Management Tips for Continuous Soybean Production
Carrie Knott, Carl Bradley, and Raul Villanueva
There is a lot of interest this year in second year and even third or more year soybean. If you are thinking about growing continuous soybean this year there are several management decisions you should consider.
First and foremost, it is important to know what the soybean cyst nematode (SCN) populations are in your fields. Soybean cyst nematode is often referred to as a “silent yield thief” because SCN generally does not cause obvious symptoms on modern varieties, and soybean yields can be reduced by as much as 30% without visual symptoms. Therefore, before considering continuous soybean, it is essential to test fields for SCN, even if an SCN problem is not expected. Fall is the best time to test, but if necessary spring sampling can also be useful.
Many fields in the western part of the state are likely to be infested with SCN, but some fields may not be. For fields with no detectable levels of SCN, management of this pathogen is not necessary; however, it is important to continue monitoring, as SCN populations will grow under continuous soybean. For fields with SCN, it is important to know the actual population (number of SCN eggs per ½ pint of soil). Fields with relatively low SCN egg populations (less than 500 eggs per ½ pint of soil) may not be affected greatly by SCN and could be potential fields for back-to-back soybean production. However, it is important to realize that SCN populations will increase in continuous soybean, and that SCN egg levels could soon grow to over 500 eggs per ½ pint of soil. Planting soybean varieties with resistance to SCN is one way to help manage SCN egg populations. Most SCN-resistant soybean varieties rely on a single source of resistance, known as the “PI 88788” source of resistance, and unfortunately, there are HG types (historically known as races) in Kentucky that can reproduce fairly well on varieties that utilize this source of resistance. These concerns raise a caution flag in regards to continuous soybean production, and heighten the importance of monitoring SCN egg populations in these fields every year.
In addition to SCN, most other soybean pathogens will increase in population under continuous soybean production. Variety selection can help minimize potential yield reductions caused by diseases that these pathogens incite. For Kentucky, a couple of these important diseases are frogeye leaf spot (caused by Cercospora sojina) and southern stem canker (caused by Diaporthe aspalathi). Selecting varieties with high levels of resistance to these diseases and SCN (as described above) is essential in minimizing yield reductions under a continuous soybean production scenario.
It will also be important to consider the expected yield loss of continuous soybean and to increase seeding rates and use seed treatments. It is well established, at local and national levels, that a 5-10% yield loss is expected for continuous soybean. In Kentucky, research conducted by Dr. John Grove at Spindletop Research Farm in Lexington from 2008 to 2016 indicates that a 5% yield loss occurs for second year soybean. Depending on the field, a 5% yield loss may be a significant penalty, which results in unprofitable soybean production.
Seeding rates should be increased in continuous soybean production. The University of Kentucky (and most other states) recommend a harvest population of 100,000 plants per acre for maximum yield and profitability for a corn-soybean rotation. Typically with a 90% seed germination rate and assuming a 10% stand loss for no-till, that will result in about 120,000 seeds per acre. In continuous soybean production, greater seeding rates are needed to establish adequate plant populations, typically due to pathogens. It is best to assume a 25-30% stand loss. This will give you a seeding rate of about 160,000, with a 90% seed germination rate. Seed treatments will also be important to help with seed germination and seedling establishment. Fungicide seed treatments, in particular, will help protect seeds and seedlings against pathogens that cause seedling diseases (i.e. Pythium and Fusarium seedling blights). Make sure to use high quality fungicide and insecticide seed treatments at the highest recommended rates for effective control.
Ensuring adequate soil fertility is also a key component to a profitable continuous soybean production system. It is not uncommon to drive throughout Kentucky and see potash deficiency from the road. The cause of that “windshield” diagnosis cannot be determined sitting a vehicle. However, according to Dr. Edwin Ritchey, Extension Soil Specialist, the majority of the soybean samples submitted to the Plant Disease Diagnostic Lab in Princeton have very low soil test potassium levels. There is also new research out of Wisconsin indicating that an 80 bu per acre soybean crop has a total P2O5 uptake of more than 70 lbs per acre and K2O uptake of more than 200 lbs per acre in one growing season. A 60 bu per acre soybean crop was found to uptake more than 60 lbs P2O5 per acre and more than 160 lbs lbs K2O per acre. However, only a percentage of that is removed from the field in the seed. This new research found that about 45 lbs P2O5 per acre and 70 lbs K2O per acre were removed in the seed for a 60 bu per acre soybean crop, while 60 lbs P2O5 per acre and 95 lbs K2O per acre were removed for an 80 bu per acre soybean crop. That is about 20 to 40 lbs K2O per acre more than a 150 bushel per acre corn crop and about 5 lbs K2O per acre more for the 80 bu per acre soybean yield than a 250 bushel per acre corn crop. Therefore, to ensure maximum yields in a continuous, or even a corn-soybean rotation, it is very important to ensure adequate fertility, particularly potash.
Diligent scouting will also be essential to monitor and manage pathogen and insect pest populations. For pathogens, applying fungicides once diseases are detected will be important until about the R3 (beginning pod) growth stage. It is important not to simply apply fungicides according to a calendar in the absence of disease problems, because this can lead to pathogen populations resistant to fungicides. For insects, foliar insecticide application should be made once economic thresholds are found.
For fields with a recent history of Dectes stem borer, do not plant into continuous soybean. The adult stage can emerge from June to August and females are oviposit (able to lay eggs) for at least 45 days. The adult Dectes make small holes in leaves and petioles however, the larval stage cause most of damage. After egg hatch, Dectes larva bores into the stem and moves to the lower parts of the plant. Soybean plants can be girdled and fall under heavy rains and wind. Chemical control is difficult to impossible; once the larvae enter the plant there are no control options. In addition, there are no known varieties resistant to Dectes stem borer. Therefore, crop rotation is the only way to control the Dectes stem borer. If you happen to have a problem with this pest, timely and even early harvesting have been suggested as strategies to mitigate yield loss.
a) Dectes stem borer larva in a green stem; (b) larva in bottom of plant; and (c) stem girdled by Dectes larva (photos: a, Yaziri Gonzalex; b and c, Doug Johnson).
The last consideration for continuous soybean is the cost. Below is an example of estimated additional costs for a continuous soybean rotation compared to soybean grown in a corn-soybean rotation.
Table 1. Estimated additional costs to produce continuous soybean in Kentucky for two yield levels: 60 and 80 bushels per acre. Foliar pesticide estimates were not included because of the wide range in application costs. However, these could be considered to determine the total additional cost for continuous soybean production compared to soybean produced in a crop rotation.
