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Critical reproductive period begins soon for northern corn

Lon Tonneson Close up of thread-like fibers, know as corn silk, grows from an ear of a corn
SILKING STAGE: Corn enters its first reproductive stage when it begins silking. At this stage, hot weather can significantly reduce yield potential.
Learn what’s happening with corn from silking through physiological maturity.

When corn begins silking it enters the critical reproductive growth stages. According to South Dakota State University Extension’s publication, “Best Management Practices For Corn,” here’s what going on from silking (reproductive growth stage R1) through physiological maturity (reproductive growth stage R6):

Silking (R1). The emergence of silk marks the first stage of the reproductive period. Every potential kernel (ovule) on the ear grows its own silk. Silks begin to elongate soon after the V12 stage.

At the R1 stage, the silks emerge and capture pollen shed from the tassel. Pollen captured by the silks fertilizes ovules on the cob within 24 hours, which then develop into kernels. Pollen shed typically occurs during early or mid-morning, when moisture and temperature conditions are favorable.

This stage is one of the most crucial reproductive stages, and unfavorable environmental conditions can severely reduce yield. Dry (low humidity) and hot (95 degrees F and up) conditions result in reduced fertilization because of the drying of the exposed silks and killed pollen.

With no fertilization, ears are barren. Silks grow at a rate of approximately 1.5 inches a day. The silks continue to grow until pollen is captured and germinate or until they degrade as they mature. Environmental conditions such as drought stress can result in delayed silk elongation and emergence. Generally, silks remain receptive to pollen for up to 10 days after silk emergence, though they start to deteriorate only five days after emergence.

Under favorable environmental conditions, there is synchrony between pollen-shed and silk emergence, making silk receptivity of little concern. Insect pests such as corn rootworm destroy silks through feeding and can produce reduced yields. To minimize losses, fields should be scouted for corn rootworm beetles at silking (R1) and controlled if populations exceed the economic threshold.

Potassium (K) uptake is complete at silking, but nitrogen (N) and phosphorus (P) uptake continues. If N and P are limiting, the plant will attempt to compensate by moving these nutrients from older leaves into upper leaves or the developing grain. At this stage, N- and P-deficiency symptoms can be observed in lower leaves. Unfortunately, nutrient application either at this time or later in development will not make up for these deficiencies.

Kernel blister stage (R2). After pollination, kernel formation begins. The kernels at the R2 stage are whitish and shaped like blisters. They appear approximately 10 to 14 days after silking.

At this stage, silks turn brown and dry rapidly. Starch begins to accumulate in the kernel as the plant initiates a period of kernel fill.

At the R2 growth stage, the radicle, coleoptile, and the first embryonic leaf have formed in the embryo. The kernel moisture content at the R2 stage is about 85%. Any severe stress at pre-blister and blister stage can result in aborted kernels and reduce the number of kernels on the cob.

At this stage, the plant will need 960 growing-degree days (GDD), also called growing-degree units, to reach physiological maturity. Additional water at or after R2 does not enhance yield, slows dry-down, and may encourage stalk and grain diseases.

Kernel milk stage (R3). The kernel milk stage occurs approximately 22 days after silking.

At this stage, kernels are mostly yellow on the outside, starch accumulation occurs rapidly, kernels contain a milky white fluid, and cell division in the endosperm is complete. Observable kernel growth is mainly due to cell expansion and starch accumulation, severe stress can cause kernel abortion. The kernel moisture content is about 80%, and approximately 880 GDD are required to reach physiological maturity.

Although not as critical as the R1 growth stage, stress at this time can reduce kernel size and weight.

Kernel dough stage (R4). As the kernels mature to the dough (R4) stage, they change from a milky consistency to soft and sticky. At R4, the kernels have accumulated nearly half of their mature weight and the cob has a color ranging from light red to pink.

At this stage, four embryonic leaves are formed, and the kernel moisture content is approximately 70%. Unfavorable environmental conditions or nutrient deficiencies can reduce kernel weight.

Kernel dent stage (R5). At the R5 growth stage, nearly all of the kernel crowns are denting, the moisture content is approximately 55%, and a distinct horizontal line called the milky line can be seen between the yellow (starchy-solid) and white (milky-liquid) areas on the kernel.

As the kernel matures and starch hardens, this line slowly progresses to the tip end of the kernel. A hard frost at R5 can kill the plant, thus reducing yield and kernel development.

Corn plants killed at this stage generally have low test weight and a slower dry-down rate. Selecting a hybrid that matures 2 to 3 weeks before fall frost reduces these risks. If early frost kills the plant, the crop can be harvested and ensiled as high-moisture grain for animal feed.

Physiological maturity (R6). The corn plant is at physiological maturity (R6) about 55 to 65 days after silking.

At this stage, kernel dry-weight has reached its maximum, the kernels are physiologically mature and safe from frost damage, the moisture content ranges from 30% to 35%, the starch line has advanced to the kernel tip, and a black layer has formed at the base of the mature kernels. The black layer forms from the tip of the kernels to the basal kernels.

Severe stress after this stage has little effect on grain yield, unless the integrity of the stalk or ear is compromised by disease such as stalk rots or insect feeding. At this time, allowing the crop to dry in the field reduces drying costs if the crop is to be harvested for grain. Moisture content of 15% allows corn to be stored safely for less than six months. For long-term storage, corn should be dried to 12% moisture to avoid spoilage.

Hybrids have subtle differences in growth and development (with respect to number of leaves, ears, maturity, dry-down and other traits). Early harvest is rarely profitable because of drying costs or dockage.

Corn can be left in the field if stalks maintain strength, ear drop is not a problem, and there is limited risk of ear and kernel rots — especially under hot, dry conditions. Harvest loss from lodging and ear drop can be significant in fields damaged by European corn borer or Western bean cutworm. In these situations, early harvesting to reduce harvest losses should be weighed against drying costs. Scouting to assess stalk condition, ear retention, ear rots and grain moisture is recommended.

Source: SDSU, which is solely responsible for the information provided and is wholly owned by the source. Informa Business Media and all its subsidiaries are not responsible for any of the content contained in this information asset.
TAGS: Reproduction
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