip end of the kernel over about three weeks. This is called the milk line which marks the boundary between the liquid (milky) and solid (starchy) areas of the maturing kernel (Figure 5). Stress at this point can reduce kernel weight, but not kernel number. An early frost can stop the grain filling process causing corn to dry slowly and delay harvesting.

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Figure 5

Figure 5. Milk line on corn kernels.

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R6 (Physiological Maturity) - This stage occurs about 55 to 65 days after silking when maximum kernel dry weight is reached and the crop is safe from frost. The milk line disappears as starch dry matter progresses to the base of the kernel and a black layer forms at the tip of the kernels. Stress after black layer formation has little effect on grain yield, unless the integrity of the stalk or ear is compromised by insects or stalk rots. At black layer, kernel moisture is about 30 to 35%. The plant should remain alive for some time after black layer formation to help it dry down faster. Harvest maturity can be defined as the grain moisture content when harvest can occur with minimal kernel damage and mechanical harvest loss, typically near 25% grain moisture. Often times, you may find harvesting the driest grain from the greenest corn, because the plant remained alive long enough to easily draw down moisture out of the kernels. If the plant dies at black layer, yield will not be affected, but drydown can be very slow.

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In summary, the grain fill period begins with successful pollination and initiation of kernel development, and ends about 60 days later when the kernels are physiologically mature. A stress-free grain fill period can help to maximize yield potential while severe stress can cause poor pollination, kernel abortion, lightweight grain, and encourage the development of stalk rots. The upper leaf canopy from the ear leaf to the uppermost leaf is particularly important for achieving maximum grain filling capacity.