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Index

• Planting Dates and Effects on Yield
• Row Spacing and Seeding Rates for Double-Crop
• Planting into Dry Soils
• Estimating Stands/Replanting
• Reducing Deer Damage
• Soybean Insect Pest Update
• American Soybean Association News

Planting Dates and Effects on Yield

The optimum time to plant soybeans in Virginia for maximum canopy development and yield is usually from mid-May to early June. Little or no decrease in yield will occur with delayed planting until mid-June. Afterwards, yields begin to decrease about 3 bu/A per week, or about _ bu/A per day. Average seed yields for soybean following wheat and planted before July 4 has yielded about 10-20% less than May planted soybeans. On the other hand, soybean following barley generally does not show any large, if any, yield decrease. Many producers even report higher yields for these "barley beans" than full-season soybeans. I haven't documented this, but this phenomenon is likely due to pushing the critical pod and seed development stages into late August when moisture stress is less likely.

However, the above generalities are just estimates. Variation in yield due to planting date will vary due to weather patterns, variety maturity, and location. One cannot absolutely predict soybean yield reductions due to late planting. Sometimes late-planted soybean will yield as well as earlier plantings, especially in a productive environment. (high rainfall or fertility, high water-holding capacity soils, etc.). Research from North Carolina has shown that soybean canopy develop-ment determines whether or not a yield reduction from late plantings will occur. In this research, no yield reduction was incurred if soybeans were at least 36 inches tall and the plants lapped the row middles by flowering. This would be about equal to the soybean crop obtaining a leaf area index (see Soybean Update, Vol. 4 No.2) or LAI of approximately 3.5-4.0 by the pod development stages. Therefore, if a soybean crop can produce adequate canopy growth, regardless of when planted, then maximum yield for that year and location can be obtained.

On the following page is a rather complicated figure that shows yield potential and planting date. In this figure, yield has been adjusted to % of maximum yield potential and planting date has been adjusted to weeks before or after the last planting date that will result in the needed leaf area to maintain maximum potential yield. What makes this figure complicated is there is no real date on the graph, but only weeks before and after an unknown plantin date in which yield will be either maintained or drop dramatically. This zero week could range from May to July. So, how is it useful? It's useful because it pretty much explains this entire planting date question. How do we estimate the planting date in which yield will start dropping dramatically? We cannot estimate it exactly, but we can come close. We can also recognize from this graph how increasing plant population or narrowing row spacing can push this date further back into the growing season.

For instance, let's assume that we are growing double-crop soybean following wheat on one of our "average" productive soils such as a Eunola or Emporia, which can experience some drought stress throughout the season. We will begin planting on June 18 and continue for the next 2 weeks. Let's also assume that June 10 is that "zero" date. Therefore, by looking at the graph, we can tell that we are not yet loosing much yield. But should we necessarily assume that we would automatically loose _ bu/day with each delay in planting? Not necessarily. Narrowing row spacing will definitely buy some time. Why? Because the canopy will develop faster. The same goes for plant population. For these soils, I'd normally plant enough seed on June 18 to insure a final plant population of 180,000 plants/acre (Higher than needed for full-season soybean). But, I'd increase the seeding rate to insure a population of 200,000 plants/acre by the first of July. Another option can be planting a later-maturing variety. By planting a variety that is a full maturity group (MG) later, one can increase the number of days to bloom (and the number of days of vegetative growth) by about 10 days. If you plant a variety that is _ MG later, then you can buy about 5 days. But one must be careful with planting later maturing varieties. One should not plant a variety that will not mature before the fall frost. This is usually a late MG 5 for southeast Virginia; a late MG 4 or early MG 5 for the northeast Coastal Plain; and nothing later than a MG 4 for the Northern Piedmont.

So, study this figure. Try to understand it. And try to figure out how one can minimize yield reductions from late plantings.

Planting date also affect soybean in other ways. Listed below are other agronomic characteristics affected by planting date:

• Plant height decreases as planting date is delayed.
• Lodging is related to plant height; therefore should decrease with later planting dates.
• Pod height from soil surface will decrease
• Maturity is affected by later planting dates
  • Before mid-June, a 3-day delay in planting results in about a 1-day delay in maturity.
  • After mid-June, a 5-day delay in planting results in a 1-day delay in maturity
  • After flowering, soybean development responds to growing-degree-days, therefore warmer conditions will hasten development and cool, cloudy condition will slow development.

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Row Spacing and Seeding Rates for Double-Crop

As one plants later, row spacing should be narrowed and seeding rates increased in order to minimize the yield effect shown on the figure on the previous page. Regardless of planting date, decreasing row spacing will generally increase yields. Most of the increase comes from narrowing the rows from 30 to 36 inches to 15 to 20 inches. However, smaller increased in yield can be obtained from even narrower row spacing. Once again the response is related to leaf area development, therefore factors such as planting date, soil type, and maturity group will cause the response to vary. Full-season plantings are less likely to respond to row spacing, but double-crop soybeans will nearly always respond. Similarly, earlier MG soybean are more likely to respond to narrow rows than later maturing varieties. Narrow row spacing is beneficial in productive soils, but the percentage response may be less than on non-productive soils. Finally, a greater response to increased seeding rates will be realized with narrow rows. This is due to less competition between plants in the same row (plants are spread out more in narrow rows). Therefore, a synergistic response to narrow rows and higher populations occurs. Regardless of seeding rate, soil type, etc., narrow rows (<20 inches) will always be beneficial in double-crop plantings.

The appropriate seeding rate to obtain the highest potential yield will again depend on the environment, planting date, soil type, etc. For double-crop plantings, we recommend a seeding rated to obtain a final plant population of 160,000 to 220,000 plants per acre. Why such a large range? Again, it depends on several factors. Only use the 160,000-plants/acre recommendation if you are planting on our most productive soils such as a Pamunkey loam. Use the 180,000 plants/A on moderately productive soils when planting in June. Use the 200,000-plants/A population on less productive soils or if planting begins in July. The higher rate of 220,000 plants per acre is reserved for very late plantings, poor soils, and early maturing varieties (i.e., MG 3).

Be mindful that these rates are for the final plant stand, not seeding rates. The final population goal should be adjusted by the expected emergence rate (use the germination rate on the bag as a guideline). Soybean seed should be seeded to obtain the desired number of seed per foot of row. Soybeans should never be seeded by pounds of seed per acre because seed size can vary greatly between varieties. Planters should be calibrated before planting and recalibrated when changing seed lots. Adjustments in seeding rate should be made when adverse soil conditions are present as described below.

• Increase 5% for each rotary hoeing planned
• Increase 10% for rough seedbeds.
• Increase 10% for short-season varieties planted double-crop
• Decrease 10% if lodging has occurred with a specific variety and soil type in the past.

The table below lists the suggested seeding rates in seed per foot assuming 85% emergence for double-crop plantings. Below this table is a small conversion chart to help users to estimate the number of pounds of seed per acre they will need depending on estimated seed size, which can be found on the seed label or in seed company literature. Do not use this conversion chart to estimate seeding rate.

	Desired Plant Population Per Acre
Row Width	Double-Crop
	160,000*	180,000	200,000	220,000
	Seed/ft. (assuming 85% emergence)
24"	8.6	9.7	10.8	---
20"	7.2	8.1	9.0	9.9
18"	6.5	7.3	8.1	8.9
15"	5.4	6.1	6.8	7.4
7.5"	2.7	3.0	3.4	3.7

*160,000 plants/acre only recommended for most productive soils with history of high yields.

Seeding Rate = Desired population ÷ (43,560 sq. ft./acre ÷row width in ft.) ÷emergence

Example: = 180,000 ÷ (43,560 ÷ 1.25 ft) ÷ 0.85
= 180,000 ÷ 34,848 ÷ 0.85
= 6.1 seeds per foot on 15" rows.

Desired Plant Pouplation per Acre (x1000)
Seeds/lb	160	180	200	220	240
	Pounds of Seed per Acre (assuming 85% emergence
1600	118	132	147	162	176
1800	105	118	131	144	157
2000	94	106	118	129	141
2200	86	96	107	118	128
2400	78	88	98	108	118
2600	72	81	90	100	109
2800	67	76	84	92	101
3000	63	71	78	86	94
3200	59	66	74	81	88
3400	55	62	69	76	83
3600	52	59	65	72	78
3800	50	56	62	68	74
4000	47	53	59	65	71
4200	45	50	56	62	67
4400	43	48	53	59	64

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Planting into Dry Soils

Should I plant into very dry soils? This question was a hot topic back in May when most of Virginia was dry. However, rains were received and at the time that I'm writing this article, this is a non-issue for many areas. However, if weather patterns return to that typical of our area, I would expect us to run into this scenario again before planting is complete.

Back in May, the answer to the above question was easy - wait for rain. But, now that we're into our double-crop plantings, we need to be more concerned with the effects of late planting, as described in the previous sections. Remember, every day delay in planting after mid-June will cost us about _ bu. per day in yield. So, what are the options? I can think of three.

The first is the same as my May recommendation - wait for rain. My reasoning is that even under dry soil conditions, there will be significant areas in the field that have enough moisture to swell the seed and begin the germination process, but not always enough to bring the soybeans out of the ground. The soybean seed then die and a poor stand results in these portions of the field. Then, we are faced with another question. Do we replant the entire field or just these areas. To insure uniform emergence, plant into moist soil (no dry zones beneath the where the seeds are placed).

Another option may be to plant deeper than normal to hit moisture. This will work to an extent, but seed should be planted at a depth that insures rapid emergence. Rapid emergence will not usually occur when soybeans are planted 2 inches deep or more. We might get by with this if the soil is warm, but this puts a lot of pressure on the seedling even under warm conditions. During our full-season plantings, we actually used this method because of dry surface soils. Moisture was found at about 1 inch, but we knew this would rapidly dry out after disturbing the soil with our planter. So, we opted to plant 1_ to 1_ inches deep and we obtained very good stands. However, I do not recommend planting deeper than 1_ inch. Furthermore, you should not plant to a depth that just hits moisture, but _ to _ inches below this point. I generally prefer this method the least to the other two.

The third option is one that may work, but only under conditions of very dry soil in the seed zone. Plant at a normal depth (1 to 1_ inches) and hope that the next rain isn't a 0.10 to 0.15 inch rain that starts the germination process, but fails to allow continued growth. As long as the seed is in dry soil, the germination process will not begin until it rains. Furthermore, soybean seed can remain in the soil for 2-3 weeks without substantial reduction in germination. Jim Dunphy, Soybean Specialist at N.C. State University is a supporter of this method and has seen good results. He feels that the chances of getting a little shower that starts germination is less that we may believe. It's a pretty narrow window between too little rain to start germination and enough rain to sustain continued growth. Jim reminds us to think about how many fields we've seen with two stands of plants in the same row - one stand from seeds that found enough moisture to start germination and continue growth, and a second stand from seeds that didn't even start germination until it rained.

The third option above brings on another question that I continually hear from producers Ç Will I bake the seed if I plant into dry soils that are also very hot (common in late-June and July double-crop plantings)? In May and early June plantings, this is probably a non-issue because soils are not that warm. But in July, this can be a concern. Again the extent of the damage to the seed under hot soils will depend on whether the seed can find just enough moisture to begin the germination process. If it does, then high temperatures can easily kill the seed. The hotter the temperatures, the more stored energy in the seed gets used up by respiration (respiration is at a high level if the germination process begins). But, respiration is low if the seed is at 9-11% moisture, therefore the chances of "baking" the seed is less likely. The drier the seed, the more temperature stress it can withstand. On the other hand, if the seed moisture is 12-13% (common for farmer-saved seed), temperature stress is less tolerated. Seed quality is a major factor affected this and other emergence problems. Remember however, the above discussion relates to "bone-dry" soils.

So, what do we do? I still favor planting into soils moist enough to begin germination and sustain its growth. I say this even in regards to our previous discussion on the effect of planting date on yield. The seed will not emerge in dry soils; therefore planting date is delayed anyway. Plus, when soils are dry after a small grain crop (sometimes several feet down), it will be very hard to penetrate it with a planter or drill (but this can be done with adequate down-pressure and weight, see Virginia Soybean Update, Vol. 4 No.1). Rainfall will help soften the soil. The only situation which one may consider planting into dry soils is if one has very large acreages to plant. If it takes 2 or more weeks to complete double-crop planting, then one could potentially lose several bushels from the time they start planting to the time they end.

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Estimating Stands & Replant Decisions

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 feeding, poor soil to seed contact, or disease infection? Determine if the cause can be corrected to avoid a similar situation.
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 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. Use Table 5.4 to determine remaining plant population. Or use "hula hoop" method (Table 5.5) to determine population if 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.
3. Estimate the yield of the poor stand. Use Table 5.6 or Table 5.7 to determine percent of full 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. Up until mid-June, decrease the expected yield 1 bu/A per week from the original planting date. After mid-June, decrease the expected yield an additional 1/2 bu/A per day. 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) to obtain gain or loss in $/A.
6. Estimate the cost of replanting. Include per acre cost of tillage, herbicide, seed and labor.
7. Determine profitability of replanting. Subtract your cost of replanting from your estimated gain from replanting.

Table 5.4. Plant populations of different row spacing with different plant counts per foot.

	Row Spacing
Plants/foot	30	24	20	15	7.5
	-------------------- Plant Population (1,000's/acre) ---------------
1	17	22	26	35	70
2	35	44	52	70	140
3	52	65	78	105	210
4	70	87	105	139	278
5	87	109	131	174	---
6	105	131	157	209	---
7	122	152	183	244	---

Table 5.5. Hula-hoop method for determining drilled soybean populations.

No. of Plants	Inside Diameter of Hula Hoop
	30"	32"	34"	36"	38"
	(Plants in 1,000's per acre)*
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

* Plants/acre = no. plants ÷ (3.14 * r² ÷ 43,560 ft²) where r = radius of hula hoop in feet.

Table 5.6. Yield response of full-season soybeans to deficit stands.

% Stand lost to gaps	Remaining Plant Pop (1,000's/A)
	140	105	70
0	100	97	95
10	98	96	93
20	96	93	91
30	93	90	88
40	89	86	83
50	84	81	78
60	78	75	73

*Source: Illinois Agronomy Handbook (Anonymous, 1986), University of Illinois.

Table 5.7. Yield response of double-crop soybeans to deficit stands.

% Stand lost to gaps	Remaining Plant Pop (1,000's/A)
	180	140	100
0	100	95	88
10	98	93	86
20	96	90	83
30	93	87	80
40	89	83	76
50	84	78	71
60	78	73	65

*Yield losses assume that soybeans were planting soon after wheat harvest (mid- to late-June).

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Reducing Deer Damage

The table to the lower left shows the influence of clipping (to simulate deer depredation) at various growth stages on yield and height of 'Hagood' soybean. Clemson University conducted the research in Pendleton, SC. The clipping treatments removed the upper one-quarter to one-third of each plant at the shown soybean development stages. Yields were severely reduced by any clippings in the late pod stage (R4). This 2-year study was conducted under good crop growth and weed-free conditions. The lack of canopy closure due to deer damage could lead to further losses if late-season weed flushes occurred or under droughty conditions. (Source: Clemson Extension Publication IL 59 (June 1996), Strategies for Reducing Deer Damage to Soybeans)

In fields with perennial histories of heavy deer depredation, it is recommended not to grow soybeans. The only practices that were found to lead to profitable soybean yields with heavy deer depredation are to reduce the deer population through hunting and kill permits or with electric fencing, which can be rather expensive. With moderate levels of deer depredation, later maturing varieties planted in narrow rows at high populations will help alleviate the damage. Once again, the reason for yield reductions is lack of canopy growth; therefore, any practice that increases leaf area will help the soybean to tolerate the damage.

V4	V6	V10	R4	Yield (bu/A)	Height (in.)
-	-	-	-	40.2 a	37.0 a
+	-	-	-	40.2 a	33.1 b
-	+	-	-	39.7 a	29.0 c
-	-	+	-	40.9 a	27.2 cd
+	+	-	-	41.8 a	26.5 cd
+	-	+	-	40.8 a	22.4 ef
-	+	+	-	36.0 a	20.7 fg
+	+	+	-	39.7 a	20.9 fg
+	-	-	+	26.7 b	24.4 de
-	-	-	+	24.0 bc	29.2 c
-	+	-	+	21.2 bc	22.5 ef
-	-	+	+	20.8 bc	22.1 ef
+	+	-	+	18.5 bc	17.9 gh
+	-	+	+	16.3 c	16.8 hi
-	+	+	+	16.5 c	15.5 hi
+	+	+	+	16.4 c	14.4 i

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Soybean Insect Pest Update

In past years, we haven't had to be too concerned about early season insect problems in Virginia soybeans. In fact, corn earworm has traditionally been the single most troublesome insect pest. We hope that doesn't change, as we have a good program for managing earworm and for making sound economic decisions.

This year has already gotten off to a strange start in several southern states with very early and high numbers of unusual pests like saltmarsh caterpillar, grass hoppers and armyworms. These are all common species in Virginia soybean fields, but rarely, if ever, occur in outbreak proportions. However, given what has already occurred in some other states, we should be on the lookout.

Early season insect problems, if they occur at all, will more likely be due to outbreaks of spider mites, bean leaf beetles, thrips, potato leafhoppers Ç or potentially, aphids. Taken in that order, here is an update.

Spider mite outbreaks have already occurred in a few of our cotton fields in the southeastern counties. Mites are infesting cotyledon cotton leaves causing feeding damage and compromising plant vigor. To my knowledge, this is the first time this has ever occurred. We think this problem was set in motion by the overall dry weather conditions during April and May. Mite populations built up on alternate weed hosts and moved onto cotton seedlings when those weed hosts dried down. Mites also tend to be worse in minimum tillage cotton fields indicating that they may have increased on field weeds and moved to cotton after burndown herbicides were applied. If dry weather persists, mites could begin showing up in soybean fields. This would be a good year to stay alert to this, especially in those areas where soybeans have a history of mite infestations.

Bean leaf beetles have always been present in soybean fields in Virginia, but mostly at low enough numbers not to present a threat to bean yields or seed quality. They can be early season pests as first generation adult beetles feed on the leaves of seedlings. We have not considered this to be a problem unless damage is so severe that seedlings are being destroyed. As the season progresses, beetles will go through more generations and continue to feed on leaves Ç even scarring the outer walls of pods in extreme outbreak cases. However, it has been very unusual to have enough leaf damage or leaf area loss to beetles to warrant an insecticide treatment.

Thrips are perhaps the most 'talked about' potential early season insect pest. We have a lot of information and data that shows that early season thrips damage can reduce yields in both peanuts and cotton. It has been much more difficult to verify that for soybean. In two years of replicated field testing, we have not been able to demonstrate a significant yield reduction where thrips were not controlled with insecticide sprays compared with where they were. Thrips may slow seedling growth somewhat, but with the long growing season, plants fully recover. Soybean Entomologists across all soybean states are in agreement that thrips damage has to be both severe and combined with prolonged dry weather conditions to result in significant yield loss. We are continuing to pursue this problem in hopes of developing better management guidelines.

Potato leafhopper populations can cause seedling damage if populations are large enough. This pest is typically worse in dry years, and is especially worse in soybean varieties that have few or sparse hairs on the leaves and stems. As with thrips damage, early season leafhopper damage may slow seedling maturity somewhat, but plants have a long season to recover, and yield loss to hopper is rare across all soybean states.

Aphids have never been a problem on soybeans in the U.S. until the discovery of a new species, the soybean aphid (Aphis glycines) that was introduced from China. We have enclosed a new bulletin with details on this problem. Thus far, it is most common the mid-West, although it was detected as close to us as West Virginia. We are initiating a survey this summer to determine if the aphid is in our state. More on this later.

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Sincerely,

David L. Holshouser

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