Sorghum Schools offer nutrient and pest information

Kansas State University’s department of agronomy and the Kansas Grain Sorghum Commission hosted a Sorghum School Feb. 5 in Garden City, Kansas, and topics ranged from crop production and fertility to insects and weed control.

Dorivar Ruiz Diaz, professor of soil fertility and nutrient management at K-State, said there are several key nutrients in sorghum production including nitrogen, phosphorus, zinc and sulfur.

“Phosphorus always is a key nutrient that will often times limit production in sorghum,” he said. “Sorghum is a crop that can respond to things like zinc.”

K-State has conducted trials looking at the influence of sulfur.

“We haven’t seen a lot of consistent response yet like we do in winter wheat and corn,” Diaz said. “Those crops tend to be a lot more responsive to sulfur. However, there are situations where we are seeing slight benefits to sulfur application in sorghum.”

In some soils in western Kansas, producers need to be more aware of the micronutrient levels they have available to their crops.

“For this part of the state, one thing we always talk about is high pH and iron chlorosis,”Diaz said. K-State has a number of studies on iron chlorosis and tips for managing it in high pH situations.

Looking ahead, Diaz sees nitrogen and phosphorus to continue to be the most limiting nutrients for sorghum.

“You have to watch those very closely,” Diaz said. “Especially again, when talking return on investment. That’s really the key here.”

Chloride and sulfur are becoming an issue but that depends on the field.

“My suggestion in this case is that maybe we don’t need to be too quick at putting these as part of the fertility program, necessarily but we have to be kind of looking at specific fields that maybe benefiting from some of these,” he said.

Sorghum production management

Lucas Haag, Northwest Area Agronomist at Northwest Research-Extension Center in Colby, Kansas, said when looking to improve sorghum production revisit the basics. Think about what the critical growth stages for grain sorghum are in terms of setting yield components.

“When do we need to make sure we’re doing everything we can to make sure that plant is not under stress?” Haag said. He stressed that the growing point differentiation for sorghum is about 30 days after planting

At this point, the maximum seed number potential of the head is being set. Tillers aren’t being set because they haven’t been formed yet.

“We know it’s critical to make sure the plant’s not under stress at that point in time,” Haag said. “We’re setting what’s that upper limit and it’s just a matter of what’s going to happen after that.”

The next growth point, boot stage is really where the sorghum plant is hitting its peak in terms of rapid growth and nutrient uptake.

“If we’re in an irrigated situation we have that ability to fertigate some nitrogen on and this is a good opportunity to do that,” Haag said. “Make sure you’re supplying the needs of that plant.”

By that time, it’s mid-bloom and in the critical phase for pollination.

Haag said if the plant is going into stress at this point some of those potential seeds could be lost.

One key for irrigating sorghum is when it’s watered during the first stages of plant development.

“You water it at 30, 60 and 90 days after planting,” Haag said. “If you really wanted to find irrigation management in one sentence, that’s the simplest way to put it. Because it goes back to those key growth stages.”

Maximizing water use and yields

When it comes to managing sorghum, row spacing and seeding rate should be determined with careful thought and consideration.

“This applies to all of our crops. We get into a lot of conversations about water use and how do we maximize, how do we match water use in order to maximize yields,” Haag said.

But when thinking about seeding rates, don’t fall into a common misconception. Sorghum doesn’t grow isolated plants; instead they end up as a community. What’s the key driver in water use of the plant? Light interception. When there’s more plants in a community, the leaves end up being shaded and the canopy closes up, affecting the water use.

“It’s really driven by not how many plants you have out there but by the light you’re intercepting,” Haag said.

It’s a little foreign to think about it this way, but Haag insists it makes sense.

“Why does this matter?” he said. “If we think about in crop production, if were managing fertility, if we’re managing pests, what are the two things I have to have to grow a crop? What two resources? Water and sunlight. And so if we’re managing everything else and we’re limited by one of two things—were either light-limited or water-limited.”

Insect control

J.P. Michaud, entomology professor with K-State, said within the last few years, there’s been a lot of insect action in sorghum. The sugarcane aphid has been the primary concern, especially in Kansas and states to the south. He believes the SCA is in decline.

“When we get endemic, it means it’s like all the other aphids in sorghum and we probably won’t need to spray for it hardly ever,” Michaud said. “Problems continue to diminish annually. Fewer and fewer acres treated.”

A few producers at the meeting admitted to spraying their fields in 2018 and the entomologist said many of the infestations were economic, but it was like looking for a needle in a haystack in central Kansas.

“What happened is we’ve now got really good natural biological control established year round in Mexico,” Michaud said. “That’s really where everything starts every year because they plant sorghum all year long.”

So fewer and fewer winged aphids are traveling north with the wind.

“Again, as long as you have a few migrants it’s not as dangerous as when you have a lot,” Michaud said.

The winged aphids have a lower rate of reproduction and in order to survive they must congregate. The more they congregate the sooner they can get some of their wingless daughters mature. That’s if the predators stay away.

“One lady beetle can come by and scarf down that whole patch of aphids,” he said. “The fewer and fewer allies that come, the more and more chances they’ll be taken off by natural enemies before they establish colonies.”

Michaud said when there’s a new pest that bursts onto the scene, there’s huge population numbers. But it doesn’t always stay that way.

“So gradually as the natural enemies adapt and start picking off the colonies earlier, you have fewer migrants moving north every year, these cycles diminish over time,” he said.

Michaud expects there to always be a few aphids out in the fields, but not enough to merit spraying for them. Similar to what happened with the green bug several years ago.

For more information about grain sorghum in Kansas visit ksgrainsorghum.org.

Kylene Scott can be reached at 620-227-1804 or [email protected].