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As we approach the heat of the summer, it is important to know when crop water demands will become greater than precipitation, which often occurs during the critical reproductive periods. Under hot, dry conditions, peak corn water demand can reach up to 0.5 inches/day in semi-arid regions like western Nebraska.1 Plan ahead by knowing the capacity of the irrigation system to ensure that adequate water is available to the crop through maturity.
Corn water use rates peak during the early reproductive growth stages, while soybean water use peaks during the mid- to late reproductive stages. When crops do not receive enough water to meet evapotranspiration demands during the reproductive growth stages, significant reductions in yield can occur.
Figure 1. Example of long-term daily average (A) and individual year (B) corn water use. Source: Kranz, W.L. et al. 2008. Irrigation management for corn. NebGuide G1850. University of Nebraska-Lincoln Extension.
Early reproductive stages. Water stress should be avoided during the reproductive stages (tasseling, silking, and pollination). Water stress during silking can have the greatest impact on yield potential due to desiccation of the silks and pollen grains, which will result in poor pollination.2
Dough through dent stages. Corn water use rates steadily decrease from the dough stage through maturity due to a lower evaporative demand (shorter days, lower temperatures, lower solar radiation), a loss of transpiring leaf area as lower leaves begin to die, and changes in plant physiology.2 At the beginning of the dough stage, corn will still require roughly 1/3 of its seasonal water requirement (Table 1). For optimal grain development and maximum yield potential, corn requires water right up until physiological maturity. Water stress during dough through dent stages can accelerate maturity, prohibiting kernels from reaching their full potential size and weight.3
Maturity. After physiological maturity (black layer), water is no longer needed for kernel growth and no yield benefits can be achieved with additional irrigation. Allowing soils to dry at maturity is a good strategy for avoiding compaction that can be caused by harvesting machinery on wet soils.
Figure 2. Example of long-term daily average and individual year soybean water use. Source: Kranz, W.L. and Specht, J.E. 2012. Irrigating soybean. NebGuide G1367. University of Nebraska-Lincoln Extension.
Reproductive stages. Soybeans are most sensitive to water stress during the mid- to late reproductive stages: pod development (R3 to R4) and seed fill (R5 to R6). Water stress during pod development and early seed fill can have the greatest impact on yield potential and result in a reduced number of seeds per pod and reduced seed size. Irrigation may be required during flowering on soils with an insufficient water holding capacity (sandy soils) or when conditions are exceptionally dry.3 When water is applied during flowering, it is especially important to supply adequate water during seed fill. This is because irrigation during flowering usually increases the number of seeds produced, but subsequent water stress during seed fill will reduce the seed size which can result in greater yield penalties than would have occurred if the crop had not been watered at all during flowering.3
Maturity. Soybean requires adequate water through the reproductive stages in order for seeds to achieve their maximum weight (Table 2). Discontinuing irrigation before physiological maturity can result in yield penalties if the soil water content is not sufficient.
1 Schneekloth, J. and Andales, A. Seasonal water needs and opportunities for limited irrigation for Colorado crops. No. 4.718. Colorado State University Extension. www.ext.colostate.edu. 2 Kranz, W.L., Irmak, S., van Donk, S.J., Yonts, C.D., and Martin, D.L. 2008. Irrigation management for corn. NebGuide G1850. University of Nebraska-Lincoln Extension. 3 Kranz, W.L. and Specht, J.E. 2012. Irrigating soybean. NebGuide G1367. University of Nebraska-Lincoln Extension. Other sources: Yonts, C.D., Melvin, S.R., and Eisenhauer, D.E. 2008. Predicting the last irrigation of the season. NebGuide G1871. University of Nebraska-Lincoln Extension. Web sources verified 3/31/15. 160604154820