Strategic Farming: Let’s talk crops! session talks short-statured corn and soybean cyst nematode resistance breeding

By: Angie Peltier, UMN Extension crops educator

On February 1, 2023, Aaron Lorenz, Ph.D., UMN soybean breeder and geneticist and Rex Bernardo, Ph.D., UMN professor, endowed chair and corn geneticist, joined UMN Extension crops educator Lizabeth Stahl for a conversation about short-statured corn and SCN-resistant soybean breeding efforts. This was the fourth episode of the 2023 Strategic Farming: Let’s talk crops! webinars in this series.

 

Alternatively, watch the episode here: https://youtu.be/_R9Hbb7M5lA

Short-statured corn

While known by different names such as dwarf, semi-dwarf, or short corn, short-statured corn is corn that does not grow as tall as the typical Midwestern field corn hybrid. The concept of breeding shorter crops to combat production challenges has been around for long time; for example, short-statured rice supports adding additional nitrogen to improve grain without increasing lodging potential. While research on short-statured corn began in in the 1960’s on corn destined for dairy cows, Bernardo’s interest and research in short-statured corn began in the early 2000’s. He and a former student, Chris Schaeffer, Ph.D., were interested in investigating whether corn could be grown similar to a small grains crop and researched a 62 day, open-pollinated corn variety and found that on average the corn grew 50 inches tall and set ears approximately 16 inches from the ground when grown in 12 inch rows at 71,000 plants per acre. As expected from open-pollinated varieties, the dwarf corn studied at UMN had a much lower yield than commercial corn hybrids.

Bayer and Stine seed companies have also been breeding short-statured corn in an effort to increase resistance to lodging and green-snap and increase the period of time that inputs can be applied using ground-based equipment, with release expected in a few years. Bayer’s short-statured hybrids were ‘put to the test’ during the historic derecho (straight-line high speed winds) that downed so many acres of hybrid corn throughout Iowa a couple of years back – with more of the short-statured corn standing after the derecho moved through than full-sized hybrid corn. Bayer’s effort relies on mutations that reduce height from 8-9 feet to 6-7 feet with ears set approximately 2 feet from the soil. Stine’s short hybrids grow about 6 feet tall with ears set approximately 40 inches from the ground, similar to the ear height of current full-statured dent corn hybrids.

Soybean cyst nematode (SCN)

Soybean cyst nematode (SCN, Heterodera glycines) is a microscopic worm that parasitizes soybean roots and is the top yield-limiting pathogen of soybean in the US. SCN is thought to have been brought to this country in soil that had been collected and brought to the US from east Asia long before commercial Bradyrhizobium inoculants (a bacterium that lives in soil, forms a symbiotic relationship with soybean roots and converts atmospheric nitrogen into a form that the plant can use) were available. SCN moves any way that infested soil moves: with wind and water, on wildlife and machinery. SCN was first identified in the US in 1954 in North Carolina, but between the 1970’s, when it was first found in Minnesota’s Faribault County, and today SCN can now be found as far north as the Canadian border. SCN eggs are long-lived in the soil, because many are contained within the body of dead female SCN that have hardened to form survival structures. As a consequence, eggs can remain viable in soil for 10 years or longer.

SCN is capable of causing up to 30% yield loss without causing obvious above-ground symptoms. Although by no means specific only to SCN infection, symptoms of SCN infection include stunted, yellow plants that may never close rows. Symptoms caused by SCN infection tend to be most severe when the crop is undergoing drought stress and can lead to early maturing crops in severe infestations. Symptoms caused by the fungi that cause sudden death syndrome (SDS) and brown stem rot (BSR) can be more severe when the crop is also infected with SCN.

SCN management

SCN management involves periodic soil testing to determine 1) whether and to what extent a field is infested and 2) once a field is infected how well one is managing population densities. Rotating to a non-host crop can reduce population densities and growing SCN resistant varieties reduces the number of eggs that are produced when growing soybean when compared with growing an SCN susceptible variety. Several soybean lines resistant to SCN were identified decades ago in the USDA’s soybean germplasm collection, although only one source of resistance (called PI 88788) is present in the vast majority of SCN-resistant soybean varieties grown in the Midwestern U.S.. While six additional soybean lines were identified from the germplasm collection, PI 88788 proved very effective and was easiest to breed for as it is conferred by genes in one location on a single chromosome and these genes aren’t associated with regions of the genome creating yield drag as may be the case with some other sources.

However, relying on a single management tactic for decades will eventually cause pest populations to shift from one that is sensitive to and easily controlled with the tactic to one that is resistant to the tactic. While easier to see with shifts in weed populations over time from sensitivity to resistance to glyphosate in glyphosate-tolerant soybean fields, this same shift is taking place below-ground with SCN populations shifting from PI 88788 sensitivity to resistance after decades of growing soybean varieties with only the PI 88788 source of SCN resistance.

Rotating sources of SCN resistance

In an effort to forestall shifts from sensitive to resistance to a single source of varietal SCN resistance in a field’s SCN population, rotating among sources of SCN resistance is recommended. It can be difficult however, to find the Peking source of SCN resistance (the other source of resistance available commercially) in varieties available from all soybean seed companies or in soybeans of all maturities.

Due to severe reductions in soybean yield potential, soybean production is not recommended when SCN egg counts are greater than 10,000 eggs/100 cubic centimeters (cc) of soil; non-host crops are recommended in a field until SCN population densities drop below 10,000/100cc. The suggested rotation between 2,000 and 10,000 eggs is to alternate SCN-resistant soybean and non-host crops between years, but to also rotate between sources of SCN resistance, for example growing a variety with the PI 88788 source of resistance in year 1, growing corn or spring wheat in year 2, growing a soybean variety with the Peking source of resistance in year 3, and another non-host crop in year 4, etc. When SCN population densities drop below 200 eggs/100 cc, one can rotate with an SCN susceptible soybean variety, expecting an SCN population increase, but no yield in that year. This tactic may help slow the shift of SCN populations to those that are more virulent on resistant varieties.

In Dr. Lorenz’s breeding program, they are working with the Peking, PI 667516C and PI 90763 sources of SCN resistance. Their goal is to provide additional sources of resistance with which farmers can rotate to both maintain soybean yield potential and slow SCN population shifts toward resistance to an SCN source of resistance.

BASF scientists are developing Bt soybeans for SCN management. For those that aren’t familiar with Bt, it stands for a naturally occurring soil-borne bacteria called Bacillus thuringiensis that produces proteins toxic to certain animals (ex. European corn borer, western corn rootworm, black cutworm, tobacco hornworm, and in this instance SCN) when ingested. Scientists used genetic modification to insert genes from Bacillus thuringiensis that code for a Bt toxin into the soybean genome. When that gene is expressed in soybean roots, a Bt toxin toxic to SCN is produced and available for ingestion by feeding SCN. These varieties are still being researched and are projected for being available commercially in the latter half of this decade.

Fielding audience questions

Bernardo and Lorenz answered the numerous audience questions including: will the short-statured hybrids have rootworm Bt traits in them?, how will short-statured corn differ in fertility needs?, do we expect more ear molds with ears being set closer to the soil in short-statured corn? will there be more weed management challenges due to fewer leaves being produced in short-statured hybrids?, is yield drag the reason why Peking varieties are harder to come by?, does tillage impact SCN?, and how quickly do SCN egg counts drop when growing a non-host crops?, why do we not see company ratings for the two main commercial sources of SCN resistance (Peking and PI 88788)?.

Thanks to the Minnesota Soybean Research & Promotion Council and the Minnesota Corn Research & Promotion Council for their generous support of this program!