Corn and Soybean Science Newsletter

Knowing the growth stage of corn is critical to understanding the management practices and potential yield impact from wet weather and/or hail damage.

There are a couple methods for determining vegetative growth stages in corn. These different staging methods are used by different disciplines and often occur on different herbicide labels. Knowing the differences between these staging methods will help to reduce confusion when determining corn growth and development. These stages are determined either by the number of visible leaf collars or the number of leaves.

Collars and V-Stages

The collar is the part of the leaf that wraps around the sheath of the plant. A corn plant with one collar visible is at V1. The first leaf of the corn plant is usually oval-shaped, while all subsequent leaves will be longer and come to a sharper point. The V1 corn plant may have three or four leaves visible. However, only one collar is visible. Similarly a corn at V6 will have six collars visible. However, 8 or 9 leaves may be visible on the V6 corn plant.

Once the corn plant reaches V6, the stalk will grow rapidly. This rapid growth often tears some of the lower leaves and collars off of the plant, making staging more difficult. Vegetative stages can be determined at these later stages with some practice. Corn plants must be dug and the stalk split lengthwise through the roots. The first elongated internode is usually 0.4 inches in length. The first node above this internode is often connected to the fifth leaf (fifth collar). Once this node has been determined as a reference point, the remaining visible collars can be counted.

Leafs and Leaf Stages

Hail adjuster’s determine vegetative stages by counting the number of leaves that are bending over. The last leaf to bend over is referred to as the indicator leaf. The first leaf of a corn plant likely will not be long enough to bend over, but each subsequent leaf will start to bend over. For example, a corn plant with the first leaf pointing upwards and the second leaf bending over is at the 2-leaf stage. Corn at the 2-leaf stage will have at least two more leaves that are visible. The third leaf and fourth leaves will be pointing upwards.

Just as with the collar method, determining leaf stage becomes more difficult when the lower leaves are torn away from the stalk. Follow the procedures discussed above to find the fifth leaf. Use the fifth leaf as a reference for counting the remaining leaves that are bending over.

Comparing the Methods

Because one method uses collars and the other uses an indicator leaf, the two stages do not equal each other. For example, V1 corn is usually equal to 2-leaf corn. However, V6 corn is usually equal to 8-leaf corn. Corn at the V12 stage is typically equal to 14-leaf corn.

Corn Development

The growing point on most corn hybrids will remain below ground until the V6 growth stage (8-leaf stage). By the V6 growth stage, tassel development and ear shoot development have both started. Once the corn reaches V6, the stalk will begin to grow rapidly. When the corn reaches V12 (usually 14- or 15-leaf), kernel number and size are being determined. When the corn reaches V15, it is passing through the most critical stage for yield determination. Once the corn reaches tasseling, it becomes the most susceptible to hail damage.

Any stress factors that occur during these phases of growth can impact yields. For this reason, many postemergence herbicides have restrictions against certain growth stages. Flooding, hail damage or weed flushes also can impact yields by stressing plants at these different growth stages.

Herbicide Considerations

The wet weather will prevent optimal timing of some postemergence herbicide applications. The wet weather may stunt corn growth, keeping height limits under herbicide label requirements. However, the stage of growth may be past the limit. Drop nozzles will need to be used in some cases. For more details on proper growth stages for herbicide applications, consult pages 38-39 of AGR-6, “Weed Control Recommendations for Kentucky Farm Crops 2004” as well as the respective herbicide labels.

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2. Assessing Damaged Corn Populations

Chad Lee and Jim Herbek, Agronomy

Determining how many corn plants survived wet weather and/or hail damage will be one of the first steps taken to consider management options. The following information also occurs in the last newsletter regarding stand counts.

Estimated stand of surviving plants should be measured about 5 days after the weather event. Waiting 5 days should allow time for new growth to appear on the surviving plants. Multiple stand counts should be made in both injured and non-injured areas of the field. Use Table 1 to determine how long of a row to count to estimate plant stand. Compare the number obtained in Table 1 to the population numbers in Table 2 to help determine maximum yield. The information in Table 2 was obtained and adapted from the National Corn Handbook, NCH-30, "Guidelines for Making Corn Replanting Decisions” and is Table 5 in ID-139, “A Comprehensive Guide to Corn Management in Kentucky”.

Table 2 should be viewed as a general guide. Most of the data in the table is averaged across the Midwest and may need adjustment for your particular area. For example, populations above 25,000 plants per acre should provide yields comparable to stands at 25,000 plants per acre based on Kentucky research.

Table 1. Estimating Corn Stand. Determine the length of row to count. Count the plants within that row. Multiply that number by 1,000. The product is the estimated number of plants per acre. This process should be repeated throughout the field in injured areas and non-injured areas.
Row Width (inches)	Length of Row to Count	Number of Plants in Row	Multiplication Factor	Estimated plants / acre
38	13’ 9”		x 1,000
36	14’ 6”		x 1,000
30	17’ 5”		x 1,000
20	26’ 2”		x 1,000
15	34’ 10”		x 1,000

Table 2. Grain yields for various planting dates and population rates, expressed as a percent of optimum planting date and population rate (uniformly spaced within row).
Planting date	Plants per acre at harvest
Planting date	12,000	14,000	16,000	18,000	20,000	22,500	25,000
	(% of optimum yield)
May 6	78	83	88	92	95	98	100
May 11	77	83	88	92	95	98	99
May 16	75	81	86	90	93	96	98
May 21	73	78	83	87	91	94	95
May 26	69	75	80	84	87	90	92
May 31	64	70	75	79	82	85	87
June 5	59	64	69	73	77	80	81
June 10	52	58	63	67	70	73	75

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3. Assessing Corn Hail Damage

Chad Lee, Agronomy

Assessing hail damage on corn requires a knowing several factors, such as the growth stage, leaf area destroyed and surviving stand.

Growth Stage

See the related article in this newsletter for determining growth stages. The growing point usually begins to move above ground as corn approaches the V6 growth stage. Corn is highly tolerant to hail damage at these early stages of growth. As the growing point moves above ground and the corn plant gets closer to tasseling, it becomes more susceptible to hail damage. Corn is most susceptible to hail damage just prior to tasseling through early milk. Once corn passes through the early milk stage, it becomes more tolerant to hail damage.

Percent Leaf Area Destroyed

Estimate the amount of leaf material that is missing or no longer green. Shredded leaves that are still green are considered viable. Any green leaf material should not be counted as destroyed. Refer to Table 1 to estimate yield losses. Table 1 is adapted from the National Crop Insurance Association’s “Corn Loss Instructions” (Rev. 1984). The entire table can be found in a Nebraska Extension publication (Vorst, 1986). The 8-leaf stage as determined by a hail adjuster is usually equal to a V6 leaf stage (six collars visible), whereas the 10-leaf stage is usually equal to V8 corn. The numbers in Table 1 assumes that the corn population was not reduced by hail.

For farmers raising corn for silage, the actual yield losses may be slightly less than the estimated yield losses for grain yield. Corn damaged at early stages will have the opportunity to produce more leaves and add to the total dry matter yields.

Table 1. Estimated percent yield loss occurring from defoliation.
		Percent Leaf Area Destroyed
Growth Stage*	10		20	30	40	50	60	70	80	90	100
7-leaf	0		0	0	1	2	4	5	6	8	9
8-leaf	0		0	0	1	3	5	6	7	9	11
9-leaf	0		0	1	2	4	6	7	9	11	13
10-leaf	0		0	2	4	6	8	9	11	14	16
11-leaf	0		1	2	5	7	9	11	14	18	22
12-leaf	0		1	3	5	9	11	15	18	23	28
13-leaf	0		1	3	6	10	13	17	22	28	34
*Hail adjusters’ method for staging corn. (8-leaf stage is usually similar to V6 corn, where 6 collars are visible).

Estimating Population

Reductions in corn stand may reduce yields. For most of Kentucky, stands ranging from 22,000 to 30,000 plants/acre are ideal. Stands below 22,000 plants/acre will cause yield reductions in most cases. The article “Assessing Damaged Corn Populations” in this newsletter provides more details regarding stand assessment.

Tied Whorls and Silking

Corn plants damaged by hail early in the season usually recover from tied whorls and bruised stalks. The corn will usually grow through these tied whorls within 3 or 4 weeks (Mangen and Thomison, 2000). Stalk bruising had little impact on lodging. Silking will be delayed on severely damaged corn plants. Corn from V5 through V7 that received 100% leaf damage from hail had a 1 to 1.5 week delay in silking compared with plants that received 80 to 90% leaf damage.

Final Thoughts

Hail damage early in the growing season almost always looks worse than it really is. Wait about five days before making any kind of stand assessment. If replanting does become necessary, then switch to an earlier hybrid.

References:

Vorst, J. J. 1986. Assessing Hail Damage to Corn, G86-803-A: http://ianrupus.unl.edu/fieldcrops/g803.htm

Mangen, T. and P. Thomison, 2002. Early season hail damage in corn: effects of stalk bruising and tied whorls. Agronomic Crops Team On-Farm Research Projects 2000. Special Circular 179-01. http://ohioline.osu.edu/sc179/sc179_16.html

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4. Corn Flood Survival

Chad Lee, Agronomy

Corn in some fields across Kentucky were completely submerged for several hours to several days. The question now is whether or not these plants will survive. When soil temperatures are in the higher 70’s F, then corn will survive about 24 hours of submersion (Thelen, 2001). Corn can survive submerged conditions for up to four days in cooler soil temperatures. Water over the top of corn for a few hours likely will do little harm to the crop.

Corn that was submerged may show yellowing and/or stunting symptoms. These symptoms are likely due to an inability to uptake N in saturated conditions (Nafziger, 1998). These symptoms are prolonged even after water has receded from the field because the soils will remain saturated. The length of time required for the soil to dry out, and allow oxygen back into the root zone will affect the rate of recovery. Mud and other debris on the corn can delay recovery as well.

To determine if the corn is surviving, select several plants and cut down the length of the stalks to the growing point (at or below the soil surface on V6 and younger corn). Healthy growing points will be white or cream-colored. Darkening and/or flaccid growing points indicate death of the growing point and the plant. The darkening or flaccid growing points will not occur for several days after the flooding.

Surviving plants could suffer a yield loss from flooding conditions. The lack of oxygen experienced by the plant could stunt root and shoot development during critical stages of development. The stage of growth affected by the flooding event could affect ear development (usually V6 corn) or kernel size and number (usually V12 corn). As mentioned earlier, the yellowing and stunting often are result of a lack of N. This does not mean that all the N has been removed from the soil, but that the corn cannot take up N in saturated conditions. Much of the N is still available to the corn once the soils dry out. See the article on “N Losses in Wet Soils” for more information about N management options following a flood.

References:

Nafzinger, E. 1998. Water- and wind-damage corn. Illinois Pest and Crop Development Bulletin. No. 15. http://www.ag.uiuc.edu/cespubs/pest/articles/v9815g.html

Thelen, K. 2001. Managing corn and soybean fields submerged by recent heavy rains. Field Crop Advisory Team Alert. Vol. 16 no. 6. http://www.ipm.msu.edu/CAT01_field/FC05-17-01.htm

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5. Estimating N Losses from Wet Soils

Greg Schwab and Lloyd Murdock, Agronomy

How much N was lost from corn fields which were fertilized before the heavy rains? Because of the high intensity of the rain (high runoff), leaching losses will probably be low. Therefore, the main N loss mechanism to be concerned with is denitrification, which occurs when soil nitrate is converted to nitrogen gas by soil bacteria. Well drained upland soils which have been wet from a series of rains probably have not experienced much denitrification because it takes 2-3 days of saturation for the bacteria to begin the process. For lower landscape positions that stay saturated for less than five days, the amount of N lost is still not as great as one might assume. Applications up to an additional 50 lb N/acre is the most economical in these situations.

If the soil says saturated for an extended period, you can calculate the amount of N lost. First, estimate the amount of applied N that was in the NO₃-N form when the flooding occurred (Table 1). Second, assume that 3 to 4 % of the NO₃-N is lost by denitrification for each day of saturation beyond 2 days. Table 1 and the example below will help with calculations

Table 1. Estimated NO₃-N remaining after fertilizer application.
	% of Fertilizer as NO₃-N
N Source	Weeks After Application
	0	3	6
Anhydrous Ammonia (AA)	0	20	65
AA with N-Serve	0	10	50
Urea	0	50	75
Urea with N-Serve	0	30	70
UAN (solution 28 and 32%)	25	60	80
Ammonium Nitrate	50	80	90

Example

A farmer applied 175 lbs N/acre as urea to corn grown on poorly drained soil. Three weeks after application due to a series of heavy rains, the field became saturated and remained saturated for 7 days. How much N was lost?

Step 1. Determine the amount of applied N in the nitrate form. According to the table, 50% of the urea will