Effects of Stress on Corn Throughout the Season

Transcription

1 Effects of on Corn Throughout the Season R.L. (Bob) Nielsen Purdue Univ, Agronomy Dept. West Lafayette, Indiana Web: Purdue Univ 1 Grain yield is the product of the seasonlong development of the four individual components of yield. Plants per acre X Optimizing yield requires optimizing each component Ears per plant X Kernels per ear Purdue Univ 2 Image RLNielsen, Purdue Univ X Weight per kernel Purdue Univ 1

2 The story begins with germination, emergence, & stand establishment Season-long development of yield components Productive # of plants Germ. and emergence Stand establishment V6 The success or failure of stand establishment largely determines the final harvest population unless you replant. Health & uniformity of the stand by V6 has large impact on future potential growth rates & tolerance to stress. Purdue Univ 3 Source of graphic: Nielsen s imagination Image RLNielsen, Purdue Univ Next, ear size determination Season-long development of yield components Productive # of plants Potential # of rows & kernels per row Germ. and emergence Stand establishment Ear size determination V6 V14 Image source: Somewhere on the Internet Minimal stress from ~V6 to ~V14 enables good potential ear size; i.e., # of ovules Source of graphic: Nielsen s imagination Purdue Univ 4 Purdue Univ 2

3 Then, pollination & kernel set Season-long development of yield components Productive # of plants Potential # of rows & kernels per row Actual # of kernels per ear Germ. and emergence Stand establishment Ear size determination Success of pollination Kernel survival V6 V14 R3 Silk+Pollen nick is crucial, especially for silks from upper ovules on ears. Post-silk stress leads to kernel abortion. Purdue Univ 5 Source of graphic: Nielsen s imagination Image RLNielsen, Purdue Univ Grain filling completes the story Season-long development of yield components Productive # of plants Potential # of rows & kernels per row Actual # of kernels per ear Dry weight per kernel Germ. and emergence Stand establishment Ear size determination Success of pollination Kernel survival Grain filling V6 V14 R3 Minimal late-season stress enables maximum deposition of dry matter (starch) in the developing kernels, but earlier growth rates may determine max. potential kernel weight. Purdue Univ 6 Source of graphic: Nielsen s imagination Image RLNielsen, Purdue Univ Purdue Univ 3

4 Kernel black layer closes the book Season-long development of yield components Productive # of plants Potential # of rows & kernels per row Actual # of kernels per ear Dry weight per kernel Physiol. maturity Germ. and emergence Stand establishment Ear size determination Success of pollination Kernel survival Grain filling Kernel black layer V6 V14 R3 Season-long risk of plant mortality and, thus, loss of harvestable plant population Source of graphic: Nielsen s imagination Purdue Univ 7 Because yield is the product of the season-long development of the individual components of yield achieving high yield requires minimizing stress all season long. Purdue Univ 8 Image RLNielsen, Purdue Univ Purdue Univ 4

5 Ear size determination Pollination & kernel set Stand establishment Yield Grain fill period Purdue Univ 9 Effects of Stress Throughout the Season GERMINATION, EMERGENCE, AND STAND ESTABLISHMENT Purdue Univ 10 Image: Purdue, RLNielsen Purdue Univ 5

6 Germination & Emergence (G&E) Seeds imbibe water within the first 24 to 36 hrs after planting into moist soil, which renews the cellular activity of the embryo (germination). Emergence of the embryo through the seed coat occurs in a distinct visual sequence. Image RLNielsen, Purdue Univ Purdue Univ 11 ~ 35 GDD after planting How many days to acquire 35 GDDs? Radicle root ruptures through the seed coat near the tip end of kernel, effectively creating an open entry for pathogens. Important consideration when G & E occurs slowly. Image RLNielsen, Purdue Univ Purdue Univ 12 Purdue Univ 6

7 GDD Calculation for Corn Modified 86/50 Cutoff Method Daily average temperature minus 50 Calculation of average includes limits: Daily high temperature no greater than 86F Daily low temperature no less than 50F Example based on 60F high and 45F low [( ) / 2 ] 50 = 5 GDD for the day About 82 GDDs per leaf collar from VE to V10 After that, about 50 GDDs per leaf collar Purdue Univ 13 Remainder of embryo also enlarging Mesocotyl & coleoptile Coleoptile encases the PLUMULE (4 or 5 embryonic leaves + apical meristem) Radicle root Image RLNielsen, Purdue Univ Purdue Univ 14 Purdue Univ 7

8 ~ 60 GDD after planting Mesocotyl & coleoptile clearly visible Image RLNielsen, Purdue Univ Radicle root Purdue Univ 15 ~ 70 GDD after planting Mesocotyl & coleoptile Lateral seminal roots are now visible Image RLNielsen, Purdue Univ Purdue Univ 16 Purdue Univ 8

9 The mesocotyl is the tubular, white, stem-like tissue that connects kernel and base of coleoptile or crown. Technically, is the first internode of the seedling. Elongation of the mesocotyl elevates the coleoptile toward the soil surface. Purdue Univ 17 ~ 100 GDD after planting Coleoptile/plumule Not quite at VE Mesocotyl Radicle root Lateral seminal roots Image RLNielsen, Purdue Univ Purdue Univ 18 Purdue Univ 9

10 ~ 115 GDD after planting Growth stage VE: Emergence Coleoptile 1 st true leaf emerging from coleoptile Mesocotyl Lateral seminal roots Radicle root Purdue Univ 19 Image RLNielsen, Purdue Univ ~ 115 GDD after planting Growth stage VE: Emergence 1 st true leaf emerging from coleoptile Coleoptile Image RLNielsen, Purdue Univ Purdue Univ 20 Purdue Univ 10

11 Rapid germination & emergence shortens the calendar window of vulnerability to stresses prior to stand establishment. Rapidly developing seedlings are more resilient to stress than slower stragglers. Purdue Univ 21 Soil temperature is the driver Seedling emergence occurs ~ 115 growing degree days (GDDs) after planting. The number of GDDs to emergence is fairly consistent, especially when based on soil temperature, unless other stresses exist. For corn to emerge 7 days after planting requires AVERAGE daily soil temperatures during that time period no less than 66F. Purdue Univ Image: littlebigplanet.com 22 Purdue Univ 11

12 Days from planting to VE Days to Emergence vs Average Soil Temperature 14 Late March planting Assuming soil moisture is adequate, soil temperature is the key driver for emergence. Late Apr Late May Mid Jun 5 Data: RLNielsen, DTC Avg daily soil temperature from planting to VE Purdue Univ 23 Variable seedbed temperatures result in variable Germination & Emergence Variable seedbed temperatures can result from variable Soil color and texture. Seedbed moisture. Seeding depths. Distribution of surface trash in no-till systems. Especially important when soil temperatures are only hovering around 50F (10C). Purdue Univ 24 Image: blogs.mprnews.org Purdue Univ 12

13 Variable G & E can lower yields Young plants 2 or more leaves behind in development cannot compete with older, more established plants once the latter begin their rapid growth phase ~ V6 stage. If the effective population is reduced below the agronomic optimum, yield loss will occur relative to the stand loss. Purdue Univ 25 Manage the risks of cool soils Avoid planting early in fields with poor drainage and/or high residue (slower to warm). Avoid using less than optimum seed quality. Hedge bets by avoiding extremely small seed lots (80,000 kernel bags weighing less than about 40 lbs). Consider extra seed treatments, though limited data to say with certainty. Purdue Univ 26 Purdue Univ 13

14 Chilling injury In addition to simply slowing the physiological processes, soil temperatures lower than 50F can cause outright injury. Imbibitional chilling injury to the embryonic tissue that arrests germination. Chilling injury to the exterior surfaces of the mesocotyl that interferes with normal elongation during emergence. Purdue Univ 27 Image source: Somewhere on the Internet Imbibitional chilling injury Seeds swell in response to imbibition. Under cool temperatures, cell walls become less elastic Can rupture or tear with swelling Causing irreversible physiological damage. Purdue Univ 28 Photo courtesy of J Nagel, CERES Solutions Purdue Univ 14

15 Imbibitional chilling injury Photo courtesy of J Nagel, CERES Solutions One consequence is simply arrested development. (image taken 3 wks after mid-april planting) Purdue Univ 29 Image RLNielsen, Purdue Univ Cold temperatures during emergence can cause Corkscrews & Curly Q s Caused by chilling injury to surface layers of the mesocotyl, but occurring unevenly around the circumference. Purdue Univ 30 Image RLNielsen, Purdue Univ Purdue Univ 15

16 Example of full corkscrew symptom Purdue Univ 31 Image RLNielsen, Purdue Univ Seedbed moisture for G & E Kernels need to imbibe enough soil moisture to bring seed moisture content to about 30% on fresh weight basis. Optimum soil moisture for germination generally considered to be field capacity. Excessively dry soils à Inert seed Excessively wet soils à Inert seed and, eventually, death of embryo Purdue Univ 32 Purdue Univ 16

17 Uneven G & E due to variable soil wetness Purdue Univ 33 Causes of uneven seedbed moisture... Soil variability for texture and natural or artificial drainage. Soil drying patterns due to soil compaction. Uneven distribution of surface trash in no-till systems. Uneven seeding depth. Purdue Univ 34 Purdue Univ 17

18 Good seed-2-soil contact facilitates the initial imbibition of moisture by the kernels Poor substitutes Seed-to-residue Seed-to-rock Seed-to-clod Purdue Univ 35 Variable seed-to-soil contact... Rough, cloddy seedbeds. Uneven distrib. of surface trash in no-till. Hairpinning of surface trash into furrow. Coulters set deeper than seed placement. Incorrect furrow opener adjustment. Incorrect furrow closer adjustment. Excessively fast planting speed. Purdue Univ 36 Purdue Univ 18

19 Delayed or failed emergence Can occur when surface crusts or clods physically impede emergence of the coleoptile (spike) Images RLNielsen, Purdue Univ Purdue Univ 37 Delayed or failed emergence Symptom looks similar to chilling injury to the mesocotyl, but was due to something else Compacted, smeared, seed furrow caused by planting in soils that were a bit on the wet side Purdue Univ 38 Image RLNielsen, Purdue Univ Purdue Univ 19

20 Recognize that successful emergence is not the same thing as successful stand establishment. The success of stand establishment is determined by the success of the initial establishment of the nodal root system AND the transition of the young plants from reliance on kernel reserves to reliance on the nodal root system. Purdue Univ 39 Image RLNielsen, Purdue Univ Until nodal roots are well established Seedlings depend primarily on the energy reserves of the kernel, translocated through the connecting mesocotyl pipeline to the young stalk and leaves. Therefore, a healthy kernel, seed roots, and mesocotyl are absolutely vital until the nodal roots are well established by about V6. Image RLNielsen, Purdue Univ Purdue Univ 40 Purdue Univ 20

21 Damage to kernel or mesocotyl Up to about V3, will kill seedlings. From about V3 to about V6, will stunt seedlings but usually not kill them. After about V6, will have little to no effect. Understanding this helps you determine the approximate time period when stress occurred in a field, which then may help you identify the cause of a stand problem you are investigating. Purdue Univ 41 Seminal (seed) roots originate from a node located within the embryo Consist of radicle root + lateral seminal roots Anchor the seedling Minimal uptake of water & nutrients Cease new growth shortly after seedling emergence Purdue Univ 42 Purdue Univ 21

22 The other root system Nodal roots originate from axillary meristems at each of the stalk nodes. The individual whorls of nodal roots develop sequentially & acropetally over time. One set or whorl of roots develops from every below-ground stalk node plus 1 or more above ground stalk nodes (brace roots). Purdue Univ 43 Image RLNielsen, Purdue Univ Nodal Roots at VE (emergence) Seminal (seed) roots Swelling of 1st nodal roots Nodal roots begin elongating from 1 st node shortly after seedling emergence. Purdue Univ 44 Images RLNielsen, Purdue Univ Purdue Univ 22

23 Nodal Roots at V2 Three to four roots from Node #1 are visible by the V2 or 2-leaf collar stage. Purdue Univ 45 Images RLNielsen, Purdue Univ Roots & their environment Roots develop downward naturally Why? Roots will develop wherever growing conditions are conducive. Favorable temperature Favorable moisture Adequate soil nutrients / ph Absence of physical limitations Purdue Univ 46 Image source: Purdue Univ 23

24 Roots exposed at the soil surface. Why? Drier surface soil = More oxygen Purdue Univ 47 Image RLNielsen, Purdue Univ Growing point of a root is located near the end of the root, just behind the root cap Axillary meristems eventually develop along the length of the root that facilitate root branching. Purdue Univ 48 Purdue Univ 24

25 Desiccated & dead nodal roots Excessively dry and/or hot soils during initial elongation of nodal roots. Purdue Univ 49 Image RLNielsen, Purdue Univ Nodal Root Morphology Purdue Univ 50 Purdue Univ 25

26 Relative positions of nodal roots and their respective nodes Purdue Univ 51 Roots can develop from higher nodes In this example, a result of hormonal changes in response to root lodged stalk angle. Purdue Univ 52 Image RLNielsen, Purdue Univ Purdue Univ 26

27 Seedlings wean themselves from reliance on kernel reserves for sustenance to 3 increasing reliance on nodal roots beginning at about V Purdue Univ 53 Image RLNielsen, Purdue Univ A true transition period Success or failure largely determines whether stand establishment will end with a healthy, vigorous, uniform stand of corn or a crappy stand of corn. Subsequently influences how well the crop will tolerate stresses later in the growing season and, thus, can have major bearing on yield potential. Purdue Univ 54 Image: Purdue, RLNielsen Purdue Univ 27

28 Traditional 2x2 starter fertilizer is a form of crop insurance and can aid young corn plants when they struggle to wean themselves from the kernel reserves to the nodal roots. Yield response admittedly does not always occur. When no yield benefit, crop still uses the nutrients and cost per bushel remains the same. Grain is often drier at harvest and so fewer $$ are spent on post-harvest grain drying. Image RLNielsen, Purdue Univ Purdue Univ 55 Nodal root development In the absence of severe stress, nodal roots will be wellestablished by the 6-leaf collar stage and the plant will have completely weaned itself from support from the kernel reserves. Purdue Univ 56 Image RLNielsen, Purdue Univ Purdue Univ 28

29 Sucessful stand establishment relies on Excellent seed quality Germination ratings Excellent genetic seedling vigor Company ratings Seed protection from insects or diseases Seed treatments Seedbed and soil free of crust or compaction Adequate soil nutrients Starter fertilizer (esp N) Error-free seeding Planter maintenance Planter adjustments Planting speed Adequate & uniform Soil temperatures Soil moisture Seed-soil contact Purdue Univ 57 Image RLNielsen, Purdue Univ The sins of planting will haunt you all season. Ozzie Luetkemeier, former superintendent of the Purdue Agronomy Farm Purdue Univ 58 Bug Scout image: Purdue Univ 29

30 Ear size determination Pollination & kernel set Stand establishment Yield Grain fill period Purdue Univ 59 Ear size determination Yield components develop all season long Productive # of plants Potential # of rows & kernels per row Germ. and emergence Stand establishment Ear size determination Source of graphic: Nielsen s imagination V6 V14 Purdue Univ 60 Image source: Somewhere on the Internet Purdue Univ 30

31 Coincides with rapid growth phase Shortly after V5, a healthy corn field turns the corner and seemingly overnight its growth begins to accelerate as it enters the rapid growth phase (RGP). Above ground Below ground Reproductively Purdue Univ 61 Above-ground dry matter (lbs / ac) Above-ground Dry Matter Accumulation in Corn R1 V15 V10 V8 V4 V Growing degree days since planting Miller et al., M.S. thesis, Purdue Univ. 2010, NW Indiana Purdue Univ 62 Purdue Univ 31

32 Above-ground nitrogen (lbs / ac) Above-ground Nitrogen Accumulation in Corn V15 V10 V8 V7 V4 R1 ~ 61% of season total N Growing degree days since planting Miller et al., M.S. thesis, Purdue Univ. 2010, NW Indiana Purdue Univ 63 The start of the RGP race coincides with the differentiation of the tassel initial from the apical meristem of the stalk AND the differentiation of the uppermost (harvestable) ear initial from its axillary meristem at about V5. and is driven by hormonal changes in the plant that encourage rapid growth. Purdue Univ 64 Image: hdwallpapers.in Purdue Univ 32

33 Meristems = Growing points Physiologically active regions where cell division & cell differentiation occur. The shoot apical meristem (SAM) in corn initiates Primordia for all the leaves except the 4 5 preformed leaves in the embryo. Following initiation of the final leaf primordium, the apical meristem initiates the tassel primordium. Scanning electron micrograph of an apical meristem (SAM) in corn and two leaf primordia (P1 & P2). Source: Tsiantis & Hay Nature Reviews Genetics 4, Purdue Univ 65 Initially, GP is located below ground Purdue Univ 66 Purdue Univ 33

34 While the GP is below ground it is relatively impervious to aboveground damage to the plant. Hail, insect feeding, frost, UAN burn, etc. Purdue Univ 67 While the GP is below ground is vulnerable to below-ground damage from soggy soils, disease, insects, lethal cold temperatures, pounding hail stones, etc. Purdue Univ 68 Injury to sweet corn from lethal cold temperatures prior to emergence, Image courtesy of C Gunter, Purdue Univ Purdue Univ 34

35 Stalk elongation begins ~ V4 Prior to about V4, very little elongation of the stalk tissue occurs. During this time, the above-ground stem consists of visible leaves and rolled-up leaves. Purdue Univ 69 Split stalk of V6 plant Is the growing point label on this image technically correct? By V6, elongation of stalk internodes has elevated stalk sections and tassel above the soil surface. Purdue Univ 70 Purdue Univ 35

36 Split stalk of V7 plant As internode elongation continues, the stalk nodes or joints become increasingly visible. As do the elongating internode sections. Purdue Univ 71 Split stalk of V9 plant The top of the stalk tissue is about 9 inches above the soil surface in this V9 image >>> About 7.5 inches of stalk elongation during two leaf stages of development. Purdue Univ 72 Purdue Univ 36

37 Axillary meristems are initiated acropetally, one per stalk node, in an alternating fashion, beginning at the lowermost stalk node and finishing at about the 7th stalk node below the tassel. Beginning shortly after emergence and ending around the V5 stage of development. #1 #1 leaf Purdue Univ 73 Axillary meristems differentiate into either tillers or ears, depending on hybrid genetics and/or hormone activity Tiller differentiation tends to occur at the lowermost axillary meristems. Ear differentiation tends to occur at the uppermost axillary meristems. Tiller vs. ear is indistinguishable to the naked eye at early stages of development. Sheridan Annual Rev. Genetics 22: Purdue Univ 74 Image RLNielsen, Purdue Univ Purdue Univ 37

38 Tillers / ears are located behind the base of their respective leaf sheaths and can be found even at the lowermost nodes below ground. Remember, the first 4 nodes comprise the triangle of pithy stalk tissue. The #5 node is also usually below ground. Purdue Univ 75 Tillers / ears on a V6 Plant Tiller/ear at stalk node #4 Tiller/ear at stalk node #5 Remember; tillers vs ear shoots essentially indistinguishable to naked eye at early stages. Tiller/ear at stalk node #6 Purdue Univ 76 Purdue Univ 38

39 Ear shoot seniority Ear primordia are formed at multiple stalk nodes, but the upper one or two ear shoots eventually take priority because Of their closer proximity to the uppermost active photosynthetic leaves of the plant. Hormonal apical dominance expressed by the uppermost ears on the lower ones. Damage to an uppermost ear can allow lower ear shoots to develop more aggressively. Purdue Univ 77 Tassel & upper ear are linked The uppermost, harvestable, ear is initiated around V5 at the same time that the tassel is initiated at the apical meristem (Lejeune & Bernier, 1996). No further ears are initiated at upper nodes after the tassel initiates (apical dominance). For typical central Corn Belt hybrid maturities, the node # of the uppermost ear ranges from #11 through #14 on the stalk. Purdue Univ 78 Purdue Univ 39

40 Postlethwait & Nelson (1964) An elegant article on ear development. An organism matures through a series of sequential steps ordered by the genetic complement. The succession of stages in development can be diverted by hormones, surgery, control of nutrition, and by light and temperature. Postlethwait & Nelson, 1964, Amer. J. Bot. 51(3): Purdue Univ 79 Postlethwait & Nelson (1964) There is no point at which a meristem can be considered to be irrevocably determined for a particular course of development. The meristem is, rather, a plastic system which must be programmed at successive intervals. Interpretation: Severe stress can screw up the process. Postlethwait & Nelson, 1964, Amer. J. Bot. 51(3): Purdue Univ 80 Purdue Univ 40

41 Meristems initiate leaves The vegetative meristem (vm) of the main stalk or the ear shank differentiates into leaf primordia (lp) that develop into leaves on the main stalk or husk leaves on the ear shank. Postlethwait & Nelson, 1964, Amer. J. Bot. 51(3): Purdue Univ 81 Tassel forms at apical meristem Ears form at axillary meristems Following initiation of the final husk leaf, the vegetative meristem transitions to become the ear meristem (em) with longitudinal rows of branch primordia (bp) that differentiate acropetally. Postlethwait & Nelson, 1964, Amer. J. Bot. 51(3): Purdue Univ 82 Purdue Univ 41

42 Branch vs. spikelet primordia Each branch primordium differentiates into paired spikelet primordia (sp). Each individual spikelet primordium has its own spikelet meristem (sm) that initiates two glume primordia (g) and then ultimately initiates an ovule. Postlethwait & Nelson, 1964, Amer. J. Bot. 51(3): Purdue Univ 83 Interesting trivia The fact that each branch primordium eventually differentiates into two spikelet primordia explains why kernel row number on an ear of corn is always an even number. Image: Cheng et al., 1983 Amer. J. Bot. 70(3): Purdue Univ 84 Purdue Univ 42

43 Ovule Development ~ V10 Ear meristem Spikelet & glume primordia Branch primordia Paired spikelet primordia Purdue Univ 85 Image RLNielsen, Purdue Univ Potential ear size is determined during the rapid growth phase Maximum row number is set no later than V8 (Strachan, 2004). Row number is influenced more by the hybrid s genetics than by growing conditions. Maximum ovule number per row is determined by at least V15, perhaps as early as V12 (Strachan, 2004). Ovule number per row is influenced less by genetics and so is influenced more easily by growing conditions. Purdue Univ Image: Nebr. Historical Society 86 Purdue Univ 43

44 Not much to look at By V9 (about thigh-high), the uppermost ear shoots and the tassel can be easily dissected. Fraction of an inch long. Visible tassel branches. Ears are mostly husk leaves at this point, yet cob length is about half-way complete. Purdue Univ 87 Severe stress during RGP can directly reduce yield potential by limiting the potential number of kernels (ovules) on the developing ears (no. of rows or no. of ovules per row). Although, our research suggests that ovule formation appears to be amazingly resilient to stress from factors like severe N deficiency and excessively high plant populations. Purdue Univ 88 Purdue Univ 44

45 Perhaps more importantly Severe stress during RGP can indirectly reduce yield potential by stunting the potential size of the photosynthetic factory prior to pollination and thus limiting the potential photosynthate output during pollination and grain filling. Incomplete pollination success Abortion of young kernels Low kernel weights Purdue Univ 89 So, certainly makes sense to minimize the risk of severe stress during the rapid growth phase that would significantly decrease photosynthesis. Excessively wet or dry soils Significant competition with weeds Root-limiting soil compaction Significant nutrient deficiency Significant herbicide injury Purdue Univ 90 Purdue Univ 45

46 Corn leaves change during RGP Image RLNielsen, Purdue Univ Prior to the RGP, corn plants are light green and have dull leaf surfaces. Purdue Univ 91 Once Rapid Growth Period begins Leaves become darker green and shiny. Video RLNielsen, Purdue Univ Purdue Univ 92 Purdue Univ 46

47 Impact of dull to shiny conversion From VE to ~ V4: Cuticular leaf wax (leaf surface) is crystalline in nature low spray retention low leaf wettability herbicide tends to run off leaves. Shortly after RGP begins Cuticular leaf wax changes to a smoother nature greater spray retention greater leaf wettability greater risk of herbicide absorption into corn leaves. Source: Hartzler, Bob Purdue Univ [URL accessed Jan 18] 93 The RGP is a sensitive time period Young, developing ear shoots are physiologically & hormonally very active. Herbicides often naturally move towards physiologically active meristematic areas. Herbicides and/or additives may penetrate upper husk leaf tissue & physically contact the ear shoots (e.g., arrested ears & NIS). Purdue Univ 94 Image: hdwallpapers.in Purdue Univ 47

48 Pinched ear example Image RLNielsen, Purdue Univ Steadfast ATZ (nicosulfuron, rimsulfuron, atrazine) applied to abt V10 V11 corn (label restriction at the time was V6 or 20 inches tall) Purdue Univ 95 Arrested ear examples Status, Durango, AMS, Array (drift control), Fraction (acidifier), and NIS applied w/ 18-inch drops to shoulder-high corn. From 30-40% affected plants. Purdue Univ 96 Image RLNielsen, Purdue Univ Purdue Univ 48

49 Mendelian Genetics & Herbicide Injury Purdue Univ 97 Many RoundupReady hybrids are heterozygous dominant (Rr) for the trait Ovule gametes Dominant allele R r R RR Rr Pollen gametes r Rr rr Recessive allele 25% of the embryos of the fertilized ovules expected to be susceptible to glyphosate Purdue Univ 98 Purdue Univ 49

50 Jumbled kernel symptom AKA bubble kernel symptom caused by death of embryo, while endosperm & maternal seed coat remain alive. Image: Mike Vose, University of Illinois Orr Research Center Glyphosate + AMS applied with drop nozzles to head-high RoundupReady corn. EVERY SINGLE EAR in this field was affected. Yield loss was 25% at the minimum. Purdue Univ 99 Image RLNielsen, Purdue Univ To summarize Potential # of rows & kernels per row Ear size determination The uppermost ear forms ~ V5 at the same time that the tassel is initiated. Max. row # is determined no later than V8. Max. ovule # per row determined by at least V15, perhaps as early as V12. Many herbicide labels restrict app s after certain leaf stages because of the risk of physiological damage to developing ears. Purdue Univ 100 Purdue Univ 50

51 Ear size determination Pollination & kernel set Stand establishment Yield Grain fill period Purdue Univ 101 Next, pollination & kernel set Yield components develop all season long Productive # of plants Potential # of rows & kernels per row Actual # of kernels per ear Germ. and emergence Stand establishment Ear size determination Success of pollination Kernel survival V6 V14 R1 R3 Purdue Univ 102 Source of graphic: Nielsen s imagination Purdue Univ 51

52 A review of corny sex Gravity, wind or human effort transports the pollen to the silks to eventually fertilize the ovules. Pollen produced in the tassel anthers contains the male genetic material. Ovules produced on the ears contain the female genetic material. Purdue Univ 103 R1 Silking stage Silks typically emerge in close synchrony with pollen shed from tassels. First silks to emerge are typically those from basal third of the cob. Silks are most receptive to pollen during the first four days of exposure. Purdue Univ 104 Purdue Univ 52

53 Female flowers of corn Every ovule (potential kernel) on the cob develops a single silk (functional style of the female flower). Approx. 800 to 1000 ovules per ear. Usually 400 to 600 develop into harvestable kernels. Purdue Univ 105 Silk emergence timing Basal (butt-end) silks typically develop & emerge first and apical (tip-end) silks develop and emerge later. Tip silks usually pollinated last; younger tip kernels are more susceptible to abortion under stress. Purdue Univ 106 Purdue Univ 53

54 Pollen capture by silks Pollen grains are captured by the small hairs or trichomes located along the entire length of the exposed silks. Survival of the fastest determines which one fertilizes the ovary. Purdue Univ 107 Pollen & silks A pollen grain germinates within 30 minutes after landing on receptive silk, A pollen tube (containing male sperm cells) develops and penetrates the silk, The pollen tube elongates the length of the silk within 24 hours and fertilizes the egg & endosperm of the ovary. Purdue Univ 108 Purdue Univ 54

55 Pregnancy Test for Corn The base of a silk collapses a few days after successful fertilization of the ovary and detaches from the developing kernel. Pollination success can be estimated by % of silks detached. Purdue Univ 109 Silks elongate until they are pollinated or until they deteriorate with age. Emerged silks initially lengthen from 1 to 2 inches per day, but then slow over the next few days due to natural aging or the inhibition caused by "captured" pollen grains as they germinate and initiate pollen tubes that penetrate the silk and elongate toward the ovule. Purdue Univ 110 Purdue Univ 55

56 If not pollinated early on Silks may become quite long. Earliest emerging silks may deteriorate and become non-receptive to pollen; resulting in blanks on the cob. Later emerging silks may shade earlier emerging silks from pollen; resulting in blanks on the cob. Purdue Univ 111 Symptoms of long silks Purdue Univ 112 Purdue Univ 56

57 Symptoms of long silks Purdue Univ 113 Unusually long silks. Why? Drought stress commonly delays silk emergence & hastens pollen shed. Cool temperatures & ample soil moisture favor prolonged rapid silk elongation. With some hybrids, silk emergence seems to occur 1 to 4 days prior to pollen shed. An unintended consequence of genetic selection for drought resistance (aggressive silking). Purdue Univ 114 Purdue Univ 57

58 Male flowers of corn Image RLNielsen, Purdue Univ A single tassel produces about 6,000 pollen-bearing anthers. Purdue Univ 115 Male flowers of corn About 1000 spikelets form, in pairs, along the length of the tassel. Purdue Univ 116 Purdue Univ 58

59 Male flowers of corn Two florets form per spikelet. Purdue Univ 117 Male flowers of corn Three anthers form per floret, total of six anthers per spikelet. Purdue Univ 118 Purdue Univ 59

60 Corn pollen is nearly microscopic, spherical, yellowish or whitish translucent Purdue Univ 119 Anthers & pollen shed Anthers first appear & shed pollen from the middle of the central tassel spike. Then, slowly over the remainder of the tassel & its branches over about a 7 day period. Whole field takes 10 to 14 days to shed all of its pollen due to plant-to-plant variability for development. Purdue Univ 120 Purdue Univ 60

61 Pollen dispersal Anthers exsert from the florets of the tassel in the early morning hours. n As the humidity drops & temperature rises, small pores develop at the tips of the anthers and pollen disperses into the air. n aka Poricidal dehiscence n Pollen dispersal occurs mostly by gravity & wind, but also by virtue of Insect movement Plant breeders, agronomists Crop scouts, Field inspectors Purdue Univ 121 The rate of pollen shed Usually peaks by mid-morning, then tapers off in the afternoon as temperatures rise. Once dispersed, pollen grains may remain viable for 1 to 2 hrs before they desiccate and collapse. Estimates of total pollen production from single tassel range from 2 to 25 million pollen grains. Purdue Univ 122 Purdue Univ 61

62 Silks deteriorate over time Purdue Univ 123 Kernel set w/ 1-day abstinence Purdue Univ 124 Purdue Univ 62

63 Kernel set w/ 2-day abstinence Purdue Univ 125 Kernel set w/ 3-day abstinence Purdue Univ 126 Purdue Univ 63

64 Kernel set w/ 4-day abstinence Purdue Univ 127 Kernel set w/ 5-day abstinence Purdue Univ 128 Purdue Univ 64

65 Kernel set w/ 7-day abstinence Purdue Univ 129 Pollen & silks mate quickly Purdue Univ 130 Purdue Univ 65

66 Kernel set w/ 1 day of pollen Purdue Univ 131 Kernel set w/ 2 days of pollen Purdue Univ 132 Purdue Univ 66

67 Kernel set w/ 3 days of pollen Purdue Univ 133 Kernel set w/ 4 days of pollen Purdue Univ 134 Purdue Univ 67

68 Kernel set w/ 5 days of pollen Purdue Univ 135 So, flowering is THE critical period for determining grain yield More yield potential can be lost per day of severe stress during pollination than any other developmental period. Drought + heat are especially deadly during pollination. Purdue Univ 136 Purdue Univ 68

69 Daily ET (inches) s 90s Daily Water Use by Corn at Two Temperature Ranges V8 Pollination R Data: Univ of Minnesota Week after emergence Purdue Univ 137 Pollination can fail due to Persistent silk clipping by insects. Silk emergence delay due to drought stress. Desiccation of exposed silks due to heat & low humidity. Photosynthetic stress in general. Herbicide injury to the developing ovules or tassel. Unavailability of pollen. Silk balling. Old age. Purdue Univ 138 Purdue Univ 69

70 Minimal stress during pollination Ensures successful ovule fertilization and maximum kernel set Minimizes risk of kernel abortion Purdue Univ 139 Favorable conditions for pollination Minimal stress from moisture deficits. Stored soil moisture Rainfall or irrigation Moisture conservation by zero tillage Moderate day/night temperatures (86/64F). Plenty of solar radiation. Minimal interference of pollination by silk clipping insects. Rooting profile free of soil compaction or other rooting restrictions. Healthy crop canopy capable of intercepting 95% or more solar radiation. Purdue Univ 140 Purdue Univ 70

71 Ear size determination Pollination & kernel set Stand establishment Yield Grain fill period Purdue Univ 141 Grain filling completes the story Yield components develop all season long Productive # of plants Potential # of rows & kernels per row Actual # of kernels per ear Dry weight per kernel Germ. and emergence Stand establishment Ear size determination Success of pollination Kernel survival Grain filling V6 V14 R1 R3 Purdue Univ 142 Source of graphic: Nielsen s imagination Purdue Univ 71

72 Grain Filling Period Technically begins with silk emergence and ends at physiological maturity. Silking (R1) Blister kernel (R2) Milk kernel (R3) Dough kernel (R4) Dent kernel (R5) Physiological maturity (R6) For much of Indiana, R6 (maturity) occurs approximately 60 days after silking. Purdue Univ 143 The grain filling sequence Begins with embryo development. Including oil, protein Finishes with endosperm development. Starch (dry matter) Purdue Univ 144 Purdue Univ 72

73 Optimal grain fill requires a photosynthetic plant factory capable of harvesting no less than 95% of available sunlight during grain fill. Possible ½ to ¾ percent yield increase for each percentage point increase in sunlight capture up to about 95% capture. (Andrade et al., 2002) Purdue Univ 145 Image RLNielsen, Purdue Univ Maintaining canopy health throughout grain filling is important to maintain the necessary photosynthetic output to maximize both kernel survival and kernel dry weight. Plant nutrition, diseases, insects, temperature, soil moisture. Purdue Univ 146 Purdue Univ 73

74 Temperature during grain fill Purdue Univ 147 Temperature during grain fill Affects the length of entire grain fill period. Warm = shorter (fewer days) grain fill period. Cool = longer grain fill period. Desirable Affects daily rate of photosynthetic output. Warm = More carb s per day. Cool = Fewer carb s per day. Desirable The overall balance favors cool grain fill periods for optimizing grain yields. Purdue Univ 148 Purdue Univ 74

75 Windshield surveys May help spot problems in the field early in the season, but cannot easily assess kernel set success early in the grain fill period. Kernel set failure occurs from a combination of: Pollination problems Kernel abortion during grain fill Source: Purdue Univ 149 R2 Blister kernel stage About 10 to 12 days after silking begins, the fertilized ovules develop into blisters filled with clear sugary fluid. By the end of R2, the embryonic radicle root, coleoptile, and 1 st leaf have formed. Kernel moisture content at beginning of R2 is about 85%. Severe stress can easily abort R2 kernels. Purdue Univ 150 Purdue Univ 75

76 The so-called brown silk stage referenced by fungicide application recommendations Purdue Univ 151 Purdue Univ 152 Purdue Univ 76

77 R3 Milk kernel stage About 18 to 20 days after silking, the developing kernels are mostly yellow and contain milky white sugary fluid. Kernel moisture content at the beginning of R3 is about 80%. Severe stress can still cause kernel abortion. Purdue Univ 153 Purdue Univ 154 Purdue Univ 77

78 R3 Milk kernel stage Purdue Univ 155 Kernel abortion Kernels can abort in response to severe stress during blister and early milk stages of kernel development. Symptoms are shrunken, white or yellow kernels, often with a visible yellow embryo. Purdue Univ 156 Purdue Univ 78

79 Causes of kernel abortion? Primarily severe photosynthesis problems Severe heat or drought Severe nutrient deficiency Severe leaf disease Leaf loss due to hail Severe ECB stalk tunneling Excessively warm nights during or shortly after pollination Consecutive cloudy days during or shortly after pollination Purdue Univ 157 Most commonly occurs at tip Youngest kernels are most susceptible to abortion. Purdue Univ 158 Purdue Univ 79

80 R4 Dough kernel stage About 24 to 26 days after silking, continued starch accumulation in the endosperm creates a doughy consistency to the kernels. Embryo development is nearly complete. Kernels are ~ 50% of mature dry weight. Kernel moisture content at beginning of R4 is about 70%. Purdue Univ 159 Purdue Univ 160 Purdue Univ 80

81 R4 Dough kernel stage Purdue Univ 161 Stress at dough and beyond Risk of kernel abortion decreases once kernels have reached early dough stage. Subsequent severe photosynthetic stress late in the season decreases yield by decreasing kernel dry weight (primarily starch). Drought & heat Corn borer damage Hail defoliation Disease defoliation Stalk rots Early killing frost Purdue Univ 162 Purdue Univ 81

82 R5 Dent kernel stage About 31 to 33 days after silking, nearly all kernels are denting near their crowns. Embryo development is complete. Dry matter content ~ 45% of mature at beginning of R5 and 88% of mature by late dent (half-milkline). Kernel moisture content ~ 60% at early dent stage and ~ 40% by half-milkline. Purdue Univ 163 R5 Dent kernel stage Purdue Univ 164 Purdue Univ 82

83 R5 Dent kernel stage Purdue Univ 165 Kernel milkline By about mid-r5, the kernel milkline becomes visible near the dent end as sugary fluids convert to solid starch and progresses down over time. The half-milkline stage of kernel development occurs approximately 250 GDD or about 10 to 14 days prior to physiological maturity. Grain moisture content ~ 40% Purdue Univ 166 Purdue Univ 83

84 Kernel milkline Purdue Univ 167 Kernel milkline Purdue Univ 168 Purdue Univ 84

85 Kernel milkline Purdue Univ 169 Kernel milkline Purdue Univ 170 Purdue Univ 85

86 Premature plant death Severity of yield loss depends on timing and extent of death. If only leaf death (e.g., light frost) Plant may be capable of remobilizing stored carbohydrates from stalk tissue to the immature ear. If whole plant death (e.g., killing frost) Remobilization not possible. Kernel black layer soon forms. Purdue Univ 171 Yield loss & timing of death 55 Leaves only Whole plant 41 Yield loss (%) Dough Dent Half-milkline Source: Purdue Univ 172 Purdue Univ 86

87 Grain fill stress & stalk rots Stress during grain fill impairs the ability of the photosynthetic factory to produce adequate amounts of carbohydrates to satisfy the plants carbohydrate needs. Cell maintenance & repair throughout the plant, including the root system. Kernel development. Purdue Univ 173 Carbohydrate availability Physiologically, kernel development takes priority over all the maintenance needs of the plants (i.e., kernel development is a strong physiological sink ). Under severe photosynthetic stress, plants will remobilize stored carbohydrates from lower stalk and leaf tissue and send them to the developing ear. Purdue Univ 174 Purdue Univ 87

88 Opens the door for rots The combination of inadequate cell maintenance of root and lower stalk tissue; plus the additional reduction in carbohydrate levels due to remobilization not only compromises the integrity of the roots and stalk tissue, but lowers the natural resistance against the root and stalk rot fungi. Purdue Univ 175 Remobilization, but little rot Purdue Univ 176 Purdue Univ 88

89 Remobilization and rot Purdue Univ 177 Photosynthetic stress & rots Defoliated at R1 on 13 July Defoliated at R3 on 27 July Photos taken on 19 August Purdue Univ 178 Purdue Univ 89

90 Grain fill stress & Kernel BL Half-milkline stage on plants not stressed during grain fill. Premature BL resulting from severe defoliation at R3. Photos taken same day. Purdue Univ 179 Kernel black layer = Physiological maturity The kernel BL effectively closes the door for any further movement of photosynthate into the kernel. Moisture at R6 averages 30% and ranges from 25 to 40%. Purdue Univ 180 Purdue Univ 90

91 Kernel black layer always forms Kernel BL forms on time when grain fill occurs with minimal stress. Kernel BL forms prematurely when severe stress during grain fill causes early death of the plant (e.g., drought, leaf disease). Kernel BL eventually forms following lethal frost or freeze injury to immature corn plants. Purdue Univ 181 Grain moisture loss occurs slowly throughout the entire grain filling period 80% Grain moisture content (%) 70% 55% 40% R3 R4 R5 Half-milkline Purdue Univ 182 Purdue Univ 91

92 Important drydown factors Temperature, humidity, sunshine, wind. Consequently, calendar date of physiological maturity greatly influences rate of subsequent grain drydown. Hybrid characteristics Image RLNielsen, Purdue Univ Purdue Univ 183 Hybrid characteristics & drying Kernel pericarp characteristics Husk leaf number Husk leaf thickness Husk leaf senescence Husk coverage Husk tightness Ear declination Grain moisture loss occurs primarily through the kernel pericarp; not through the pedicel and cob. Image RLNielsen, Purdue Univ Purdue Univ 184 Purdue Univ 92

93 Examples of drydown rates Purdue Univ 185 Ear size determination Pollination & kernel set Stand establishment Yield Grain fill period Purdue Univ 186 Purdue Univ 93

94 Web: Purdue Univ 187 Herbicide label language Up to 20-inch corn Up to 6 collars 30-inch or 8-leaf corn 4 to 20-inch corn Up to V8 corn V1 to V6 stage Apply through layby stage Avoid spraying into the whorl or leaf axils Purdue Univ 188 Herbicide label screenshots from labels Purdue Univ 94

95 Plant height restrictions 1. Large corn canopies intercept more of a broadcast-applied herbicide targeted weeds intercept less and so weed control will be less effective. Image RLNielsen, Purdue Univ Purdue Univ 189 Plant height restrictions 2. Greater interception of broadcast herbicide by a large corn canopy also increases the risk of herbicide injury to the corn crop. Purdue Univ 190 Image RLNielsen, Purdue Univ Purdue Univ 95

96 Label Language Fuzziness Often, label language is not clear How plant height should be measured or What is meant by a particular leaf stage. Purdue Univ 191 Corn Plant Height Most agronomists agree that corn plant height = that of freestanding plants. Height from the soil surface to the arch of the uppermost leaf that is at least 50% emerged from the whorl. Herbicide Labels? Usually not clear whether plant height refers to free-standing plants Purdue Univ 192 Image RLNielsen, Purdue Univ Purdue Univ 96

97 Leaf staging in corn Corn leaf staging is technically quite simple and straight forward. All it requires is the ability to correctly identify the parts of a leaf and the ability to count. Image source: Purdue Univ 193 Leaf staging method Purdue Univ 194 Logo: Purdue Univ 97

98 Three distinct morphological leaf parts Purdue Univ 195 Leaf collar staging method begins with the lowermost leaf that is shorter than the others and has a rounded tip. Some uncertainty whether herbicide label growth stage definitions include 1 st true leaf. Purdue Univ 196 Purdue Univ 98

99 Leaf collar staging method ends with the uppermost leaf with a visible leaf collar. Purdue Univ 197 Staging older plants can be difficult once lower leaves have disappeared Yet, even the missing lower leaves must be included. Purdue Univ 198 Purdue Univ 99

100 Once stalk elongation begins to occur, individual stalk nodes become visible Purdue Univ 199 The first recognizable individual node on a split stalk is usually Node #5 Purdue Univ 200 Purdue Univ 100

101 Once Node #5 is identified Identify the leaf sheath that connects to that node, then count upward to uppermost leaf with visible leaf collar to determine leaf stage. Leaf sheath attachment to stalk node Purdue Univ 201 Image: RLNielsen The leaf stage for an entire field is equal to that of the majority of the individually staged plants. E.g., if 51 of 100 sampled plants are V10, then the field is defined as V10. Purdue Univ 202 Image RLNielsen, Purdue Univ Purdue Univ 101

102 Phenology can be Phrustrating From the perspective of herbicide label restrictions, how should one deal with variable leaf stages across a field? Let s say label restriction = Apply up to V8 If 15% of the sampled plants are V7 and 55% are V8 and 30% are V9, should you spray? Purdue Univ 203 Image RLNielsen, Purdue Univ To spray or not to spray? Unofficial OISC response... No one has asked that question before, but the majority Vstage probably prevails. Some industry colleagues responses Variable Vstages are more important than variable heights relative to label. When was the field staged and when will the applicator arrive to spray? Depends on corn sensitivity to chemical. What is the cost of being wrong? Purdue Univ 204 Image: RLNielsen Purdue Univ 102

103 Monitor leaf stages by Visiting fields regularly prior to herbicide applications to document leaf stages. Predicting leaf stages at your computer using dates of planting (or better yet, emergence) and accumulated numbers of Growing Degree Days (aka heat units). Generally about 82 GDDs per leaf collar appearance up to about V10. DISCLAIMIER: Actual rate can vary among hybrids and growing conditions. Purdue Univ 205 Image RLNielsen, Purdue Univ Online options to track leaf stage U2U Corn GDD DST tool Agrible s Morning Farm Report Others I ve not personally used DuPont Pioneer Encirca Yes Becks FarmServer GDDs, but no leaf stage AgriReliant Genetics Advantage Acre? Climate Fieldview? Images from respective source URLs Purdue Univ 206 Purdue Univ 103