Nitrogen and Irrigation Management for Sorghum in South Carolina

Sorghum is a drought-tolerant crop suitable for growing in South Carolina. Although it can survive well under adverse conditions, nitrogen (N) insufficiency and water stress can reduce sorghum grain yield. This publication summarizes how the negative impacts of water stress and N insufficiency can be mitigated with supplemental irrigation (SI) and N fertilization.

Background

Potential shifts in precipitation patterns due to climate change could alter how crops fulfill water requirements, reduce water availability and crop productivity, and increase the costs of water access across the agricultural landscape. Adopting alternative crops with lower water requirements and increased crop yield per unit of water applied is important for agricultural sustainability. Grain sorghum (Sorghum bicolor L.) is an ideal grain crop suitable for growing in regions drier than those for corn (Zea mays, L.)^1,2 In many humid regions, corn is usually a better choice than sorghum. However, interest in sorghum production has increased because of recent climate changes resulting in hotter and drier conditions which creates water scarcity. Although sorghum can survive and produce yield under adverse conditions, environmental stress and improper management can reduce yield considerably. This is because drought conditions decrease the plant’s ability to utilize solar radiation, and plant growth depends on water availability during different growth stages. Adequate water supply increases the photosynthetic rate and gives plants extra time to translate carbohydrates into grains.³ According to the 2012 US Census of Agriculture report, US producers planted grain sorghum on approximately 5.2 million acres with 0.50 million acres irrigated. Sorghum can be an alternative to corn in the southeastern United States, especially in South Carolina. However, poor and uneven rainfall distribution and marginal soil fertility of South Carolina’s humid region can affect crop production.^2,4

In many regions where sorghum is grown, farmers use irrigation to supplement rainfall. Previous research studies have found that integrated irrigation and fertilization can improve biomass and grain production in cereal crops.⁵ Irrigation methods affect the distribution, content, and movement of soil water, strongly influencing plant growth, and root system development, and water uptake.⁶ Increased N content can enhance root density and contribute to increased shoot biomass and grain yield. In 2013, in Arizona, Texas, and North Carolina, sorghum growers applied an average of approximately 43, 14, and 2.4 inches of irrigation water to grain sorghum, respectively.⁷ Deficit irrigation is a common practice for sorghum grown in the Texas Southern High Plains region because precipitation rates are below well-watered crop requirements. In Mississippi, Bruns (2015)⁸ concluded that supplemental irrigation did not significantly increase the sorghum grain yields in the Mid-South under normal seasonal conditions. However, limited information is available on providing supplemental irrigation to sorghum in humid regions like South Carolina. Understanding the optimum supplemental irrigation level and N fertilization will help farmers achieve sorghum grain or silage yield or sorghum in South Carolina.

Nitrogen and Supplemental Irrigation Effects on Sorghum Aboveground Biomass

Sigua et al. (2018)⁹ observed in South Carolina to determine the combined effects of N fertilization at rates of 0 lb, 76 lb, and 152 lb N ac^-1 and supplemental irrigation applied at rates of 0%, 50%, and 100% of the full irrigation rate in South Carolina. The researchers measured the effect on aboveground biomass (both grain and straw) for two grain sorghum varieties (Dekalb A571 and Pioneer 84P80) grown in South Carolina.

Supplemental irrigation combined with the N fertilizer application at the rates of 76 lb and 152 lb N ac^-1 increased the sorghum aboveground biomass, N use efficiency, and N uptake in this research study (table 1). On the other hand, the application of 76 lb and 152 lb N ac^-1 produced a statistically similar yield. These results confirm that the application of supplemental irrigation along with optimum N fertilization (76 lb N ac^-1) mitigated the impact of water stress and N deficiency in sorghum grown in South Carolina. The results also implied that sorghum grown under N deficient conditions (0 lb N ac^-1) slows the rate of accumulation of dry matter accumulation. Reducing the N content below the threshold level may lead to vegetative tissue death and restrict the sorghum crop’s growth, productivity, and grain yield.

Table 1. Aboveground biomass (grain and straw) as influenced by supplemental irrigation and N fertilization in 2013 and 2014 cropping.⁹

^†Values within the same column followed by letters are significantly different at p<0.05 within the treatments. *p<0.01.

Combining supplemental irrigation (100%) with 76 lb N ac^-1 resulted in significantly higher aboveground biomass in this grain sorghum study. The study demonstrated that sorghum growers could mitigate the negative impacts of water stress and N deficiency by providing supplemental irrigation and optimum N fertilization.

On-farm research trials conducted near Lawrence, Nebraska over three years (2009-2011)¹⁰ showed that in two out of three years, sorghum had the highest crop water use efficiency (grain produced per unit of water used) followed by corn and soybean (tables 2a, 2b, 2c). ¹⁰

Table 2a. Crop evapotranspiration (in) of corn, soybean, and sorghum in on-farm trials conducted near Lawrence, Nebraska, 2009-2011.¹⁰

Table 2b. Crop yield (bu/ac) of corn, soybean, and sorghum in on-farm trials conducted near Lawrence, Nebraska, 2011-2011.¹⁰

Table 2c. Crop water use efficiency (bu/in) of corn, soybean, and sorghum in on-farm trials conducted near Lawrence, Nebraska, 2011-2011.¹⁰

Ways to Improve Crop Productivity Under Water-Scarce Conditions

There are two fundamental approaches to improve and sustain crop productivity under water-scarce conditions:

1. Selecting and planting varieties that are well-adapted to drought conditions.
2. Providing supplemental irrigation and reducing soil water loss by managing the soil environment.

Improving water use efficiency can be achieved by using efficient irrigation systems (such as center pivots, and drip irrigation), using proper irrigation scheduling techniques (such as using soil moisture sensors), and using site-specific irrigation technologies (such as variable-rate irrigation (VRI)) to apply water when, where, and in the amount needed to meet crop requirements while minimizing water losses. Application of these technologies enables water to stay in the crop root zone, available to the crop, minimizing water losses by runoff, surface evaporation, and deep drainage. The USDA Environmental Quality Incentives Program (EQIP) website provides information about financial opportunities for growers to implement some of these water conservation technologies.

Conclusions

Poor and uneven rainfall distribution and soil fertility in humid coastal plain regions like South Carolina may affect the production of grain crops. A crop with lower water requirements, like grain sorghum, is well adapted to various adverse environmental conditions and can replace corn in areas where soil fertility is low and water supply is limited. The use of supplemental irrigation (100%) along with optimum N fertilization (76 lb N ac^-1) to sorghum could mitigate the negative impacts of water stress and nutrient deficiency. This study concludes that effective use of irrigation water and maintaining soil N level will improve the biomass of grain sorghum in South Carolina.

Acknowledgments

We would like to extend our utmost appreciation to the following volunteer reviewers of the original version of this document:

Dr. Ariel A. Szogi, Research Leader
U.S. Department of Agriculture, Coastal Plains Soil, Water and Plant Research Center, Florence, SC

Dr. Gilbert C. Sigua, Research Soil Scientist
U.S. Department of Agriculture, Coastal Plains Soil, Water and Plant Research Center, Florence, SC

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