by Gerhard Eloff, Omnia Area Manager Mkushi

Stress can affect grain yield and in this article we are going to look at critical growth stages where stress can play a role in the eventual yield per hectare. The farmer can do little about stress brought about by natural conditions, but he can at least follow practices to minimize other contributing problems which might reduce yield potential.

Components that define grain yield are plants per unit area and the maximum amount of marketable grain per unit area. It can be further broken down into number of cobs, rows per cob, number of kernels per row and weight per kernel. The leaf collar method of staging maize uses a counting system of “collars” (when the leaf partially unclasps the stem) during the vegetative growth stages and includes the first round-tipped leaf in the count.

Estimated Weeks After Emergence (WAE) have also been included for a typical 120 day hybrid, but one must remember that values will vary over seasons, production environments and locations. Some stages have been singled out because of the importance of certain developments at a particular growth stage.

    Germination and VE (emergence):

• Germination begins when the seed contains 30% moisture.
• The seedling primarily depends on the energy reserves of the kernel at this stage.²
• Stresses to the kernel, radicle (young root) or coleoptile (emerging plant) at this stage can easily kill or stunt the maize seedlings.²
• Soil moisture should be sufficient to support a rapid and continuous germination process.³

V1 to V4 (roughly 1 to 3 WAE):

• The root system is relatively small and banded fertiliser will stimulate early growth.¹
• All the leaves the plant will ever have are formed during the first 3 weeks of growth.⁴
• The growing point is still below the soil surface so damage to the plant above the soil surface at this time usually results in very little reduction in yield.^{1, 2, 3}_.
• The maize plant will recover from injury in this stage if later conditions are favourable. Foliar applications will assist in the recovery.⁴

V5 (roughly 3 WAE):

• The growing point is just below the soil surface and hail or light frost will cause little long term damage to the plant. However, flooding at this stage can kill the plant, especially if temperatures are high.⁵
• Tassel and ear initiation takes place. From about V5 to V15 stress can limit ear size potential.²
• Cold soil temperatures can increase the number of leaves formed, delay tassel formation and reduce nutrient availability.¹

V6 to V11 (roughly 4 to 6 WAE)

• The growing point is above the soil surface and the stalk and roots begin a period of rapid growth.^1,4
• Nutrient and water uptake increase dramatically. Any shortages will have a serious effect on yield.^1,2,4
• Row applied fertiliser is less critical now as nodal roots have proliferated throughout the soil.¹
• Nutrient deficiencies at this stage seriously restrict leaf growth.
• Potential ear shoots will form at every above ground node except the upper six to eight. However, only the upper one or two ear shoots eventually form harvestable ears.^1,5

V12 (roughly 6 WAE)

• The number of rows per ear and the number of potential kernels per row is determined.^1,2,3,4,5,6
• Moisture or nutrient deficiencies will seriously reduce the ear size and the number of potential kernels.^1,2,3,4,5,6

V13 – V17 (roughly 7 to 8 WAE)

• Roots eventually reach a depth of 1,8 meters and spreading in all directions.₁
• Early maturing hybrids progress from the 13-leaf through the 17-leaf stage faster than late maturing hybrids.
• If possible, the soil moisture profile should be at field capacity prior to tassel emergence.

V18 and other numbered V-stages to VT (tasseling)

• The most critical stage of development for maize plants is the V15 (10 to 12 days pre-tassel) through R2 (blister) stages.
• During this period the final number of kernels per row is determined.³
• Stage VT occurs two to three days before silking, when the last branch of the tassel is completely visible but silks have not yet emerged from the ear shoot. This is the beginning of a critical four week period.^1,5

R1 (silking)

• This stage begins when silks are visible and pollination occurs.^1,5
• Depending on climatic conditions, one tassel can shed pollen for a period of 5 to 10 days.⁶
• Pollination in a particular field can happen over a fourteen day period.⁶
• It is the most critical stage in the development of the maize plant and the largest yield reduction occurs with stress during silking.^1,5
• A pollen grain grows down the silk and fertilises the ovule in 24 hours. It generally takes two to three days for all silks to be fertilised on a single ear.^1,5

R2 to R6 (blister to physiological maturity): Stress at any of these stages will greatly affect yield, either because of a reduction in kernel numbers or because of a reduction in kernel weight.

References:

1. Corn Growth and Management: North Dakota State University
2. Stress & the Common Corn Plant: Bob Nielsen, Purdue University Agronomy Department
3. Critical Growth Stages of Corn: Cloyce G Coffman, Texas Agricultural Extension Service
4. Growth Potential, Corn Growers Workshop: Pioneer Seeds
5. Corn Development and Key Growth Stages: Brent Bean and Carl Patrick, Texas Agricultural Research & Extension Centre.
6. Know the Maize Plant: PANNAR