Objective:

1. Improve grain sorghum yields by testing nitrogen fertilizer rates with different plant populations.
2. Develop tissue N content, chlorophyll meter, and green color chart thresholds in grain sorghum for applying midseason N by fertigation.

Current status/importance of research area:

Grain sorghum can be a profitable crop when high yields are achieved. Nitrogen is the nutrient that most grain sorghum farmers have the most questions. Grain sorghum is often planted on marginal soils where achieving an optimal plant stand is sometimes difficult. For this reason, about 80% of the farmers in Southeast Missouri delay N applications until grain sorghum seedlings emerge. If a poor stand occurs, less N is applied. If excessive rain occurs after N is applied, additional N fertilizer may be applied by fertigation or by aerial application of dry fertilizer. This project will supply growers with much needed information on grain sorghum nutrient requirements.

Procedure:

Nitrogen experiments will be conducted on three soil types at the University of Missouri-Lee Farm at Portageville, Missouri. A randomized complete block design with four replications will be used in each test. Grain sorghum will be planted on 30-inch rows in each of the tests. Plots will be 10 feet wide and 35 feet long. Plant populations will be measured when grain sorghum is 4 inches tall. Plots will be watered with a center pivot or flood irrigation. Visual lodging rating will be recorded at harvest. All plots will be mechanically harvested for yield with a plot combine.

Plant population. In the first experiment, the effect of plant population on yield response to early-season N rates will be determined. This test will be conducted on Tiptonville silt loam and Sharkey clay soils. Grain sorghum will be planted in two seeding rates to achieve low and high plant populations. After emergence, grain sorghum plants will be hand thinned to 35,000 and 105,000 plants per acre. Five N rates will be tested at each plant population. Early-season N treatments will be ammonium nitrate applied at rates of 0, 50, 100, 150, and 200 lb N/acre when grain sorghum is 5 inches tall.

Nitrogen timing. In the second experiment, midseason tissue N contents, Minota SPAD chlorophyll meter, and University of California green color chart measurements will be correlated to grain sorghum response to N applied through a sprinkler irrigation system. This test will be conducted on a Crevasse loamy sand soil. Ammonium nitrate will be applied preplant at the rates of 0, 25, 50, and 75 lb N/acre. Two preplant treatments will be included with urea at 50 lb N/acre with and without AGROTAIN. Each preplant N rate will have additional N applications with 0 and 30 lb N/acre by fertigation (32% UAN+ AGROTAIN) late vegetative and soft dough stages. Two fertigation treatments without AGROTAIN will be included. Before making the fertigation N application, grain sorghum leaves will be sampled from each plot. Each sample will be tested for tissue nitrate content and digested for total N using a Hach Digesdahl. SPAD chlorophyll meter and green color chart readings will also be recorded. After harvest, we will compare yields between plots with and without fertigation N. Chlorophyll meter, color chart readings and tissue test results will be correlated with yield increases from fertigation N to develop critical levels for recommending additional N applications on grain sorghum.

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