Phyllis Bongard, Educational content development and communications specialist, Bruce Potter, Extension IPM specialist, and Melissa Wilson, Extension manure management specialist
Wet weather continues to impact the soybean crop in the state. Meanwhile, high fertilizer prices may affect how we think about manure management this late summer and fall. To address these issues and discuss management strategies, Bruce Potter, IPM specialist, and Dr. Melissa Wilson, Extension manure management specialist, joined moderator, Dave Nicolai, Extension educator-crops, in the July 24 session of Strategic Farming: Field Notes.
Wet soil can aggravate the yellowing and stunting caused by iron deficiency chlorosis (IDC), especially when soils are cold. However, symptoms related to IDC typically initially show up earlier, rather than later, in the season.
If soybeans are yellowing now, a couple of potential causes may be at play. One of them, soybean cyst nematode (SCN), is a good candidate for causing stunted and yellow soybeans and is assumed to be at some level in most, if not all fields, in southern Minnesota. Nematodes don’t mind wet soils and can even survive flooded conditions for several months, so hopes that they may have drowned will be dashed. Other potential causes include diseases like pod and stem blight and top dieback. Although these diseases frequently occur alongside SCN, they can also occur independently.
Because there are several potential causes for yellow soybeans, the trick is to identify the main reason for these issues now.
If you find cysts, that indicates that reproduction is occurring. If you suspect SCN, but don’t see any cysts, sample again in a couple of weeks or take a soil sample. SCN has more than one generation a season and it’s possible that the eggs or larvae are inside of the root. If you take soil samples, include edges of a heavily infested area where there are still enough roots to support the nematode.
Taking action
Take samples at the same time and same areas when soil sampling to help determine the success of your SCN management. When you see a lot of SCN on a PI88788 resistant variety or even on a Peking variety, that’s an indication that management strategies should change. When this occurs, either the variety doesn’t have good resistance or the nematodes have evolved to be able to reproduce on it, or some of each. At minimum, make sure you plant genetically different varieties of a particular resistance source in consecutive soybean crops. Including more than one source of resistance, such as including a Peking variety occasionally, is especially important when resistant nematodes are found. It is prudent to include an extra year(s) of a non-host such as corn , alfalfa, or small grains, when SCN populations are very high. For more information, visit Soybean cyst nematode management guide.
Overall, when there is ample soil moisture, disease and SCN symptoms associated with stressed root systems will be less pronounced since water is readily available. However, if conditions become hot and dry, symptoms that had been masked may become more obvious.
It’s also time to start evaluating soybean aphid (SBA) populations. As the soybeans go through the R3 and R4 stages, SBA populations are expected to ramp up quickly. The threshold for treatment is still 250 aphids per plant on 80% of the plants in the field. There is no economic nor agronomic reason to treat aphids if the threshold has not been reached. See Scouting for soybean aphid for more information.
Commercial fertilizer is usually in the ammonium or nitrate form already – or in the urea form which quickly converts to ammonium. The amounts of ammonium and organic N in manure will vary depending on the type of livestock and how it was stored and applied. For example, liquid swine manure may have 70-80% of its nitrogen in the ammonium form, so it performs more like a fertilizer. In contrast, liquid beef manure may contain 50% ammonium and 50% organic nitrogen (N), making it a slower-release N source.
Manure should be injected or incorporated to reduce N losses. The ammonium in surface-applied manure can quickly volatilize and be lost as ammonia gas. If you can smell manure after its application, nutrients are being lost.
Manure analysis should be provided for any purchased manure, as well. If it is custom-applied, application records should include rates and exact locations.
N-based vs. P-based rates
Manure application rates based on phosphorus needs will be lower than those for N needs. In this scenario, supplemental fertilizer N will be needed. On the other hand, if manure is applied to satisfy N needs, phosphorus will likely be over-applied. In either case, consider what you want to do for your fertility plan and then fine-tune that.
Thanks to the Soybean Research and Promotion Council and the Corn Research and Promotion Council for their generous support of this program.
 |
| SCN-resistant soybean on left and SCN- susceptible soybean on right. |
Got yellow soybeans?
Soybeans have been under a great deal of water stress this season, leading to a high degree of in-field variability and increased vulnerability to disease. While some soybeans purely drowned, other hidden causes may be at work on this season’s crop.The potential culprits
Excessive moisture and flooding have caused an unusual amount of soybean root disease this year. Pathogens like Pythium, Phytopthora, and Rhizoctonia can be responsible for dead or missing plants, stunting, and yellowing. As hot and dry weather resumes, these diseases should slow down.Wet soil can aggravate the yellowing and stunting caused by iron deficiency chlorosis (IDC), especially when soils are cold. However, symptoms related to IDC typically initially show up earlier, rather than later, in the season.
If soybeans are yellowing now, a couple of potential causes may be at play. One of them, soybean cyst nematode (SCN), is a good candidate for causing stunted and yellow soybeans and is assumed to be at some level in most, if not all fields, in southern Minnesota. Nematodes don’t mind wet soils and can even survive flooded conditions for several months, so hopes that they may have drowned will be dashed. Other potential causes include diseases like pod and stem blight and top dieback. Although these diseases frequently occur alongside SCN, they can also occur independently.
Because there are several potential causes for yellow soybeans, the trick is to identify the main reason for these issues now.
Differentiating between the culprits
To help differentiate between the potential causes of soybean yellowing, it’s time to take your good hand lens out to the field. If you suspect SCN and have stunted or stunted and yellow plants, dig or pull up some roots so that you get a good chunk of the root system to examine. Look for the female SCN cysts on the roots. Compared to the larger and more irregularly shaped N-fixing nodules, SCN cysts are small, smooth, and lemon-shaped.If you find cysts, that indicates that reproduction is occurring. If you suspect SCN, but don’t see any cysts, sample again in a couple of weeks or take a soil sample. SCN has more than one generation a season and it’s possible that the eggs or larvae are inside of the root. If you take soil samples, include edges of a heavily infested area where there are still enough roots to support the nematode.
Taking action
Take samples at the same time and same areas when soil sampling to help determine the success of your SCN management. When you see a lot of SCN on a PI88788 resistant variety or even on a Peking variety, that’s an indication that management strategies should change. When this occurs, either the variety doesn’t have good resistance or the nematodes have evolved to be able to reproduce on it, or some of each. At minimum, make sure you plant genetically different varieties of a particular resistance source in consecutive soybean crops. Including more than one source of resistance, such as including a Peking variety occasionally, is especially important when resistant nematodes are found. It is prudent to include an extra year(s) of a non-host such as corn , alfalfa, or small grains, when SCN populations are very high. For more information, visit Soybean cyst nematode management guide.
What else to watch for
Sudden death syndrome infection occurs early in the growing season and is favored by cool, wet conditions. Since conditions were favorable for this disease, foliar symptoms may start showing up in late July – early August if it becomes hot and dry. White mold may also be favored this year.Overall, when there is ample soil moisture, disease and SCN symptoms associated with stressed root systems will be less pronounced since water is readily available. However, if conditions become hot and dry, symptoms that had been masked may become more obvious.
It’s also time to start evaluating soybean aphid (SBA) populations. As the soybeans go through the R3 and R4 stages, SBA populations are expected to ramp up quickly. The threshold for treatment is still 250 aphids per plant on 80% of the plants in the field. There is no economic nor agronomic reason to treat aphids if the threshold has not been reached. See Scouting for soybean aphid for more information.
Looking ahead in manure management
Last fall’s manure applications were followed by an unusually warm winter. As a result, the corn crop is also showing a lot of in-field variability and yellowing in fields, particularly in low spots. Much of that is due to nitrogen loss, regardless of whether it was from a commercial fertilizer or manure source. Between ample soil moisture and warm conditions, the soil microbes responsible for converting nitrogen into its different forms have been ramped up into overdrive. Ammonium can be converted into nitrate rapidly – in just days or weeks – and become susceptible to leaching. Nitrate is also vulnerable to denitrification, a process that converts nitrate to nitrogen gas under saturated soil conditions, situations that are common in low spots.Commercial fertilizer is usually in the ammonium or nitrate form already – or in the urea form which quickly converts to ammonium. The amounts of ammonium and organic N in manure will vary depending on the type of livestock and how it was stored and applied. For example, liquid swine manure may have 70-80% of its nitrogen in the ammonium form, so it performs more like a fertilizer. In contrast, liquid beef manure may contain 50% ammonium and 50% organic nitrogen (N), making it a slower-release N source.
Performance of fall-applied manure
In comparisons between fall applied manure and spring applied fertilizer in Waseca, corn yields were higher in the manure treatments for the first two years of the study. However, those results were flipped during the past two years. The nitrogen in the fall applied manure had converted to nitrate and then was lost in the wet spring. Fall applied manure can be at a disadvantage when conditions – such as warm winters and significant spring rains - favor this conversion to nitrate.Fall application considerations
To avoid N losses, fall applications of manure should be delayed until soil temperatures are cool. Ideally, that soil temperature is 50F or less, because that’s when microbial activity and subsequent nitrogen conversions slow down. Applications in August and September after small grains or a canning crop run the risk of N loss, even with a nitrification inhibitor, because soil temperatures are still quite warm. One possible early application scenario would include seeding a cover, like hybrid rye, that could take advantage of the nitrogen that might otherwise be lost.Manure should be injected or incorporated to reduce N losses. The ammonium in surface-applied manure can quickly volatilize and be lost as ammonia gas. If you can smell manure after its application, nutrients are being lost.
Get manure tested
Manure nutrient content can be quite variable, so getting it tested is critical. When the barn and feed are consistent, testing the nutrient content every couple of years after getting a three-year baseline would be adequate. However, if feeding or watering practices change, then testing annually may be more appropriate.Manure analysis should be provided for any purchased manure, as well. If it is custom-applied, application records should include rates and exact locations.
N-based vs. P-based rates
Manure application rates based on phosphorus needs will be lower than those for N needs. In this scenario, supplemental fertilizer N will be needed. On the other hand, if manure is applied to satisfy N needs, phosphorus will likely be over-applied. In either case, consider what you want to do for your fertility plan and then fine-tune that.
Thanks to the Soybean Research and Promotion Council and the Corn Research and Promotion Council for their generous support of this program.
Comments
Post a Comment