Laura Schmidt, Extension Coordinator, MPSG
Annual crop water use is low at the beginning of the growing season and increases as the crop progresses through the vegetative growth stages. It peaks during reproductive development and gradually declines at maturity.
Soybeans require significantly more water than wheat, canola and pulse crops, taking up 400–500 mm (16–20 inches) over the entire growing season (Table 1). The actual amount will vary with maturity group, planting date and environmental conditions.
For most of agro-Manitoba, the normal accumulated precipitation during the growing season is between 300–350 mm (12–14 inches). This means we often rely on moisture held in the soil from the previous year to support our soybean crops. Our other pulse crops like peas and dry beans require less water overall, as they are cool-season legumes (Table 1).
Pulse and soybean crops are susceptible to water limitations during germination. Soybean seeds need to imbibe half of their weight in water for germination and seedling elongation. Peas and dry beans also need to have sufficient water for germination and root development during the early stages of growth.
Inadequate soil water in these early growth stages may result in reduced plant populations, which can reduce final seed yield. Soybean and dry bean plants will try to compensate under low density conditions once sufficient moisture is available.
Plants will produce more branches, leaf area and pods per plant. However, in soybeans, these compensatory mechanisms are not always enough to combat severely reduced plant stands and yield reductions may still occur.
Emergence may also be uneven, due to delayed emergence. Whole seeds in dry soil will eventually germinate and emerge once adequate moisture is present, filling in stand densities. Soybean maturity is largely determined by photoperiod, meaning these late-emerging plants will be ready for harvest at the same time as early-emerging plants. However, these later-emerging plants will have less growing time, accumulating less biomass and producing less seed yield.
During the vegetative growth stages, short-term, moderate water deficits generally do not influence yield.1 Compensatory growth will occur following precipitation. Interestingly, one study evaluated the response of soybeans to water stress induced at different developmental stages and found plants subjected to water stress during the V4 stage showed an increased tolerance to water shortages in later stages.4
Typically, soybean root distribution is concentrated in the top 16 inches (40 cm) of soil, with over half of the root mass growing in the top eight inches (20 cm). Peas also obtain the majority of their seasonal water from the top 14 inches (35 cm) of soil, with an effective water extraction depth of 2.3 feet (70 cm).
Roots in this upper soil profile grow slowly under water limitations. Plants tend to compensate by partitioning photosynthates to the roots, enabling them to grow more rapidly into the lower soil profile. This results in a reduction or cessation in shoot growth, allowing roots to penetrate further into the soil in search of moisture. Under normal field conditions in Iowa, soybeans have been measured to grow as deep as 4.9 to 6.5 feet (1.5–2 m). Growth rates return to normal once soil water levels return to normal.
Above ground, symptoms of heat stress may be visible. Growth may be reduced, the distance between internodes shortens and leaves may be shed to conserve moisture. Soybeans also turn their leaves to reflect solar energy, reducing heat as well as reducing photosynthesis. Reduced photosynthesis allows plant stomates to remain closed and reduces water loss through respiration.
Critical timing for water uptake by pulse and soybean crops occurs during flowering and pod fill (R1–R6). Water deficits during these reproductive development stages will reduce yield. While water demand of soybeans is highest at flower initiation (R1), water limitations during pod initiation (R3) and filling (R5–R6) are more critical to yield.
In one study, each millimetre of rain between flowering and pod emergence caused a yield increase 0.06 bu/ac. During grain filling, each millimetre of rain resulted in a yield increase of 0.19 bu/ac.4
During these reproductive development stages, water deficits can result in flower abortion, leading to a reduction in pod number and yield. Water-stressed plants also often mature earlier, shortening the grain-filling period, resulting in reduced seed weight and seeds per pod. In August, during pod formation and seed fill, soybeans and peas will take up approximately 6–8 mm (1/4–1/3 inch) of water per day.
Prolonged water limitations in pulse and soybean crops also reduces activity in the root nodules, reducing nitrogen fixation. Water deficits result in limited water and oxygen supply to the root nodules, as well as a build up of the nitrogen compounds synthesized during nitrogen fixation (ureides) in the stem tissue.
The ureide accumulation in the stem tissue signals back to the nodules, reducing nitrogen fixation. The subsequent decline in nitrogen supply leads to further yield reductions. Some evidence has reported that soybean plants that produce larger nodules are less susceptible to reductions in nitrogen fixation under water stress.4
Water deficits induced during reproductive development may result in a higher protein content in soybean seeds, according to some studies.4 However, water limitations during this time significantly reduce soybean yield. Lower seed number and weight means fewer sinks for protein within the plant, leading to a higher protein content in the grain.
Management practices that leave high amounts of residue on the soil surface and improve soil structure can increase water infiltration rate. Adopting production practices that reduce soil compaction can also benefit yield when drought conditions occur, through improved root penetration and nodule function.
Adequate plant nutrition can further combat the deleterious effects of water stress. Potassium, phosphorus and calcium have direct beneficial effects on plant metabolism under water limitations and improve plant recovery following water deficits. •