C.O.R.N. Newsletter 2005-26

Dates Covered: 
August 15, 2005 - August 23, 2005
Harold Watters

Late Spraying for Soybean Aphid

Authors: Ron Hammond, Bruce Eisley

This past week we receive numerous phone calls about how late in the summer should soybeans be protected from soybean aphids. The following guidelines are a general consensus on our thinking. As long as the soybeans are in growth stage R4 and early R5 which is the situation with many late planted fields, we believe that the threshold should remain at 250 aphids per plant with an INCREASING population. Nothing has changed. However, as the plants reach late R5 and R6 when the seeds are reaching full size, the decision to treat should change. It will probably take a lot more aphids to cause a concern, and again, only if the population is still rising. Also, there is more of a concern if the plants are under other stresses, such as drought stress. As the soybeans enter the R6 stage, we would suggest spraying only if the aphid populations are between 500-1000 aphids per plant and STILL rising, and the soybeans are not growing well because of other stresses. If the plants appear to be healthy and well growing at this stage, it is questionable if a treatment would be worthwhile unless the aphid population was reaching extremely high levels. As the soybeans reach the late R6 and R7 stages, and begin to mature, spraying is not recommended.

Another question has been what are those large numbers of small, whitish aphids throughout the entire plant canopy? These are soybean aphids! At this time, we are not sure why these newly laid aphids are different in appearance. It is perhaps because of environmental conditions or plant nutrition; we are not sure. However, these small white aphids were also observed in 2003 in later summer, so it is nothing new. And they are soybean aphids, contrary to some rumors. At this time, we believe that they should be included in your counts for the threshold. But as noted above and numerous other times, the population should still be on the rise. There is some evidence that predators are keeping these small aphids at lower or at least, constant population levels.

In summary, with these late growth stage soybeans and mostly small white aphids throughout the plant, care should be taken before a decision to treat is made.

Soybean Pod Feeding Insects

Authors: Ron Hammond, Bruce Eisley

With all the concern over soybean aphids, we should not forget that there is one last insect concern on the immediate horizon for soybean growers. Growers need to remember that grasshoppers and second generation bean leaf beetle adults will become a concern from now until harvest when feeding on pods. Both can cause significant yield loss when feeding on pods as well as opening the pods up to entrance for secondary pathogens. Although the following information is more for the bean leaf beetle, much of it also pertains to pod feeding by grasshoppers.

When pod injury occurs on 10-15% of pods, seed injury will become evident and yield losses are possible. If the pod injury occurs during periods of wet conditions this may enable infection by seed diseases, the development of moldy bean seeds may then lead to a loss in seed quality. The assessment of a field infestation depends on (1) determination of the current level of pod injury, (2) the abundance of adult BLB activity using a sweep net, (3) consideration of weather factors that may enable infection of the damaged pods by disease agents, and (4) the amount of time remaining before total leaf drop and dispersal of a BLB population from the field. In terms of this last point, many early-planted fields are entering the full pod stage, and should be maturing in the next few weeks. Although growers should continue to monitor these fields, these soybeans should mature before much pod feeding occurs. Later planted fields, however, will stay green with succulent pods well into September, and thus, growers are advised to pay close attention to their late maturing fields.

Growers should continue to sample their fields weekly for the remainder of the summer. The percentage pod injury should be determined by randomly counting the total number of pods and those with feeding injury on about twenty plants (see http://ohioline.osu.edu/icm-fact/images/55.html). Adult BLB population should be estimated by taking 10 sweeps with a sweep net at three to four locations in a field.

Rescue treatment to prevent excessive development of seed damage may be warranted when pod injury exceeds 10%, fresh feeding scars predominate, and adult BLB are still present and actively feeding (or as it pertains to grasshoppers). Less than two BLB per sweep (20 per 10 sweep sample) are unlikely to cause significant injury. Three to five BLB per sweep (30 to 50 per 10 sweep sample) indicates a potential problem possibly warranting rescue treatment. More than five BLB per sweep (50 per 10 sweep sample) may result in significant injury especially if two or more weeks remain until leaf drop. When the foliage dries and drops, beetles exit the field. Thus, the time remaining for BLB feeding is a key factor in the occurrence of pod injury. Growers also should be aware of the harvest interval with the insecticide that is used for BLB control. Many of the materials have waiting periods of between 45 and 60 days, which would limit their use at this time.

Soybean Rust Update August 15

Authors: Anne Dorrance

As of Sunday, only six commercial soybean fields in total scattered through Florida, Georgia and Alabama have been identified with soybean rust. Both mobile search teams and sentinel plots have been very effective in monitoring the slow development of soybean rust this season. What does this mean for Ohio? - for full season and most likely the double crop, we do not have to worry about soybean rust for the remainder of 2005.

What will happen for the remainder of the year? We will continue to monitor our sentinel plots, and new sites will be located in areas of the state where double crop soybeans are doing well. The same will occur in the states to the south of us, what we can expect now – is to see how many and how much kudzu becomes infected in the south and much inoculum builds-up through the winter. This past year was a good example of some basic plant pathology principles, focusing primarily on the disease triangle. Where we have a susceptible plant, environment was poor, and only a smidgen of inoculum was just not enough to get this started to affect the northern states.

Sampling for Western Corn Rootworm in Soybeans

Authors: Bruce Eisley, Ron Hammond

Some of you are monitoring soybean field for western corn rootworm (WCR) beetles with the use of yellow sticky traps. This is the preferred method for monitoring but if you did not get traps in your fields, the use of a sweep net at this time will give some indication of the WCR population in the fields. We have sampled fields in western Ohio the past two years to help detect where and how large the WCR populations are in soybeans. The highest number we have ever found in a field is 52 beetles per 100 sweeps. This compares with 800 or 900 beetles per 100 sweeps that states west of Ohio find when they sweep in areas that have high populations of WCR in soybeans. There isn’t an established threshold to use regarding beetles per sweep but the sweep net will give an indication of fields with high populations.

The method we have used in the past to sample for WCR is to take 20 sweeps at a time and then either bag the sweeps in plastic bags (which is the method we use) or count the beetles as they come out of the net. We then move to another area of the field and take an additional 20 sweeps until we have repeated this five times for a total of 100 sweeps per field. As we mentioned earlier, there isn’t a threshold at this time to use for beetles per sweep or beetles per field, but large numbers would indicate a potential threat if the field were planted to corn next year.

Early reports from some of the counties that are monitoring fields with sticky traps indicate that the WCR adult numbers on the traps are low again this year and so if those soybean fields go to corn in 2006 they will probably not need a treatment for rootworm larval control. A complete report from the trapping program this year will be included in an article in CORN later this fall.

Harvesting Soybeans for Forage

Authors: Bill Weiss, Mark Sulc

The drought conditions this summer have led some producers to consider using their soybean crop for forage. Whole plant soybeans can be excellent forage for cattle. Soybeans with little pod development can have about 18% crude protein and 40 to 45% neutral detergent fiber. In other words, whole soybean plants are similar to good quality alfalfa in nutritional value. Good soybean silage can be made under the following conditions:

1. Check the pesticide labels of chemicals used to verify that the crop can be fed to livestock after their application, and observe any pre-harvest intervals. For example, for glyphosate-resistant soybeans with Weathermax Roundup, wait 14 days between the final Roundup application harvesting the soybeans for grain, forage, or hay fed to livestock.

2. Under normal conditions, the optimum time to harvest soybeans for forage for optimal yield and quality is when seeds are filling the pods and the lower leaves are just beginning to turn yellow. At this stage, soybeans will yield from 2 to 3 tons of dry matter per acre (lower yields can be expected if planted late) with a composition of 17-18% CP, 0.56 Mcal/lb NEL (55% TDN), 45% NDF, and 34% ADF. If seeds are present and are reasonably well developed, soybeans should be mixed with corn in the silo to achieve acceptable fermentation (high oil content of pure soybean silage with good seed set will prevent good fermentation). Mix one part soybean with two or more parts of corn in the silo. If soybeans are to be ensiled alone, they must be harvested before seeds are formed.

3. Under dry conditions when seeds did not develop or are small, soybeans can be ensiled alone, provided the dry matter content is in the right range (see item 4 below).

4. Prior to chopping soybeans for silage, check the forage dry matter content (use a microwave oven or Koster tester). If the plant contains less than 45% dry matter (at least 55% moisture), the plants should ferment fine in upright and bag silos and will usually ferment OK in bunker silos. If the plant contains more than 50% dry matter (less than 50% moisture), fermentation quality will probably be poor in bunker silos. When dry matter is much higher than 55%, fermentation will be poor in all types of silos except sealed structures. If plants do not contain enough moisture for fermentation, water can be added but it takes a lot of water to change the dry matter content. To increase the moisture content of 1 ton of material by 4 percentage units (for example 55 to 51% dry matter), you have to add about 190 lbs of water (24 gallons per ton).

5. Whole plant soybeans can be harvested for hay but generally the nutritional value is much lower than soybean silage. Leaf shatter is severe when soybeans are baled so protein is much lower and fiber much higher in soybean hay.

6. Because excess fat from soybeans can depress fermentation in the rumen, the maximum amount of soybean forage that can be fed should be based on its fat concentration. If seeds are well developed the forage will have about 10 or 11% fat, and the maximum concentration of soybean forage to include in the diet is 20 to 25% of the dry matter fed. If plants have little pod development, soybean forage can comprise the same proportion of a diet as can alfalfa. If soybeans and corn are mixed, fat concentrations are usually around 5% and rumen problems caused by fat are not expected to occur.

Nitrogen Credits for Wheat after Alfalfa

Authors: Robert Mullen

As the price of nitrogen increases, alternative methods of providing nitrogen (N) to our cereal crops become an attractive alternative. So how much N credit should be claimed if you are terminating an alfalfa stand this summer and planting wheat this fall?

Current recommendations do not provide an N credit for wheat following alfalfa, but perhaps N recommendations for spring application should be adjusted. While it is true that early spring temperatures are not conducive to mineralization of the N from the previous alfalfa crop, later spring temperatures are and the likelihood of running short of N is small but real. The N uptake pattern of wheat is such that early spring demand is small, but increases rapidly as we go through the first node stage (Feekes 6). Fortunately, the increase in crop demand for N coincides with an increase in soil temperature which leads to mineralization of N. Thus the need for N fertilization should decrease when wheat follows alfalfa.

Data collected in Wisconsin in the late 90s and early 00s showed that winter wheat responses to spring applied N was significantly lower (if responsive at all) when planted after alfalfa (even when the stand was quite thin). In fact on average application of N resulted in decreased yield due to increased lodging. The response to N was affected by timing of alfalfa termination, and alfalfa terminated earlier in the summer resulted in more N responsive wheat (at least on year). Wheat that followed incorporation of alfalfa by tillage showed less response (or a complete lack of response) compared to wheat that was seeded no-till after alfalfa termination (at least one year).

If planting wheat after a recently terminated alfalfa stand, go ahead and apply your fall N (20-30 lb N/acre) and adjust the spring application to take some credit. You can at least decrease the amount of N applied by half (based on Tri-State Fertilizer Recommendations) with little risk of being short of N for the crop (based on Wisconsin data). If you are little more or a risk taker you can decrease your nitrogen application rate even more.

Wheat Performance Trials Now Available

Authors: Jim Beuerlein

The 2005 Ohio Wheat Performance trial results are on the web at http://www.ag.ohio-state.edu/~perf and also available on the Agronomic Crops Team website: https://agcrops.osu.edu.

The purpose of the Ohio Wheat Performance Trial is to evaluate wheat varieties, blends, brands, and breeding lines for yield, grain quality and other important performance characteristics. This information gives wheat producers comparative information for selecting the varieties best suited for their production system and market. Varieties differ in yield potential, winter hardiness, maturity, standability, disease and insect resistance, and other agronomic characteristics.

Each entry was evaluated at five test sites. Field and weather conditions were favorable for timely planting in October, 2004. Fall growth was marginally adequate except in Wood County which was very dry with reduced fall growth and tillering before the onset of winter dormancy. Winter survival was good with almost no winterkill at any of the test sites. The weather in April and May was slightly warmer than normal and June was 4 degrees above normal. In Wayne County, the plots headed 4 days earlier than the other test sites in northern Ohio and also had a longer grain filling resulting in increased yield relative to other test sites. Dry weather and higher temperatures in late June shortened the grain fill period in Wood county resulting in reduced grain yields. There was very little disease at most of the test sites which resulted in significantly increased yield at most sites.

Depending on variety and test site, yields were between 54.9 and 130.0 bushels per acre, and average test weight ranged from 56.8 to 61.5 pounds per bushel. Yield differences between test sites are primarily due to the length of the grain fill period. The average heading date was six days later than in 2003 and 3 days later than normal. Average plant height was four inches greater than in 2004. Variety selection should be based on disease resistance, average yield across test sites and years (tables 2 & 3), winter hardiness, test weight and standability.

Archive Issue Contributors: 

Anne Dorrance, Dennis Mills and Pierce Paul (Plant Pathology), Ron Hammond and Bruce Eisley (Entomology), Peter Thomison (Corn Production), Jim Beuerlein (Soybean & Small Grains), Robert Mullen (Soil Fertility), Mark Sulc and Bill Weiss (Forage Production) and Jeff Stachler (Weed Science). Extension Agents: Keith Diedrick (Wayne), Howard Seigrist (Licking), Bruce Clevenger (Defiance), Glenn Arnold (Putman), Mark Koenig (Sandusky), Greg LaBarge (Fulton), Steve Bartels (Butler), Roger Bender (Shelby), Harold Watters (Champaign), Steve Foster (Darke), Ed Lentz (Seneca) and Steve Prochaska (Crawford).

About the C.O.R.N. Newsletter

C.O.R.N. Newsletter is a summary of crop observations, related information, and appropriate recommendations for Ohio crop producers and industry. C.O.R.N. Newsletter is produced by the Ohio State University Extension Agronomy Team, state specialists at The Ohio State University and the Ohio Agricultural Research and Development Center (OARDC). C.O.R.N. Newsletter questions are directed to Extension and OARDC state specialists and associates at Ohio State.