Monthly Archives: June 2013

Soybean Replant Decisions

Posted on June 27, 2013 by David Holshouser

Deciding whether or not to replant can be a gut-retching decision. I readily admit that the choice to leave the present stand or replant is not a simple one. With the ever-increasing cost of Roundup-Ready soybeans and diesel fuel, the profitability of replanting may not seem a good idea. On the other hand, high prices make replanting more appealing. In the end, your decision should be one based on the estimated dollar gain from replanting. This will require a careful evaluation of the soybean stand and an analysis of yield potential of the present and replanted crop.

In general, there is less benefit of replanting if stands are reduced uniformly across the field. Usually replanting can only be justified where stands have been reduced by half. However, poor stands usually include gaps in addition to merely a lower plant population. These gaps must be accounted for. In addition, one may have a 75-80% stand in parts of the field, while other parts of the field may only have 20-25% of the intended plant population; the decision may be to only replant part of the field.

Remember to take into account the yield loss from delayed planting. Up until mid-June, there is very little yield loss from delayed planting. After this, for every day delay in planting yield declines about half a bushel per day. For instance, if replanting in late-June/early-July (about 2 weeks late), your potential yield will have declined by 7 bushels per acre.

A few more questions must be asked before we can proceed with a step-by-step procedure for estimating the profit of replanting. When considering a replant, do you leave plants in the field and plant through them? In many cases, planting through the poor stand is a possibility. If you’re using rows wide enough to fit a tractor tire comfortably between them, then you can split the rows and plant enough new plants to get the final plant population up to 100 to 120 thousand plants for full-season systems (if re-planting before mid-June) or 180 to 200 thousand plants per acre for double-cropped systems (this may require equipment or tractor tire modifications).

On the other hand, a drill will cut up many healthy plants and make them less productive than the ones that you just replanted. Also, if the drill has wide gauge wheels, then you can severely damage a significant number of plants (there’s a significant amount of down pressure on those wheels – enough to crush a plant on hard soil). And a damaged plant can sometimes act more like a weed than a crop. Recognize that the plants growing in the field are going to be higher yielding that any that emerges after replanting. If you destroy or injure those plants, you’ve just writing off some profit. Therefore, I do not suggest planting through the old stand with a drill.

Another issue is that if you decide to replant, do you switch to an earlier maturity group? There is little need to plant an earlier-maturing variety. Here are two general rules of thumbs:

April/May Plantings: A 3-day delay in planting will result in a 1-day delay in planting. Therefore, if planting 30 days later, the crop will mature 10 days later.

June/July Plantings: A 5-day delay in planting will result in a 1-day delay in maturity. Therefore, if planting 15 days later, the crop will mature only about 3 days later.

Most important is to plant a maturity group that would grow as long as possible (therefore producing an adequate canopy for maximum yield) and still mature before the average frost date. If you’re destroying the old crop and starting over, just make sure that the variety you chose will mature before a frost. Alternatively, if you are planting into the old crop, you may want to choose a variety about one-third to half a maturity group earlier (depending on planting date differences) so that harvest maturity of both plantings will be more in sync. For example, if you planted a relative maturity group 5.6 on June 10, then you could choose a relative maturity of 5.2 to 5.4 if replanting on June 25.

Finally, realize that you’ll need a higher plant population when planting late. Final stand with a double-crop planting should be at least 180 thousand plants per acre. If replanting in July, increase the seeding rate to insure at least 220 thousand plants per acre.

To estimate the profitability of replanting, follow the guidelines listed below. Be sure to incorporate plant population and gaps in your calculations. We’ve found that 2- to 3-foot gaps cause as much or more yield loss that from low plant populations.

1. Determine the cause of the poor stand. Was the poor stand the result of poor seed quality, cold wet soils, hot dry soils, planting too deep or shallow, soil crusting, herbicide injury, insect or slug feeding, poor soil to seed contact, or disease infection? Determine if the cause can be corrected to avoid a similar situation. If slug or insect feeding or disease is the cause, then you might expect poor stands again.

2. Estimate the stand and percent stand loss due to gaps. Pace off the sections of row 20 paces long in at least 6 areas of the field. Determine (in number of paces) the total length of row lost to 2- to 3-foot gaps. For drilled soybean, this can be interpreted at 2- to 3-foot diameter gaps. Then determine the percent of row lost to gaps. In addition, count and determine average number of plants per foot in sections of row not reduced by gaps. The simplest method is to count the number of healthy plants (capable of recovery) in a length of row equaling 1/1000 of an acre. For instance:

· 36-inch rows = 14.5 feet

· 30-inch rows = 17.5 feet

· 20-inch rows = 26 feet

· 15-inch rows = 35 feet

· 7.5-inch rows = 70 feet

Then, just multiply your counts by 1,000 to get plants per acre.

Or, use the Tables 13.1 or 13.2 to determine remaining plant population. The “hula hoop” method (Table 13.2) is valuable with drilled soybean or when rows cannot be distinguished. This involves placing a circular measuring device such as a hula-hoop on the ground and counting the plants contained within.

Table 13.1. Plant populations of different row spacing with different plant counts per foot.
Plants/ foot	Row Spacing
Plants/ foot	36	30	24	20	15	7.5
	Plant Population (1,000’s/acre)
1	15	17	22	26	35	70
2	29	35	44	52	70	140
3	44	52	65	78	105	210
4	58	70	87	105	139	278
5	73	87	109	131	174	—
6	87	105	131	157	209	—
7	102	122	152	183	244	—

Table 13.2. Hula-hoop method for determining drilled soybean populations.
No. of Plants	Inside Diameter of Hula Hoop
No. of Plants	30”	32”	34”	36”	38”
	Plant Population (1,000’s/acre)^a
6	53	47	41	37	33
10	89	78	69	62	55
14	124	109	97	86	77
18	160	140	124	111	100
22	196	172	152	136	122
26	231	203	179	160	144
^aPlants/acre = no. plants ¸ (3.14 x r2 ¸ 43,560 ft2) where r = radius of hula hoop in feet.

3. Estimate the yield of the poor stand. Use Tables 13.3 and 13.4 to determine percent of yield potential for full-season and double-crop plantings, respectively. Multiply this percentage by the expected yield. This is the yield to expect from the deficient stand.

4. Estimate the yield from replanting. After mid-June, decrease the expected yields an additional half of a bushel per acre per day delay in planting. This is the yield to expect from delayed planting.

5. Determine the gain or loss from replanting. Subtract the expected yield of the poor stand (step 3) from the yield expected from delayed planting. This is the gain or loss in bu/A from replanting. Multiply this number by the expected price ($/bu), using future prices, to obtain gain or loss in $/A.

6. Estimate the cost of replanting. Include per acre cost of tillage, herbicide, fuel, seed, and labor.

7. Determine profitability of replanting. Subtract your cost of replanting from your estimated gain from replanting.

Table 13.3. Yield response (% of maximum) of full-season soybeans to deficit stands^a.
% Stand lost to gaps^b	Remaining Plant Pop (1,000’s/A)
% Stand lost to gaps^b	70	105	140
0	95	97	100
10	93	96	98
20	91	93	96
30	88	90	93
40	83	86	89
50	78	81	84
60	73	75	78
^aSource: Pepper and Wilmot. Managing Deficit Stands. 1991. Illinois Cooperative Extension Cir. 1317. ^bGaps of 12 inches or more; 30-inch rows

Table 13.4. Yield response (% of maximum) of double-crop soybeans to deficit stands^a.
% Stand lost to gaps^b	Remaining Plant Pop (1,000’s/A)
% Stand lost to gaps^b	100	140	180	220
0	80	88	95	100
20	71	79	86	91
40	61	69	76	81
60	48	57	64	69
^aSource: 2001-2004 experiments, Suffolk, VA. ^bGaps of 3 feet; 15-inch rows; MG 4 variety

It’s Time to Increase Soybean Seeding Rates

Posted on June 13, 2013 by David Holshouser

Wheat harvest has begun. Remember that soybean yield potential declines with delayed planting date. During the first two weeks of June, this decline is barely noticeable. After mid-June, expect yield to decline about ½ bushel per acre per day. So to minimize soybean yield loss due to late planting, immediately (or as soon as you can) plant. I like to see the combine and the planter or drill in the field at the same time.

Soybean planted after mid-June will not always develop enough leaf area to capture 90-95% of the available sunlight by early pod development (R3). Only if four layers of leaves are formed by that time will yield not be affected. Of course the growing season makes a difference. Last year in many parts of Virginia, even the latest-planted soybean were able to produce enough leaf area. But, on average, June- or July-planted soybean are unable to do so. Therefore it’s important to take actions to minimize this decline.

First, plant as soon as possible. With the recent rains, soil moisture should not be a problem. Even if the soil is dry on top, I suggest to go ahead and plant. You may have to plant a little deeper to hit moisture (don’t plant much deeper than 1.5 inches), but planting depth is not as big of an issue with double-cropped as with full-season soybean.

Second, narrow your row spacing. Narrow rows will produce more leaf area and canopy over faster.

Finally, increase your seeding rates gradually as time goes on. I’m suggesting the following plant populations for Virginia in 2013:

June 2-8 – 120,000 plants/acre
June 9-15 – 140,000 plants/acre
June 16-21 – 160,000 plants/acre
June 23-29 – 180,000 plants/acre
June 30-July 5 – 200,000 plants/acre
After July 5 – 220,000 plants/acre

See the seeding rate chart below to determine the seeds per foot of row needed to obtain a certain number of plants per acre. Note that these seeding rates assume 80% emergence. For different emergence assumptions, divide your desired plant population by the percent emergence that you expect.

	*Desired Plant Population*
*Row Width*	Early-June Planting Date			Late-June/Early July Planting Date
*Row Width*	120,000	140,000	160,000	180.000	200,000	220,000
	(Seed per foot of row assuming 80% emergence)
20	5.7	6.7	7.7	8.6	9.6	10.5
18	5.2	6.0	6.9	7.7	8.6	9.5
15	4.3	5.0	5.7	6.5	7.2	7.9
10	2.9	3.3	3.8	4.3	4.8	5.3
*7.5*	2.2	2.5	2.9	3.2	3.6	3.9
As planting is delayed, increase seeding rate. Use lower seeding rates on more productive soils. Calculation: Desired Plant Pop. ÷ (43,560 sq. ft./acre ÷ row width in ft.) ÷ % emergence Example: = 180,000 ÷ (43,560 ÷ (15 ÷ 12)) ÷ 0.80 = 6.5 seed per foot of row

In addition to planting date, I’ve found that more productive soils will tolerate lower seeding rates, and less productive soils need higher seeding rates; a more productive soil will produce more leaf area than a less productive soil over the same time period.

After the Flood: How much damage? What to Do?

Posted on June 13, 2013 by David Holshouser

Although we are currently drying out after the heavy rains over the weekend, I thought it would be a good idea to review the effects of flooding on soybean. Usually, widespread flooding is not a major issue for us due to the sandy nature of our soils and/or well-drained fields. Still, it can and has happened.

During the week ending on June 12, some parts of Virginia received 6 to 10 inches of rainfall (see map below). Plus, more may be on the way.

This heavy rainfall has resulted in saturated soils and in some cases, flooding. This field is a Soybean Research Verification Field in Dinwiddie County that I’ve been scouting. The photo was taken after a hard rain on Friday, June 7.

It’s difficult to know the long-term effect of flooding on soybean fields. Research is limited, but we do know that the fate of flooded fields will largely depend on 1) the development stage during which the flood took place; 2) the duration of the flood; 3) the temperature during and right after the flood; and 4) the drying rate after the flood.

Basically a flooded field depletes the roots of oxygen (O₂), causing photosynthesis to slow. After several days without O₂, the plant may turn yellow, grown very slowly, and possibly die. Other indirect effects of flooding can include reduced nitrogen-fixing bacteria (but they will recover), nutrient imbalances, and increased disease pressure.

Effects on Germination & Emergence. The most damaging effects of flooding on un-emerged soybean occur when the duration of the flood is greater than 24 hours and/or when the soil temperature is low. In controlled research conducted during the late 1990’s, researchers examined the effect of flood duration, soil temperature, and time after seed imbibition that the flood occurred. They found that saturated conditions decreased germination by 15 to 43 percentage points when averaged over temperatures of 59^O or 77^O F (germination was 62% in the non-flooded control, averaged over temperature treatments). When flooding lasted only 1 to 12 hours, germination was only decreased by an average of 15 percentage points, regardless of when the seed began imbibing water. Even after 48 hours of saturated conditions, germination was only reduced by 20 percentage points if the flooding occurred one day after the seed imbibed water. But, if the flooding occurred 2 or 3 days after the seed imbibed water, then the germination was lowered 33 or 43 percentage points, respectively. Therefore, the farther along the seed was in the germination process, the more susceptible they were to flood damage. Lower temperatures during the germination process increased the damage. At 59^OF, germination was lowered by 21 to 25 percentage points, regardless of the duration of the flood. But at 77^OF, germination was only lowered by this amount if the flood lasted 48 hours. The researchers suggested that damage to the seed under brief flooding was primarily physical (e.g., seed membrane damage), but damage with longer flood duration was physiological (e.g., ethanol toxicity, O₂ deprivation, CO₂ build up).

Flooding can also result in soil crusting. This will be worse in tilled fields that are low in organic matter and surface residue. Unless the crust is broken with a rotary hoe or similar implement, the emerging seedling, already stressed by the flood, will have an even more difficult time emerging and growing.

There is also an increased chance of seedling disease, but this is less of a problem at this time of the year with warm soil temperatures. Poor stands will be the biggest issue. To determine whether or not re-planting will be beneficial, refer to Virginia Soybean Update Vol. 13, No. 3 (June 2012).

Flooding After Emergence. Soybean can generally tolerate up to 2 days of flooding. But if the saturated soil conditions persist for more than 2 days, significant yield reductions may occur. The amount of yield reduction will vary with development stage, duration of the flood, the type of flood (stream overflow vs. low land depressional), temperature during the flood, the drying rate after the flood, and the overall environmental conditions after the flood.

In general, the following comments can be made. Flooding in the reproductive stages causes more damage than in the vegetative stages. Yield is reduced more with longer flooding duration and with a slower drying rate of the field (e.g., a well-drained sandy soil will recover faster than a poorly-drained or heavier textured soil). Higher temperatures after the flood will lead to greater yield losses. This is because the recovering plant will deplete its stored energy at a faster rate. In addition, high soil temperatures will result in greater microbial respiration, which lead to greater depletion of oxygen. The best conditions for a recovering soybean crop are cool, cloudy days and cool, clear nights.

In a Arkansas study conducted on two poorly drained soils with slow permeability (Sharkey clay and Crowley silt loam), researchers found that yield decreased by 1.8 and 2.3 bushels per acre per day of flooding on the Sharkey clay when soybean were flooded from 2 to 14 days at the V4 stage (4 trifoliate leaves on the main stem) and R2 (full flower) stages, respectively. On the Crowley silt loam soil, yield was reduced 0.8 and 1.5 bushels per day of flooding at V4 and R2, respectively. It’s worth noting that this research did not have an un-flooded control treatment; therefore, the 2-day period of flooding was considered the control. This research clearly showed that duration of the flood greatly impacts yield. In addition, flooding during reproductive stages caused more yield loss that if the flooding occurred at vegetative stage. Eight varieties were tested, but variety did not affect the flooding response. Other researchers have however indicated that certain varieties exhibit greater flooding tolerance.

Another study in Louisiana evaluated 7 days of waterlogging on V2, V3, V7, R1, R3, R5, R6, and R6.3 stage soybean under greenhouse conditions. The V stages represent the number of trifoliate leaves on the main stem and the R stages represent beginning flower, beginning pod, beginning seed, full seed, and the temporal midpoint of seed filing. In summary, the early vegetative stage (V2) and the early reproductive stages (R1, R3, and R5) were most sensitive to waterlogging.

Although some research has been conducted, estimating the yield loss due to flooding is nearly impossible because we don’t know what the conditions will be like afterwards. The best gauge may be ones past experience with fields that have flooded.

Fortunately, some of these experiences have been documented. An example is research conducted in Ohio in 1998, where researchers monitored several fields that flooded during that year. They reported a 20% reduction in soybean yield in one field and a complete loss in another field that was flooded for 3 days during the V2-V3 stage. The field with the complete loss was due to thick sediment coating the plants and not allowing any recovery. However, flooding for 3 days in two other fields caused no yield loss and flooding actually increased yield in a fifth field. In the field where yield was increased, sediments covered the plants but a light rain soon after the flooding washed the sediment off. In addition, greater residual soil moisture associated with the flooded area when a late-season drought occurred was probably responsible for the greater yield. After 6 days of flooding, yield losses ranged from 0% at two sites to about 60% at a third site and over 90% at another site. At one of the sites with no yield loss, the subsoil had a high sand content, which enable the root zone to quickly return to aerated conditions.

In another study, researchers found that flood irrigation for greater than 2 days reduced soybean yield by 20% as compared to a 1-day flood treatment.

What to Do After the Flood. While it may seem that there is little to do to help a flooded crop, it is very important that we minimize any other stress on the crop that can prevent recovery. First, try to stay off of the field. Wet fields compact easily. Compaction will further stress the crop and slow its recovery. Make sure that the field is dry enough before taking equipment back on it.

Evaluate the stand. If a stand reduction occurred, determine if it’s worthwhile to replant. Remember that after mid-June, every day delay in planting will cost you about ½ of a bushel in yield. The plants that remain are still higher yielding than seed that can now be planted, even if the stand has been substantially reduced.

Stress such as herbicide injury can slow the crop down further. Still, weeds need controlling. But you may want to select herbicides (usually as tank-mix partners to glyphosate) that don’t cause a significant amount of burning.

Cultivation is an option to conventionally tilled fields to help aerate the soil. However, cultivating a wet soil can do more damage than good by causing additional compaction, which in turn would further stress the crop.

You may also want to dig a few roots and inspect for adequate nodulation. Make sure the nodules are pink. I see little that can be done, but knowing that the nodules are still working may give you peace of mind. And while you’re doing that, split some stems and roots to check for any disease. If suspected, send in to one of our Plant Disease Clinics to be further evaluated.

Finally, some will want to apply some type of foliar fertilizer to the crop to “kick-start” it back to health. But, I see little advantage of this. Remember that the real problem is lack of O₂ to the roots and CO₂ buildup in the soil; only after the roots begin to receive O₂ will the recovery process start.

Hopefully you haven’t experienced severe flooding (> 24 hours). But if so, be patient and evaluate the field. Then make good decisions on how to handle it.