July 2013 UN-L Corn Growth & Development Sessions, pg 1 Corn Growth & Development UN-L Crop Mgmt Diagnostic Clinic, Mead, NE, July 2013 R.L. (Bob) Nielsen, Purdue Univ Agronomy 765.494.4802; rnielsen@purdue.edu Shameless Promotion of a Web Site: URL: http://www.kingcorn.org/cafe Corn Crop Canopy, Light Interception, and Grain Yield Goal: Develop a crop canopy that will capture ~ 95% of the available solar radiation no later than flowering, preferably by ~ 2 weeks pre-silk. Photosynthesis (Ps) captures solar energy, converts it to chemical energy; then uses that energy to create simple sugars from water and carbon dioxide. o Chlorophyll in the leaf chloroplasts absorbs the visible light spectrum. o Water is taken in by the roots and moved to the leaves. o Carbon dioxide is absorbed through the leaf stomata. For corn, leaf area index (LAI) of ~ 4 will capture 95% light. o LAI = Surface area of leaves per unit area of ground. Factors that affect crop canopy development and light capture: o Climate (temperature, rainfall, solar intensity, growing season) o Hybrid maturity (early RMs are typically also smaller plants) o Hybrid size and leaf architecture (erectophile vs. planophile)
July 2013 UN-L Corn Growth & Development Sessions, pg 2 o Plant population (density) o Planting date (early planted crops are usually shorter than late planted crops because stalk elongation occurs during relatively cooler time periods than late planted crops). o Success of stand establishment (planting through ~ V6) o Nutrient & water availability prior to silking. o Early season weed control. o Row width Andrade et al. 2002 concluded ~ 0.72% yield increase per percentage point increase in radiation interception (RI), up to ~ 95% RI (average from other studies ~ 0.54%). However, if 30-inch rows result in a canopy that can capture 95% of incoming light, then narrower rows will likely not be advantageous. Achieving canopy closure earlier than 1-2 weeks prior to flowering is not necessarily advantageous. The importance of full canopy closure lies chiefly with the flowering and grain filling periods. o This is why row spacings narrower than 30 inches do not consistently increase grain yield throughout much of the central Corn Belt. Stresses that directly reduce photosynthetic rates include o Low light intensity during cloudy or hazy days. o Excessively cool or hot temperatures. o Severe drought stress. o Loss of photosynthetically viable leaf area, including severe nutrient deficiencies, leaf damage by hail, leaf death by foliar diseases, & severe drought stress. o Restricted root systems that limit nutrient and water uptake. Strategies to maximize crop canopy begin at planting, continue through the stand establishment period (VE to V6) and the rapid growth phase (V7 to R1).
July 2013 UN-L Corn Growth & Development Sessions, pg 3 Emergence success relies on planter uniformity (seed drop, seed depth, seed coverage) and germination/emergence conditions (soil temperature & moisture, soil tilth, pest pressure). Simple decisions like seed depth can have major impact by targeting seed placement into uniformly adequate soil moisture. Success of stand establishment relies primarily on success of initial root development (soil temperature & moisture, soil tilth, pest pressure). One important step during stand establishment is the transition period that occurs at about V3 when the young seedlings wean themselves off reliance upon the kernel reserves and on to reliance upon the developing nodal root system. Seminal roots play minor role for seedlings. Nodal roots begin to establish at about V1 from nodes near the base of the crown of the seedlings. Success at the V3 transition stage helps assure success of the stand establishment period. Starter fertilizer can play an important role in assuring the success of the V3 transition period. Ears develop very early from axillary meristems located at the stalk nodes. An ear shoot initiates at every stalk node up to and including the uppermost one associated with the harvestable ear. Conceptually, each ear shoot is a replica of the main plant with its own stalk (ear shank); stalk nodes, leaves (husk), and terminal reproductive structure (ear). The uppermost (harvestable) ear is initiated at about V5 at the same time the tassel is initiated at the apical meristem. Once the tassel is initiated, the remaining axillary nodal meristems are hormonally suppressed and never initiate additional ear shoots. Consequently, there are typically 6 to 7 leaves without ears above the harvestable ear.
July 2013 UN-L Corn Growth & Development Sessions, pg 4 It is not uncommon for the potential ear size to be 800 kernels (16 rows x 50 kernels long) or greater. Compare that potential to a typical harvest ear size of 500 to 600 kernels and you can easily see the potential for increasing grain yield if you can only figure out how to preserve the already-existing potential for kernel number. Maximum kernel row number is "set" by about V7. o Each pair of rows initially forms as a single row of branch primordia that eventually divides into a pair of rows. o Governed strongly by hybrid's genetics and requires severe stress to alter. Last year's severe drought stress. This year's stress from soggy soil conditions. Maximum number of kernels per row takes longer to complete and that process is thought to be finished by V12 to V15; roughly 1 to 2 weeks prior to silking. o When not yet complete, a translucent dome of non-differentiated tissue is visible (w/ a hand lens) at tip of the developing cob. o Kernel number per row is less dependent on hybrid genetics and more easily influenced by other yield limiting factors, plus there is simply more time for stress to occur (from V5 to about V15) The rapid growth phase (RPG) begins shortly after the uppermost ear and tassel are initiated. The RPG represents a vegetative period characterized by exponential increases in rates of below- and above-ground growth, nutrient uptake, and water use. The success of the RGP determines not only the potential ear size (kernel #) but also the potential size of the Ps "factory". The RGP can be visualized as that point in the season when the leaves take on a distinct shine, their color darkens to a blue-green, and plant height increases almost exponentially day by day. The consequences of severe stress on root development prior to the RGP become more evident once the crop enters into the RGP. Root development & nutrient uptake become exponential. (Fig. 1) o Lengthy periods of soggy soils quickly deteriorate root health AND increase the risk of nitrate-n loss (leaching or denitrification).
July 2013 UN-L Corn Growth & Development Sessions, pg 5 o Soil compaction (including seed furrow compaction) can severely limit root development, especially if soil moisture is limiting. o Root damage from soil insects, nematodes or diseases also limit root development. o Severe soil nutrient deficiencies, including low soil ph, limit root development. Crop water use similarly increases during the RGP. (Fig. 2) 250 R4 200 Above-ground plant N content (lbs/ac) 150 100 50 V4 V7 V9 V12 V18 R1 0 0 200 400 600 800 1000 1200 1400 1600 1800 2000 GDDs after planting Miller, Eric. 2011. M.S. Thesis, Purdue Univ. Fig. 1. Nitrogen content of above-ground plant material in corn (lbs/ac) from V4 to R4. Northwest Indiana, 2011, 109 day corn hybrid.
July 2013 UN-L Corn Growth & Development Sessions, pg 6 Fig. 2. Long-term average daily (A) and individual year (B) corn water use throughout a growing season. Graphic source: Irrigation Management for Corn. Univ of Nebr Extension pub G1850. http://www.ianrpubs.unl.edu/epublic/live/g1850/build/g1850.pdf Sex in the Corn Field: What Really Goes On Out There? Flowering in corn relies on the developmental synchrony of physically separate male and female flowers; the tassels and the silks. This corny trivia distinguishes the process of corn field sex from that of all other major agronomic crops. Maximum kernel set (# kernels per ear) occurs with short Anther Silk intervals (ASI). In other words, the faster all the silks capture pollen, the more likely that kernel number will be maximized on the ear. Consequently, avoidance or minimizing of severe photosynthetic stress during flowering is essential to maximizing kernel numbers per ear. Female Flower Development Each ovule (potential kernel) develops a silk (the functional style of the female flower) that eventually elongates and emerges from the husk leaves. o Approximately 1000 ovules (and thus, silks) develop per ear. o Silks begin to elongate from the basal (butt) ovules by about V12. o Silk must capture pollen grain in order for ovule to be fertilized.
July 2013 UN-L Corn Growth & Development Sessions, pg 7 o Basal silks elongate and emerge first; tip silks elongate and emerge last. o Silk receptivity is greatest during the first 3 to 5 days of exposure, then slowly deteriorates until no longer receptive about 10 days after emergence. Silks continue to elongate until they capture pollen or until they deteriorate with age. o Severe drought stress can slow silk elongation and delay emergence from husk. Worst case; silks emerge after pollen shed is complete. Some drought tolerant hybrids have been bred to be aggressive silkers; meaning that silk emergence occurs early and rapidly. Other severe stresses, like nitrogen deficiency, often also result in slower silk emergence. Male Flower Development Tassel develops ~ 1000 spikelets (male flowers). o Each spikelet develops 2 florets. o Each floret develops 3 anthers. o Total of ~ 6000 anthers per tassel. Pollen grains develop inside the anthers. o Estimates of pollen grains per tassel range from 2 to 25 million. Obviously very small and difficult to see without magnification. o Susceptible to desiccation within hours of exposure to atmosphere, but can travel on wind currents a long ways before becoming inviable. o Most pollen lands within the source field. Beyond several hundred feet from a source field, the amount of pollen drift decreases to 1% or less. Anthers must emerge (exsert) from the tassel florets before pollen shed can occur. o Anther exsertion typically begins on the central part of the main tassel stalk first and then spreads up and down the main stalk and outward on the tassel branches over subsequent days. o Typically, all the anthers on a single tassel exsert and shed their pollen over a 7-day period. An entire field may take as long as 10 to 14 days to shed its pollen due to normal plant-to-plant variability in development.
July 2013 UN-L Corn Growth & Development Sessions, pg 8 o Small pores open at the tips of the anthers and pollen is dispersed by gravity and wind. o Pollen shed begins once tassels dry from morning dew or rain. Heaviest pollen shed typically occurs through mid-morning, then tapers off throughout afternoon in the heat of the day, especially if hot temperatures. Pollen grains captured by receptive (moist and undamaged) silks germinate within 30 minutes or so. o Pollen is actually captured by the small hairs or trichomes of a silk. o A pollen tube (containing the male gametes) elongates and penetrates the silk, then elongates toward the ovule and fertilizes the ovule (containing the female gametes) within 24 hours. Kernel Set Failure Kernel set failure occurs from either pollination failure or kernel abortion. Pollination failure can occur from: o Severe stress that delays silk emergence and hastens pollen shed (asynchronous flowering). o Silk clipping insects (typically CRW and Jap beetles). o Several days of incessant rains during early days of pollination period. o Older basal silks shading younger tip silks on long potential ears, especially as the older silks begin to dry and form a mat. Within a day or two of successful fertilization, the silk collapses at its basal connection to ovule and soon detaches from the developing kernel. This serves as an early pregnancy test for determining fertilization progress when making an insecticide application decision for silk clipping insects. Kernel abortion can occur through early milk stage from Ps stress of any kind, including drought, heat, leaf disease, nutrient deficiency, hail damage, cloudy weather, warm nights. o Severe Ps stress limits the production of and availability of sucrose to the newly-developing kernels.