Hybrids handle N stress differently

Nebraska growers are blessed with a resource that gives them an advantage over growers in other states: irrigation. With that comes options for not only watering crops, but also fertilizing in-season.

However, as more seed companies characterize hybrids based on factors like water-use efficiency, plant population and nitrogen-use efficiency, which hybrids are more suited to in-season fertigation?

It's a question posed by agronomists at Bayer's Water Utilization Learning Center near Gothenburg, Neb., as part of a study over the last two years testing response to nitrogen in different Dekalb hybrids. The goal, says agronomist Mark Reiman, is to determine how well different Dekalb products handle nitrogen stress upfront versus later in the season.

"It goes back to there's a product component and trying to understand what nitrogen product they like," Reiman says. "Fertigation can be a very good management practice. It's an opportunity we have in Nebraska that other states don't have."

This study tested 24 different Dekalb products with two different nitrogen treatments: one where all 240 pounds of nitrogen are applied at the start of the season, and one with 60 pounds applied up front, followed by 180 pounds applied throughout the season in 15-pound increments through fertigation. Last year, some of those treatments were as late as the last week of August.

The nitrogen, in the form of 32% UAN, is applied through a subsurface drip irrigation (SDI) system, with drip tape buried 12 inches deep on 40-inch spacing. Rates were calculated based on soil tests and a 40-pound nitrogen credit from soybeans grown on the field in 2016.

This is the second year of the study. In 2017, all products on average saw a benefit from the in-season fertigation treatments, and on average, used 15 pounds less nitrogen.

"We found even though we had 15 pounds less nitrogen, we found we had a 9- to 10-bushel difference in yield across all Dekalb products in the study," Reiman says. "One product in the study we saw pretty much had even yield. We see up to 30 pounds difference in yield for one hybrid that preferred nitrogen at the end of the season."

However, the results get more interesting when looking at how individual products performed at different points in the growing season. In some cases, there were no significant yield differences from fertigation treatments — and in some cases, there was even a small yield decrease.

"In the field where we did everything upfront, early on the corn looked great. It had all the nitrogen it needed to really shoot up," Reiman says. Meanwhile, early on in the season, the corn in the fertigation treatment showed signs of nitrogen deficiency. "It is a growing-season-long process of nitrogen accumulation, and by the end of the year, you started to see nitrogen stress in the products where you applied everything upfront. Where you were fertigating, you corrected that early-season nitrogen deficiency and then at the end of the season you had darker-green, healthier-looking plants."

The question is: What causes some hybrids or products to prefer late-season nitrogen more than others? The answer, Reiman says, isn't clear. It isn't always higher-yielding hybrids, and it isn't always due to relative maturity. Some products, for whatever reason, just seem to prefer more nitrogen later in the season.

Of course, early-season rainfall may have been a factor last year. The Gothenburg area saw several 1-inch rainfall events in May and June 2017. With more nitrogen applied upfront, more is left open to loss through leaching or denitrification.

"This year, we didn't have those really heavy rainfall events. We've had really nice rains. We don't have a lot of moisture stress out there in the field at all even on dryland acres. Maybe we're not going to see a lot of leaching this year, maybe the response won't be quite as dramatic as what we saw last year," Reiman says.

"Overall, the yield potential looks really high,” he says. “We had a few plots last year reach that 280-, 290- to 300-bushel range. I think we're looking at those same yield numbers this year if not maybe even a little bit better just by looking at kernel depth and kernel count."