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No Fun in the Sun: Droughts Influence on Corn & Soybeans
Corn and soybeans are the two major grain crops produced within the world due to their various uses among oils, use in human consumption products, and animal feed. Due to these vast and wide uses, it is important to retrieve the highest yields possible to gain the most profit. One factor that can diminish these returns is weather stress. During the last three decades, drought and heat waves caused about $145 billion in damages to crop production across the United States (Lott and Ross, 2006). Weather is naturally unpredictable and with that comes variables and numerous factors that can cause stress factors in crops that affect yield and growth. Thomson states “variability in crop yield is primarily due to local and regional weather and climate rather than large scale climate dynamics, which stresses the importance of the local and regional scale analysis (Thomson et al., 2005).” While stress can be visually obvious, the more detrimental is the signs that go unnoticed during times of plant stress. In this paper, I will discuss the effects of weather stress, specifically drought, on corn and soybean developmental stages that influence yield results. I will also dip into current management practices used to recover or halt drought factors that influence yield, as well as current research being done that allows for new management practices to be formulated.
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When considering crop inputs we often think about fertilizers, pesticides, insecticides, and many more but we often forget the simple and most important input which is water. It is important to understand how the plant uses water so that in times of need such as in conditions that cause drought, we can implement strategies to limit the amount of time the plant is under the stress conditions that could cause yield reduction. When researching drought factors you will often see the abbreviation ET. Iowa State University Extension and Outreach states “ET is the combination of soil water evaporation (E) and water used by the plant during transpiration (T) (Iowa State University, 2017).” Iowa State also defines transpiration as “the mechanism by which water moves from the soil through the plant into the atmosphere (Iowa State University, 2017).” These are caused by the process in which temperature, solar radiation and humidity evaporate water from the earth’s surface. Soil evaporation is the key factor that is the cause of water surface loss, which is exacerbated following rain and high temperatures due to solar radiation and humidity. It was determined that when time to anthesis increases so does water stress when transpiration was reduced (Blum, 1996). Soil moisture is also determined by four interactive factors: 1. Amount of moisture in the soil 2. Characteristics of the soil profile 3. Moisture capacity of the crop 4. Demand for water in the atmosphere (Purdue University). For adequate crop growth and development, soil moisture must be more than atmospheric evaporative demand or it will result in yield reduction during any time of the crop life cycle. A study performed by Purdue University Cooperative Extension Service concluded that the time that crops experience these conditions are generally when all determining factors are increased like the mid-summer months of June-July. Furthermore, when plants are under water stress they’re more susceptible to disease pathogens, insect feeding influences and exhibit weakened stem integrity.
Moisture stress has also been shown to lead to plant nutrient stress such as nutrient availability, uptake and transport through the plant system. While fertilizer is generally placed to assist the plant in nutrients, it is placed at shallow depths and under stressful conditions these depths can become dry. This then limits the amount of nutrients the plant can uptake from the soil and could affect growth. Soil conditions are also a key role in determining drought-related stress conditions. Poor soil conditions cause shallow root development, which contributes to nutrient stress by decreased nutrient availability. “The importance of proper early root development cannot be underestimated (Iowa State University, 2017).” Crops that exhibit deep root systems are able to better withstand moisture stress conditions longer than those who exhibit short root systems, because they explore a greater volume of soil for resources. Crops respond to drought stress are conducted through the process of water deficit and correlated strains such as leaf rolling at various levels of plant development (Blum, 1996).
Drought in corn is often exhibited in the leaves by leaf rolling. However, in severe stress conditions greying of the leaf tissue has been observed. When observing for drought-related stress symptoms in corn, the earlier the leaf rolling occurs in the day or the longer the rolling is observed the greater the stress the crop is under (Iowa State University, 2017). Iowa State University Extension and Outreach concluded that “yield loss estimates are assumed when drought stress occurs for four consecutive days or more (Iowa State University, 2017).” With that being said, it is obvious why we want to reduce the amount of time the crop is under stress to also reduce the potential yield reductions. Each stage of development will be influenced by drought in a different way, so it is important to understand and be able to determine which stage the crop is in and what is going on during that stage that might be influenced by drought-related stress. For corn, the grain filling period is the most critical stage, while for soybean the stages of blooming and pod setting are most influenced so any drought during these stages can reduce yields considerably (Mishra and Cherkauer, 2010).
During the emergence stages, stress is generally from cold soil temperatures rather than drought stress, because early in the season rainfall is at an adequate level (Purdue University). However, when cold and wet weather are in combination, it allows for an environment favorable to increased soil pathogens that may cause disease stress on the seedling.
“Drought stress during the vegetative stages results in reduced stem and leaf cell expansion (Iowa State University, 2017).” This stages is often very important due to the seeds shifting their nutrients stored from the seed to those available in the soil. Purdue University Cooperative Extension Service states that “Light drought stress during this stage may be beneficial to the plant, because it stimulates root growth (Purdue University).” As discussed previously, longer root systems are beneficial later when prolonged drought stress conditions are exhibited. “Any stress that occurs during the sixth to eighth leaf stage (V6-V8) can result in fewer kernel rows, whereas stress from the eighth leaf to seventeenth leaf stage (V8-V17) can result in fewer kernels per row (Iowa State University, 2017).” These stages are obviously important due to the kernel row and number of kernels per row development which is influential to total yield. It is also important to scout for weeds, lodging and diseases during this time, because if any are present especially weeds it could be competing with the already stressed plant for water and nutrient resources (Ciampitti et al., 2017). During the late stages, around V14, it is quite obvious the effects of stress as the plant will develop abnormal ears that can be seen during scouting until flowering (Ciampitti et al., 2017). Drought-related stress during the late vegetative stages can affect smaller plants more drastically than larger plants. During the late vegetative stages are more influenced by increased air temperatures, which could increase ET. Under these conditions, a two to three percent per day final grain yield reduction can be seen (Purdue University).
Effects on corn during the pollination stage is another critical stage that will influence how the corn reproduces and the number of kernels that form per ear. “Drought stress seven to ten days ahead of silking can result in delayed silk development (Iowa State University, 2017).” Hunker defines silking as “the stage when the tassels or corn silk emerges from the ear of the corn. This usually occurs around 55-66 days after the corn seedling emerges from the ground. At this stage, the corn plant is ready to be pollinated (Tun, 2011).” Drought can even result in poor anthesis silking interval (ASI). Stress during this stage increases the time required for pollination, as well as the chance that pollen being shed before the silks emerge (Purdue University). Water stress will not only delay silking, but also reduce silk elongation and if severe, impedes embryo development (Iowa State University, 2017). Water demands during this stage are at their peak and any heat or drought stress will effect final grain yield (Ciampitti et al., 2017). Claassen and Shaw discovered that “Significant reductions in kernel numbers were associated with yield reductions from drought-related stress before or during silking and after pollination (Claassen and Shaw, 1970).” Nutrient stress can also be exhibited during this time of drought-stress due to the uppermost part of the soil becoming dry and out of reach of active root extraction zone (Purdue University). We can conclude that drought-related stress during the pollination, tasseling, and silking stage can be very detrimental to the potential final yield.
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Lastly, the effects of drought-related stress on corn during grain filling stages. Grain filling and maturity occur in the last 50-60 days of the plants growth cycle and any kind of stress during this time can reduce final yield up to three to four percent per day (Purdue University). Iowa State University states that “during grain fill drought stress results in premature death of leaf tissue, shortened grain fill periods, increased lodging, fewer kernels, and light kernel weight (Iowa State University, 2017).” This is damaging to final yield due to light kernel weight affecting the price the farmer will receive for their crop. However, once the crop reaches physiological maturity the reduction in yield is almost un-heard of, so again it is important to understand and recognize the crops developmental stages to assist in determining potential yield reductions and stress in the crop. Purdue University provides statistical yield reduction percentages based on developmental stages that the crop is in: “ten days to two weeks before maturity results in four to five percent yield reduction, three weeks before maturity results in: ten to twenty percent plus reduction in grain quality and one month before maturity results in: thirty-five to fifty percent with grain generally not being marketable (Purdue University).”
Now moving onto the effects of drought-related issues in soybeans. Drought symptoms in soybeans are also exhibited in their leaves similar to corn. “Soybeans respond to drought stress by flipping their leaves over so the underside of the soybean leaf is turned up (Iowa State University, 2017).” However, like any crop under stress there are less obvious signs. One less obvious sign of soybean drought stress can simply be decreased vegetative growth.
Effects of drought-related stress during the vegetative stage in soybeans is often exhibited in the size of the plant and of its root system. “Drought stressed soybean plants are often shorter with smaller leaves due to lack of water, nutrient availability, and nutrient uptake (Iowa State University, 2017).” Similar to what I described in the corn-related effects, the root system is often affected during this stage due to the fact that fertilizer is put in a shallower depths and is out of root reach for uptake to compensate for the lack of nutrients. Soybean root growth actually increases during this time, because the plant is searching for nutrients to continue its growth. Due to this increase in root growth, the plants carbohydrates are shifted to the roots to fuel the growth rather than for storage or plant growth (Iowa State University, 2017). The early reproductive stages (R1-R5) are most sensitive to stress, which will ultimately effect strength and crop growth rate (Pederson, 1945). Pederson states that “R4-R5 is the most sensitive to moisture stress (Pederson, 1945).” While this stage is the most sensitive it does not have an effect on root growth. Hoogenboom and team state that “root growth was less affected by drought after the plants had reached he pod development stage (R4) and finally ceased during seed fill (R5) (Hoogenboom, et al., 1987).” “Under severe drought stress, soybean flowering may occur earlier than normal in an effort to produce seed before premature death (Iowa State University, 2017).” This is the plants strategy to remain viable and still be successful, but the yields will still be quite diminished.
Finally, the effects of drought-related stress on soybean plants during grain filling. This stage is generally characterized by the cell division of ovules and pod expansion (Liu et al., 2003). While drought effects on the plants can cause reduced yields, in soybeans it is generally less severe than corn due to the soybeans overlapping developmental stages (Iowa State University). As stated above, this stage is the most critical for soybeans when involving drought stress. Drought-related stress during this stage is most likely to cause increased number of pod abortions, which is directly correlated with a decrease in final yield numbers (Liu et al., 2003). “Drought can reduce pod number by up to twenty percent as a result of flower and pod abortion. Seeds per pod and seed size can also be affected by drought stress, but to a lesser extent than the number of pods. Drought stress often results in earlier maturity or shortening of grain filling period resulting in lower seed weights and yields (Iowa State University, 2017).” Dominique and their team discovered that “The number of pods per vegetative dry matter unit was significantly affected by stress during pod lengthening. Early stress during seed fill reduced the number of seeds per pod, whereas late stress decreased seed weight (Dominique et al., 2000).” This is why drought stress during this stage is the most critical, because reduce seed weight costs the farmer at the scales at the elevator. Nitration fixation is also an issue during this stage, because during dry conditions nodules cease fixation (Iowa State University, 2017). If the stress related conditions are relieved then the nodules will resume. As stated before, soybeans are marketed by their use of oils. During a stress degree day study, it was discovered that as drought stress increased in total degree days, protein content increased while oil content decreased (Dornbos and Mullen, 1992). It can be concluded that stress under prolonged periods will eventually effect marketability and usability of the crop itself, ultimately affecting the farmers’ profitability in the end.
With all the data and information presented above, we can draw conclusions and management strategies to combat most of these influences. Furthermore we see the consequences of drought-related stress in both crops, soybeans and corn. Both crops experience similar, yet difference drought-related symptoms that later affect final yield. Strategies such as irrigation, drought resistant crops, use of GMO’s, and scouting can reduce the time that the plants are under these conditions (Athar and Ashraf, 1970). Debaeke and Aboudrare state that these six objectives can reduce drought symptoms: “(i) increasing soil stored water at planting sow, (ii) increasing soil water extraction, (iii) reducing the contribution of soil evaporation to total water-use, (iv) optimizing the seasonal water use pattern between pre-and post-anthesis, (v) tolerate water stress and recover after stress alleviation, and (vi) irrigate at the most-sensitive growth phases (Debaeke and Aboudrare, 2004).” It is essential to first determine the remaining yield potential before selecting a strategy if stress also already been induced. Remember that each field is different, even if they’re right next to each other. It is important to know the combination of the soil, hybrid bring used and current water supply the crop has when determining strategies. Iowa State University states that to determine how much potential yield is remaining to perform a shake test on the corn stalk during pollination. “If here is no pollination, there are two options: (1) harvest near to pollination as possible for the highest quality forage possible or (2) leave the crop as a living cover crop until the fall before mowing or chopping (Iowa State University, 2017).” It is essential to know the crops developmental stages to determine the type of stress the crop will be under. During pollen shed and silking yields can be reduced up to nine percent per day and during pollination up to six percent per day. With the listed strategies to consider when drought-related stress is observed, we can reduce the amount of time the crop is under the stressful conditions and hopefully elevate the amount of yield reduction due to the stress and get the crop back into normal, functioning stage.
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