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Corn & Soybean Science Newsletter |
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April 2006, Volume 6, Issue 2
Cooperating Departments: Agricultural Economics, Biosystems and Agricultural Engineering, Entomology, Plant and Soil Sciences, Plant Pathology Editor: Dr. Chad Lee
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In this Issue: 2. Time and Soybean Management 4. Soybean Populations and Yield
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Marestail (Conyza canadensis) Control in Soybeans James R. Martin and J. D. Green, Plant and Soil Sciences
The glyphosate resistance issues associated with marestail makes it a challenge to manage this annual weed in soybeans. If marestail is present, it is prudent to assume that glyphosate resistance is likely and that alternative herbicides need to become a part of the weed management program.
Burndown applications of 2,4-D ester play a major role in controlling marestail in full-season no-till soybeans, yet they have their limitations. One of the primary drawbacks of using 2, 4-D in burndown applications is the risk of injuring soybeans. In order to minimize the risk, 2,4-D ester must be applied 7 to 30 days ahead of soybean planting. Treatments of 0.5 lb ae (acid equivalent) per acre, require a minimum of 7 days between application and planting; whereas, most 2,4-D ester products applied at rates > 0.5 to 1.0 lb ae/A require a waiting interval of at least 30 days. The label directions on rates and waiting periods may vary depending on specific product.
The following list of 2,4-D ester products illustrates the differences in formulations and recommended label directions concerning rates and waiting period between application and planting of no-till soybeans.
Table 1. Examples of products containing 2,4-D and waiting periods between spraying and planting soybeans.
While the persistence of 2,4-D in soil is considered a risk to injuring soybeans, it is not great enough to provide long-term preemergence control of marestail following burndown applications. Under the right conditions in Kentucky, marestail can emerge throughout much of the summer; consequently, newly emerging seedlings following burndown applications could potentially become a problem.
The following comments may help growers who are trying to manage marestail this spring:
Monitor fields well
in advance of soybean planting for emergence and size of
marestail.
2) Soil-residual herbicides When applying well in advance of soybean planting, consider including a soil-residual herbicide such as Canopy (chlorimuron + metribuzin), Canopy EX (chlorimuron + tribenuron), FirstRate (cloransulam), Sencor (metribuzin), or Valor (flumioxazin) to provide preemergence control of marestail plants. The fact these options offer alternative mode(s) of action compared with glyphosate is another benefit for including them in the overall management of marestail
3) Paraquat or glyphosate Include paraquat or glyphosate with 2,4-D as a part of the overall burndown treatment to control other weeds and help provide additional stress on marestail. While paraquat alone is not a good option for controlling marestail, it can be beneficial when using 2,4-D for the burndown control of small plants. It also provides another mode of action that is different than glyphosate. However, the benefit of paraquat may diminish as average plant size increases to 4 to 6 inches in height. While resistant marestail plants are tolerant to glyphosate, they usually show some stunting as an initial response to glyphosate. This herbicidal activity along with that from the 2,,4-D ester can be helpful in managing marestail.
4) Taller marestail Achieving consistent burndown control of marestail diminishes when plants exceed 6 inches in height. If plants exceed this height, consider applying 2,4-D ester at the high rate and include glyphosate along with FirstRate, or Canopy. Using the high rate of 2,4-D ester (1 lb ae/A) can improve control but the extra time in delay planting of soybeans is an obvious draw back when applying this rate.
5) ALS-resistance Be on the look out for ALS-resistant marestail. Although this type of resistance has not been confirmed in Kentucky, there are cases of it in Ohio and Indiana. FirstRate, Canopy EX, Classic (chlorimuron), and Synchrony XP (chlorimuron + thfensulfuron) are examples of ALS–containing products that will not control ALS-resistant marestail.
6) No-till double-cropped soybeans There are no good options for managing glyposate-tolerant marestail in no-till double-cropped soybeans. The best defense is to have a good stand of wheat to limit the emergence and growth of marestail. Marestail will probably be more of a problem in double-cropped soybeans this season due to erratic wheat stands caused by the dry weather during wheat planting last fall.
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Time- A Key to Managing Soybean for High Yields D.B. Egli, Plant and Soil Sciences
Time permeates our lives. There is not enough time. It takes too much time. I don’t have enough time. I’m not on time. Time seems to rule our very existence and it is difficult to escape from this constant obsession with time. Perhaps it is not surprising that time is also an important part of soybean production – we can’t forget about time if we want to produce high soybean yields.
There are two important aspects of time in soybean production that I want to discuss. The first relates to the growth of the plant – what is it doing and when is it doing it? The second relates to the producers’ activities – what are they doing and when?
Soybean plants (and all other grain crops for that matter) go through three phases of growth: first they grow vegetatively, producing stems, leaves and roots, then they start flowering and setting pods, and finally, the seeds grow and fill the pods and the crop is ready to harvest. In a sense, the crop first produces the machinery (leaves, stems, roots) to produce yield, then the container for yield (pods and seeds), and finally it fills the container (seed filling). Surprisingly, most of the soybean plant’s life is devoted to preliminary events, growing leaves and setting pods, while the seeds and yield are produced only during the final stages of growth. Only about 30% of the time from planting to physiological maturity is actually used to fill the seeds.
Following the soybean growth staging system (a system that all producers should be familiar with, the box at the end of this article contains more details), flowering and pod set begins at growth stage R1 (beginning bloom – one open flower at any node on the main stem) and ends at roughly growth stage R5 (beginning seed – seed 3 mm (1/8 inch) long in a pod at one of the four uppermost nodes on the main stem with a fully developed trifoliolate leaf, i.e., a leaf that has unrolled sufficiently so that the edges are not touching). Vegetative growth continues during flowering and pod set, but both end at the beginning of seed filling. Seed filling begins at R5 and ends at growth stage R7 (beginning maturity – one normal pod on the main stem has reached its mature pod color, brown or tan depending on the variety). Seed filling actually begins a little before R5 and pod set continues for a while after R5, but R5 is a good indicator of the change from one phase to the other. Growth stage R7 is a good indicator of physiological maturity, defined as maximum seed dry weight. All the yield is there at growth stage R7, but the seeds are about 55% moisture, so harvest cannot begin until the seeds dry. These comments apply, with only subtle differences, to soybean varieties with indeterminate (generally maturity group IV and earlier with only a few exceptions) or determinate (maturity group V and later) growth habits.
Which of these three phases is most important for high yields? Without a doubt, flowering and pod set is the most important because the number of pods and seeds the plant will produce is determined during this stage. Good weather (plenty of rain) during this period will produce lots of seeds (a large yield container) which is always the first step to high yield.
The yield container must be filled, so seed filling is also a very important phase. Stress (dry weather) during this phase will produce smaller seeds (often because of earlier maturity) and reduce yield. When stress during flowering and pod set reduces the number of pods and seeds the plant can compensate (assuming the weather improves during seed filling) by making larger seeds. There is a limit to this compensation (no one has ever seen a soybean seed the size of a baseball or even a golf ball), so it’s better to have a large yield container than to bet on seed size.
Surprisingly, vegetative growth is the least important of the three. Yes, you must have a certain amount of machinery to produce maximum yield, but most soybean varieties in narrow rows produce more than enough machinery (leaves). If you have more than enough you can lose a little without affecting yield. The key is that there must be enough leaf growth to provide complete ground cover by flowering or soon thereafter; if you can see the soil between the rows after flowering (growth stage R1), yield will probably not be as high as it could be. Stress during vegetative growth often has no affect on yield, because there are still enough leaves left to provide complete ground cover. An English scientist once asked the question “Is your vegetative phase necessary?” and concluded “Yes, but not as necessary as one might think”. This conclusion is definitely true for soybean.
The timing of the reproductive phases – when they occur during the summer- can also be important. In a good year with lots of rain or in a terrible drought it won’t make much difference, but in a normal Kentucky summer when it doesn’t quite rain enough and we get short dry spells, the timing of flowering and pod set and seed filling can be critical.
We can control time in some of our management decisions. When to plant and what variety to use are two decisions faced by all producers. Planting in May or early June will usually give you about the same yield. Things change about June 10 to 15 in Kentucky and yields begin to decline (~ 1.5 % per day, on average) as planting is delayed. Paying attention to when you plant can increase soybean yield without increasing input costs. Double-cropping soybean after wheat forces us to plant late and that’s why yields are often lower.
Variety maturity and planting date determine when reproductive growth occurs during the summer. Approximate dates for growth stages R1, R5 and R7 from May and late June plantings at Lexington, KY are shown in Table 1. If we could get a perfect weather forecast for the entire summer we would know what variety to plant and when to plant it to match flowering, pod set and seed filling with good weather to insure high yields. Such a forecast may be available in the future, but for now we can only play the averages that say that generally the yield of maturity group III is roughly 10% less than group IV varieties in Kentucky while maturity group V is, on the average, 10 to 15% less than group IV. Any advantage for maturity group II varieties is probably pretty small and they may yield less in some years. There can be, however, lots of variation among maturity groups in any year, depending on which one gets the most rain during the critical reproductive growth phases. Growing varieties from several maturity groups will usually give more consistent yields from year to year since a short dry spell will probably not hit the critical phases of all of them.
Time is important in soybean production. What the plant is doing and when it is doing it has something to do with yield. Thinking about time and the three phases of yield production can help you manage your soybean crop and make it more profitable. Remember, as Charles F. Kettering once said “Thinking is one thing no one has ever been able to tax”.
Table 1. Planting date and maturity group effects on timing of reproductive development, Lexington, KY. Adapted from Egli, D.B. and W.P. Bruening. 2000. Potential of Early-Maturing Soybean Cultivars in Late Plantings. Agron. J. 92: 532-537.
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Early soybeans in Kentucky Chad Lee, Dennis B. Egli and John H. Grove, University of Kentucky
In Kentucky, Maturity Group IV (also referred to as ‘group 4’) soybean varieties typically yield more than soybean varieties in other maturity groups. Early group 5 soybean varieties in the southern and western part of Kentucky and late group 3 varieties in central and northern Kentucky also typically yield well.
Group 4 soybean varieties planted in May will be filling seed during August, which is typically a dry month in Kentucky. If water becomes limiting during seed filling, yields will be reduced. Shallow soils are more prone to water-limiting conditions than deep soils because shallow soils hold less water. Group 2 varieties may be a very good fit on shallow soils by going through seed filling early enough to avoid the typically dry August period. So, on these drought-prone soils, group 2 varieties may actually yield the same as, or more than, group 4 varieties.
Studies were conducted in central Kentucky for the past three years to determine if group 2 varieties could yield as well as group 4 varieties. The varieties were planted across several landscape positions. These positions had different soil depths, which created differences in drought potential. The backslopes and shoulders have shallower soils than other slopes and are more prone to drought. The studies did not experience any extended dry period in 2003 and 2004. In 2005, the only month with surplus precipitation was August! The past three years did not follow the long-term weather trends, having a dry August, so we could not test our hypothesis regarding group 2 varieties on shallow soils.
Field landscape position did not significantly affect yield in any of the three years, but there were significant differences in yield between varieties from different maturity groups. Group 4 varieties yielded about 10% more than group 3 and 20% more than group 2 varieties (Table 1). Group 4 varieties yielded about 10% more than group 5 varieties the first two years, but group 5 varieties yielded about 12% more than group 4 soybeans in 2005. The yield advantage of the group 5 varieties in 2005 was likely due to the late season rainfall in an otherwise dry year. The earlier-maturing varieties were too mature to benefit from the late-season rainfall.
Differences in yield between varieties from different maturity groups were not as great in the Kentucky Soybean Performance Test. The three highest yields from each maturity group were averaged and compared to the three highest yields from the other maturity groups. Group 4 varieties averaged about 3% and 5% more yield than group 3 and group 5 varieties, respectively. Year-to-year differences in yield fluctuated some. For example, group 4 varieties yielded more than group 5 varieties in every year but 2005. Again, late-season rainfall benefited the group 5 varieties.
The expectation that group 2 soybean varieties will out-yield group 4 varieties on shallow soils in some years is speculation at this point. However, farmers considering group 2 varieties for their early harvest and market advantages should try to plant group 2 varieties on drought-prone soils when possible. If you try group 2 varieties, it may be a good idea to have no more than 10 to 20% of your total soybean acreage in group 2 soybeans. Also, group 2 varieties should be planted at a full season date (April 15 to May 15) to maximize growth. Late plantings of group 2 varieties will cause the plants to be too short and will reduce yield. One other concern with group 2 varieties is the difficulty in identifying high-yielding varieties for Kentucky. Group 2 varieties have not been fully tested in the Kentucky Soybean Performance Test.
Group 2 varieties may be a useful component to an overall soybean production operation; however, the current research indicates lost yield potential with group 2 varieties. Producers are advised to weigh the disadvantages of reduced yield of group 2 varieties against the benefits of time management and market premiums for an early harvest.
This research was funded, in part, by the Kentucky Soybean Promotion Board.
Table 1. Average yields of two soybean varieties from each maturity group across landscape near Lexington, KY.
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