Crops 48 www.FarmProgress.com ◆ August 2012 Prairie Farmer How drought impacts soybean yields By ANDY LENSSEN pathway for soybean yield, and N fi xation can be severely limited or stopped during IDESPREAD moderate drought stress. Soybean is an drought across annual plant, and unlike related perennial Wan entire state legumes such as alfalfa, N fi xation is rarely rarely occurs in the reinitiated once it stops. Halting N fi xation Midwest. However, the in the soybean plant essentially fi xes yield 2012 growing season is to the N previously stored in roots, stems one of those rare years. and leaves for subsequent seed develop- Normally there is more ment. The large impact of available stored concern with draining excess water. This water in soil on N fi xation and subsequent year has put more focus on conservation yield in soybean cannot be understated. of water for crop production. Numerous research studies have tried Precipitation has been below-normal to replace or augment N fi xation by ap- for much of the Midwest, and extraordi- plying N fertilizer. However, fertilizer N use narily high daytime and nighttime temper- by the plant requires adequate water for atures have put many corn and soybean root uptake. Water uptake simply is inad- plantings at risk for below-normal yields. equate during periods of drought stress, Soybean plants are most susceptible to and fertilization will be ineffective unless yield loss from drought stress at two key rainfall is very timely and substantial, and DRINK OF WATER: This fi eld of beans near Keokuk, in the southeast corner of Iowa developmental stages: germination and near the Mississippi River, had an irrigation rig running in July on a 103-degree day. soil water infi ltration rate is adequate. reproduction/seed development. For germination to occur in soybeans, bition. Water defi cit at this time results in was observed prior to the stress. This is Manage to conserve moisture seed must imbibe 50% by weight in water. poor hypocotyl growth, but root growth called compensatory growth. At this point in time, there are few options Seed coat swelling occurs during imbibi- and elongation may be unaffected. This Moderate, short-term drought stress for soybean farmers to improve available tion, causing the coat to rupture. If soil evolutionary response in soybeans allows during vegetative growth stages does not soil water status for the current crop, water is insuffi cient for radicle (root) and the seedling to search for additional soil typically impact soybean yield. unless irrigation systems are already in hypocotyl (shoot) development and emer- water while having overall low water use, place. However, soil management deci- gence, the plant will die. promoting survival and development. When crop is most vulnerable sions to improve available soil water con- When seeds are planted in dry soil, or Soybeans in the Midwest may undergo Soybean yield is most impacted by drought tent should be carefully considered by not placed into soil, germination is not relatively short-term drought stress at stress during reproductive phases. Soil soybean farmers following this year’s har- initiated. As long as seed coat integrity planting, but typically, adequate rainfall water deficits during reproduction in- vest. High-intensity tillage systems are not is maintained, seed can survive until ad- occurs shortly thereafter, and the soybean crease fl ower abscission, abortion of em- a solution to improve soil water content. equate moisture is received. Recently ger- plant responds to reinitiate shoot growth. bryos and reduced pod number; surviving Lenssen is the Iowa State University minated seedlings can undergo drought Shoot growth after short-term drought seeds are smaller. Extension soybean systems agronomist at stress as early as two days following imbi- stress can occur at a faster rate than what Nitrogen fi xation is a key biochemical Ames. Email him at [email protected].

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