Hoping For Rain: Crops At Mid-Season 2020


for both corn and soy- bean crops in Illinois was close to normal in 2020, heavy rainfall in May and again in early June in some places this year led to some replanting, and some ponded areas don’t have a stand. Stands are mostly good otherwise, but crop condition rat- ings in Illinois have been mediocre so far, with per- cent good + excellent per- centage in the low 60s for both crops in early July. Conditions in states to the east of Illinois are no better, while crop condi- tions in states west and northwest of Illinois are mostly very good. After wet soils earlier,


W


the dry weather in June helped crops to establish and develop root sys- tems. But in the parts of the state missed by rain in recent weeks, some fields are showing signs of lack of water as soils continue to dry out. The US drought map released on July 9 shows 20 per- cent of Illinois to be “ab- normally dry,” mostly centered around Peoria and with an area in southeastern Illinois. A small area of “moderate


DR. EMERSON NAFZIGER URBANA, ILL.


hile the record will show that planting progress


drought” now appears east of Peoria. Much of the rain that has fallen in recent weeks is from “popup” thunderstorms, which means that most places get none and a few small areas get a lot. While watching weather radar can help pass the time, watching as storms “just miss” may not be great for blood pressure or state of mind. As an example, three of the 17 WARM stations in Illinois reported rainfall on July 8, with amounts of 0.01, 0.02, and 0.04 inches. Roots need to grow out


into the soil to gain full access to the water in the soil, and late planting and soil compaction both decrease root growth. This can limit


the


amount of water taken up, even as leaf area in- creases along with the need for water. So we’re seeing corn leaves curl- ing in the afternoon in spots where soils are more compacted. Soy- bean leaves don’t curl but they tend to wilt and take on a grayish cast when there’s not enough water. We’ve had some reports of potassium de- ficiency in both crops, caused by restricted root growth. Water loss by evapora-


tion slows as the surface soil dries and also as the crop canopy develops to shade the soil and lower wind velocity at the sur- face. Once there’s a canopy, water taken up by roots moves through the stem and out through the leaves as water vapor by the process of transpiration. The driving force for


transpiration is “thirsty air” that acts to pull water vapor out of the leaves during the day though tiny openings called stomata. This helps to cool the leaves, and it allows carbon dioxide to move into the leaf to provide the start- ing material for photo- synthesis. Water loss is the price the plant pays for taking in carbon diox- ide needed to form sug- ars using


sunlight


energy. As the crop reaches full canopy dur- ing


late vegetative


growth, the rate of water use by the crop reaches a maximum. Water


movement


through the leaves in- creases as the canopy develops, and once corn is approaching tasseling, it can move as much water through the leaves as would evaporate from an open surface. So corn planted before mid-May that is now at or ap- proaching pollination is moving as much as a quarter inch of water every day from the soil to the air. Water use rates increase as temperature, sunlight intensity, and wind speed increase: even with low wind speeds, use rates have been high in recent weeks. Low relative hu- midity increases water use rates as well, but on a warm, humid afternoon with RH numbers of 50 to 60 percent, water loss rates are still high, espe- cially if it’s breezy. Many Illinois soils can


hold 8 to 12 inches of plant-available water for the crop, as long as the crop has a good root sys- tem to take water up. But as water loss contin-


ues and the soil dries, it gets increasingly difficult for the plant to take up water as fast as the crop uses it during the heat of the day. Stomata close at night, giving plants the chance to rehydrate. But as the water near the roots is taken up and as temperatures rise during the day, water uptake starts to lag behind crop demand for water, and we start to see stress symptoms develop. As water deeper in the soil begins to deplete, soil cracks will appear, caused by shrinkage of clay as the soil dries. Predicting the effect of


inadequate water on crop yields is not easy, espe- cially as genetic selection in recent years has im- proved the ability of both corn and soybean to set good numbers of seeds even under stress. Many fields even in dry areas have good canopy color and look good in the morning, before transpi- ration starts to exceed the ability of the crop to take up water


fast


enough. Except perhaps in the driest areas, plants now at or ap- proaching


pollination


should be able set good kernel numbers, thereby setting up good yield po- tential.


Later-planted


corn is likely to have more difficulty keeping photosynthetic


rates


high enough to maximize kernel set unless rain falls. Soybeans are having


difficulty in producing good canopy cover and good growth in dry areas. Plants in many fields are staying short, with leaves not expanding very fast. These fields haven’t used


as much soil water up to now as early-planted corn, so there’s still soil water available, but root systems of


stressed


plants may not be able to reach this water very well. Warm days and nights have brought on flowering in many fields, but with limited node numbers now,


flower


numbers may not in- crease as quickly as they normally would. Soybean plants need


enough water to make good growth over the coming weeks in order to produce and retain enough pods to set up high yield potential. The podsetting process gets underway 15 to 20 days after flowers appear, and it lasts for several weeks. This means that that any improvement in condi- tions – lower tempera- tures and more rainfall – that takes place over the next few weeks can help to increase soybean yield potential. Soybean vari- eties bred for high yield tend to set pods aggres- sively, but if stress due to inadequate water last for several more weeks, more flowers will abort and fewer pods form than we need to produce yields like those in recent years. High summertime tem-


peratures – both daytime highs and nighttime lows – have a negative effect on corn yield in Illinois, while July rainfall is very beneficial. None of those factors is pointed in a positive direction at the moment, and if that con- tinues, we can probably expect below-trendline yields. Illinois trendline corn yield for 2020 is about 190 bushels per


acre, so yields can still be good, but it will take more rain to get us there, and sooner will be better than later. The extended period of pod formation during which soybean yield potential gets estab- lished means that the “day of reckoning” comes a little later for soybean than for corn. Other than having crop


insurance in place and running irrigation sys- tems if that’s an option, there is little we can do in terms of management to mitigate yield losses from dry weather. Check fields for mites and insects in- cluding corn aphids, and check for SCN damage in soybeans, even though there isn’t much to do other than to note this for next time. In recent years some


have promoted foliar fun- gicide as a way to lower stress effects in both corn and soybean. While strobilurin fungicides can decrease respiration and increase net photo- synthesis for a week or two after application, they don’t increase yield consistently. They cer- tainly won’t counteract severe lack of water, and there’s little evidence that they will pay their cost under moderate stress. They should be used for managing dis- eases, not stress. Some plant growth regulators and other products are also sold as ways to counter stress. There is little evidence that any of these work, at least con- sistently, to do that. ∆ DR.


EMERSON


NAFZIGER: Research Ed- ucation Center Coordina- tor, Professor, University of Illinois


Midseason Soybean Checks Can Inform Next Season mid- to late-season


product selection and management approaches for next season. As soybeans enter the


F 6• MidAmerica Farmer Grower www.mafg.net / July 17, 2020


JOE BUNCK EVEREST, KAN.


armers can gather field intelligence now to inform and hone


growth stages, farmers may notice yellowing leaves, called leaf chloro- sis, which can indicate low fertility, disease or re- stricted root growth. To determine a cause, evalu- ate fields for any condi- tion that c a n jeopard- ize nu- t r i e n t uptake by the roots such as soil com- paction, water-saturated soils or insect feeding. Some soybean diseases like sudden death syn-


drome and brown stem rot can also resemble nu- trient deficiency symp- toms. Throughout the growing


season, soybean plants acquire nutrients from the soil and through ni- trogen fixation in the root nodules. Pod fill is a pe- riod of high nu- t r i e n t d e -


mand, and if the soybean plant cannot absorb needed nourishment to support seed develop- ment the plant will remo-


bilize nutrients from the stems and leaves. As nu- trients are reallocated away from the leaves they tend to yellow, which is an indication of nutrient deficiency. Channel


Seedsmen


conduct field evaluations to help farmers determine what conditions may be causing soybean leaf chlorosis. They assess the agronomic needs on farmers’ fields to help plan a fertility program for the next season’s crop and recommend seed products that are an ideal match for the anticipated


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