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SCN Affirmed as a Suspect in Soybean Monocrop Yield Decline

With soybean production in the US, two accepted tenets are 1) soybean cyst nematode (SCN) is one of the most damaging pests of soybean, and 2) a corn-soybean rotation will increase yields of both crops as a result of the “rotation effect”.

It is recognized that the most effective and economical management practices for SCN control in soybean production are: 1) planting seed of varieties with resistance to the particular race or type of nematode that infests a field; 2) rotating with non-host crops such as corn, cotton, and grain sorghum; and 3) rotating soybean varieties with differing genetic sources of resistance. These management practices must be adopted in fields infested with SCN since the pest cannot be effectively removed from those sites.

On a long-term corn-soybean rotation study site that was established in Minnesota in 1982, a companion study was established in 2010 [Agron. J. Vol. 108(3):2016] to determine the role of plant-parasitic nematodes–primarily SCN–on yield of soybean rotated with corn. The study included treatments of 1 to 5 years of SCN-susceptible soybean monofrop following 5 years of corn, continuous SCN-susceptible soybean monocrop, and continuous soybean with an SCN-resistant variety. Granular nematicides (terbufos in 2010-2011, aldicarb in 2012-2014) were applied in-furrow to one-half of each treatment at the start of the 2010 study. Only results from 2012-2014 are reported since terbufos had only limited effects on nematode populations whereas aldicarb effectively suppressed nematode populations.

A summary of the findings from this study follow.

    Across years and crop sequences, soybean yield from nematicide-treated plots average 2.8 bu/acre (37.5 vs. 34.7 bu/acre) more than yield from plots not treated with nematicides. However, this significant yield difference cannot be solely attributable to nematicide since aldicarb also affects insects.

    The nematicide resulted in fewer SCN eggs across years and crops sequences.

    Averaged across nematicide treatments and crop sequence, SCN egg numbers 1) were equally low (<400 eggs/cm3 soil in the fall) in SCN-resistant continuous soybean and first year SCN-susceptible soybean, 2) increased significantly from first- through fourth-year continuous SCN-susceptible soybean, and 3) were at levels considered to represent a moderate damage threat by the third year of SCN-susceptible soybean.

    The results suggest that alleviating SCN damage to soybean contributed to the benefit from rotating corn with soybean.

    Decreased SCN populations under a continuous monocrop of an SCN-resistant variety shows this to be an effective SCN-management strategy, even after 5 years of monocrop. Keep in mind that the development of resistance-breaking SCN populations can occur if varieties with the same source of SCN resistance are grown in a continuous monocrop.

    In another companion study [Agron. J. 108(2):2016], number of SCN eggs in the fall had decreased below threat levels by the second year of corn in a corn-soybean rotation.

These results confirm the following tenets regarding SCN management in soybean production.

    Nematicides will reduce SCN populations and result in small soybean yield increases, but the yield increase should be considered with economics in mind.

    SCN populations can increase rapidly when growing SCN-susceptible soybean varieties in monocrop, thus creating SCN management problems in this system.

    Growing an SCN-susceptible soybean variety for more than one year following corn is not advisable.

    The results from this study, coupled with results from previous studies, strongly suggest that yield loss from SCN increases as the time in soybean monocrop increases.

    A corn-soybean rotation is an effective management tool for alleviating the SCN threat to soybean.

    These results, coupled with those from previous studies, suggest that a rotation of two years corn/1 year soybean is an acceptable cropping system for sites with known populations of SCN.

Composed by Larry G. Heatherly, Nov. 2016,