Drought Sensitivity Indices for a Sorghum Crop

Grain yield in sorghum [Sorghum hicolor (L.) Moench. cv. DK‐S7] depends in general on the amount of rainfall and irrigation, and atmospheric processes affecting water use. Timing may significantly modify the yield response. The objective of this study was to derive a sorghum drought index (SD1) to express the temporal impacts of climate on grain production. Irrigation (I) and no irrigation (N) treatments were imposed on sorghum during vegetative (GS1), inflorescence (GS2), and grain fill (GS3) growth stages in a randomized, split‐factorial block design during two growing seasons in Mead, NE. The soil is Sharpsburg silty clay loam (fine, smectitic, mesic Typic Argiudolls). The irrigation strategy produced eight water treatments ranging from no irrigation in any stage (NNN) to irrigation in each stage (III). Soil water, measured by a neutron meter, was used in plot water balance simulations to estimate evapotranspiration (ET). Ratios of total actual ET (ET a and transpiration (T a to total potential evapotranspiration (ET p) were determined for each growth stage. These ratios varied from 0.317 to 0.922 across all stages and treatments. Sensitivity to water availability was depicted by A, exponents in a relative yield model (SDI = Y/Yp = II(ΣETai/ΣETpi)λi, where II indicates multiplication for each growth stage, i = 1 to 3). Yp is the potential yield, taken as 110% of yield in the III treatment. ETa, not Ta, gave the best fit. The indices (λ = 0.04 (GS1), 0.20 (GS2), and 0.18 (GS3)) are smaller for sorghum than for other grain crops, which indicates sorghum is less sensitive to drought. Even so the difference in magnitude between stages is as much as a factor of five, indicating that sorghum is sensitive to the timing of rainfall and irrigation. Validation with independent data for another variety and year was satisfactory (r2 = 0.78 and d‐index of agreement = 0.89). We conclude that the effect of short and long term drought on sorghum production for this variety can be monitored through the use of the newly defined SDI. Perhaps the fact that SDI is normalized with respect to the potential yield, a genetic characteristic, will allow it to be more representative of sorghum, irrespective of variety. This possibility seems to merit further investigation.Research QuestionDrought is a major cause of yield reduction. Indices of drought have focused on weather indicators like precipitation and temperature. Crop specific drought indices are needed that focus on the impact of drought on agricultural production. How does drought affect sorghum production? If drought occurs in short intense periods, how are yields affected in contrast to droughts that may last for a significant portion of the growing season? If water is limited when should it be applied?Literature SummaryResearch has demonstrated that seasonal crop water use, actual evapotranspiration (ETa) and transpiration (Ta), are closely related to yield. Sensitivity coeficients (λs) can be introduced to assess the relative effects of water deficit during each stage of growth. Positive λi coefficients indicate a yield loss will occur if stress occurs in the ith stage. Negative λi coeficients indicate an increase in yield with water stress and small values of λi indicate little sensitivity to water stress in the ith stage of development.Study DescriptionThe experiment was conducted in 1990 and 1991 in eastern Nebraska, USA. Soils at the site are classified as Typic Augiudoll, a deep well drained soil. Grain sorghum cv. DK‐57 was planted in 30 in. (0.76 m) rows oriented north‐south. A randomized, split‐factorial block irrigation design with eight water treatments was employed. There were three blocks (replications) and three growth states: vegetative (GS1), inflorescence to anthesis (GS2), and grain fill (GS3). Treatments consisted of irrigation or no irrigation in each of the stages and ranged from rainfed to fully irrigated. Soil water balance was used to determine ETa and Ta for each growth stage. Actual values of the sorghum drought index were defined as the ratio of yield to potential yield in each plot. The calculated sorghum drought index was taken as the product of the ratios of (ETa/ETp) λi or (T a/ETP) λi for every phase (i), where i = 1 indicates growth phase 1 (GS1), etc. and ET p is the potential evapotranspiration. The sensitivity coefficients were solved using matrix algebra.Applied QuestionsHow do reductions in actual evapotranspiration at various growth stages change yield in sorghum.The results of this study indicate that the ratio of ETa to ETp by growth stage determine the ratio of yield to potential yield. Six examples are shown in this table to illustrate the effect of short and long duration drought. For comparison, all six examples take the ratio of ETa to ETp as 0.2. This is for illustration and the actual ratio will depend on the availability of water in the soil (antecedent precipitation) and the temperature, humidity, solar radiation, and wind speed of the atmosphere. In practice, water available in the soil can be estimated by using real time weather data in water balance models developed for irrigation scheduling.The first three examples indicate the effect of a short duration drought that occurs within a single growth phase of the sorghum crop. In the first phase the effect on yield is small (6%), however, yield reduction is significant when short term drought occurs in GS2 (28%) and GS3 (25%).Examples 4 and 5 are longer term drought that spans two growth phases. Example 6 also involves two growth phases but is interrupted by adequate rainfall in GS2. Yield reductions for these longer duration droughts are from 30 to 46%, the latter represents drought effects in both GS2 and GS3. Ratios (Eta:Etp) and normalized yield Example GS1 GS2 GS3 Y/Yp 1 0.2 1 1 0.94 2 1 0.2 1 0.72 3 1 1 0.2 0.75 4 1 0.2 0.2 0.54 5 0.2 0.2 1 0.68 6 0.2 1 0.2 0.7