Contributors:
Josh Lofton, Cropping Systems Specialist
Brian Arnall, Precision Nutrient Specialist

This blog will bring in a three recent sorghum projects which will tie directly into past work highlighted the blogs https://osunpk.com/2022/04/07/can-grain-sorghum-wait-on-nitrogen-one-more-year-of-data/ and https://osunpk.com/2022/04/08/in-season-n-application-methods-for-sorghum/

Sorghum N management can be challenging.  This is especially true as growers evaluate the input cost and associated return on investment expected for every input.  Recent work at Oklahoma State University has highlighted that N applications in grain sorghum can be delayed by up to 30 days following emergence without significant yield declines.  While this information is highly valuable, trials can only be run on certain environmental conditions. Changes in these conditions could alter the results enough to impact the effect delay N could have on the crop.  Therefore, evaluating the physiological and phenotypic response of these delayed applications, especially with varied other agronomic management would be warranted.

One of the biggest agronomic management sorghum growers face yearly is planting rate.  Growers typically increase the seeding rate in systems where specific resources, especially water, will not limit yield.  At the same time, dryland growers across Oklahoma often decrease seeding rates by a large margin if adverse conditions are expected.  If seeding rates are lowered in these conditions and resources are plentiful, sorghum often will develop tillers to overcome lower populations.  However, if N is delayed, there is a potential that not enough resources will be available to develop these tillers, which could decrease yields. 

A recent set of trials, summarized below, shows that as N is delayed, the number of tillers significantly decreases over time.  Furthermore, the plant cannot overcompensate for the lower number of productive heads with significantly greater head size or grain weight. 

This information shows that delaying sorghum N applications can still be a viable strategy as growers evaluate their crop’s potential and possible returns.  However, delayed N applications will often result in a lower number of tillers without compensating with increased primary head size or grain weight.

This date on yield components is really interesting when you then consider the grain yield data. The study, which is where the above yield component data came from, was looking at population by N timing. The Cropping Systems team planted 60K seeds per acre and hand thinned the stands down to 28 K (low) and 36K (high). The N was applied at planting, 21 days after emergence, and 42 days after emergence. The rate of N applied was 75 lbs N ac. It should be noted both locations were responsive to N fertilizer.

In the data you can without question see how the delayed N management is not a tool for any of members of the Low Pop Mafia. However those at what is closer to mid 30K+ there is no yield penalty and maybe a yield boost with delayed N. The extra yield is coming from the slightly heavier berries and getting more berries per head. Which is similar to what we are seeing in winter wheat. Delaying N in wheat is resulting in fewer tillers at harvest, but more berries per head with slightly heavier berries.

Now we can throw even more data into the pot from the Precision Nutrient Management Teams 2024 trials. The first trial below is a rate, time and source project where the primary source was urea applied in front of the planter for pre in range of rates from 0-180 in 30 lbs increments. Also applied pre was 90 lbs N as Super U. Then at 30 days after planted we applied 90 lbs N as urea, SuperU, UAN, and UAN + Anvol.
Pre-plant urea topped out at 150 lbs of Pre-plant (57 bushel), but it was statistically equal to 90 lbs N 51 bushel. The use of SuperU pre did not statistically increase yield but hit 56 bushel. The in-season shots of 90 lbs of UAN, statistically outperformed 90 pre and hit our highest yeilds of 63 and 62 bushel per acre. The dry sources in-season either equaled their in preplant counter parts.

The Burn Study at Perkins, showed that the N could be applied in-season through a range of methods, and still result good yields. In this study 90 lbs of N was used and applied in a range of methods. The treatments for this study was applied on a different day than the N source. Which you can see in this case the dry untreated urea did quite well when when applied over the top of sorghum. In this case we are able to get a rain in just two days. So we did get good tissue burn but quick incorporation with limited volatilization.

Take Home:
Unless working in low population scenarios. The data show that we should not be getting into any rush with sorghum and can wait until we know we have a good stand. We also have several options in terms of nitrogen sources and method of application.

Any questions or comments feel free to contact Dr. Lofton or myself
josh.lofton@okstate.edu
b.arnall@okstate.edu

Funding Provided by The Oklahoma Fertilizer Checkoff, The Oklahoma Sorghum Commission, and the National Sorghum Growers.