ISSUE 9  June 29, 2000

    Sunflower growth and development responds to heat units similar to corn and several other crops. The base temperature of 44F is used to determine Growing Degree Days (GDD). GDD formula = [(daily maximum temperature + daily minimum temperature)/2] - 44 degrees F.

    In the table below research data was collected at the Carrington Research and Extension Center over a number of years on sunflower stage of development and heat units.

    Oil sunflower development by days and growing degree day (GDD) units, 1994-95, Carrington Research Extension Center*

    Planted on May 25 in 1994 and May 23 in 1995. Averaged over 5 hybrids each year and 5 plants per hybrid per plot were observed.

    GDD units/data for 2000 can be found on NDSU’s Extension Web site under Ag weather (NDAWN). Just click on The Daily Observation tables and maps section.

TILLERS IN CORN

    When farmers see extensive tillering in their corn hybrids, they often express concern that the tillering will have a detrimental effect on crop performance. This was the prevailing view in the early 1950's when it was widely believed that tillers if allowed to develop would "suck" nutrients from the main plant and thereby reduce yields. As a result, tillers were more frequently referred to as "suckers" and many farmers actually walked their corn fields to remove tillers. However, since then research
has shown that tillers usually have little influence on grain yields and the effects they do have are generally beneficial. So what causes tillers to appear?

    Tillers are lateral branches that form at lower, below ground nodes. Although tiller buds form at each below ground node, the number of tillers that develop is determined by plant population and spacing, soil fertility, early season growing conditions, and the genetic background of the hybrid. Nearly all hybrids will take advantage of available soil nutrients and moisture by forming one or more tillers where stands are thin in the row or at the ends of rows. Tillers are most likely to develop when
soil fertility and moisture supplies are ample during the first few weeks of the growing season. They are usually visible by the 6-leaf stage of development. Hybrids with a strong tillering trait may form one or more tillers on every plant even at relatively high populations if the environment is favorable early in the growing season.

    Do tillers deprive the main plant of nutrients?

    A number of research studies have been conducted to determine "tiller-main plant relationships." Defoliation experiments in the 1930's revealed that defoliated plants that had tillers yielded nearly twice as much grain as defoliated plants that had no tillers. These results suggested that a connection existed between the tiller leaves and the main plant that allowed sugars produced in the tiller leaves to be moved to the ears on the main plants.

    More recent research reports have found that there is little movement of plant sugars between the main plant and tillers before tasseling. However, after silking and during grainfill, a substantial amount of plant sugars may move from earless tillers to ears on the main plant. When there are ears on both the tiller and the main plant, little movement of plant sugars occurs. In this case the main plant and tiller act independently, each receiving sugars from their own leaves. The nubbin ears, which tillers may
produce, therefore have no impact on the ear development of the main plant as was once thought.

    Should tillering be ignored?

    If a particular hybrid shows excellent yield potential and also produces extensive tillers under some growing conditions, it should not be avoided. However, excessive tillering may indicate problems with stand density and plant distribution within the row. If tillering is associated with row gaps and less than optimal plant populations, these are the conditions which need to be corrected to ensure optimal yields rather than selection of the hybrid. Tillering is also caused by the fungal disease "crazy top," which also produces a range of other symptoms. Such tillering is a disease symptom and not beneficial to plant performance.

Duane R. Berglund
NDSU Extension Agronomist
dberglun@ndsuext.nodak.edu
cell number 1-701-799-7873

ONE DAY YOU WILL WIND UP REMOTE SENSING

    Recent research out of Mississippi State evaluated the accuracy of remote sensing for weed detection in soybeans. Three weed species (sicklepod, pitted morningglory, horsenettle) were selected and population counts were run across two fields.

      Seedling weed populations were sampled on a regular grid coordinate system and two days after these on-ground counts multispectral digital images of the fields were recorded by an aircraft-mounted, four-band charge-coupled device (CCD) array camera. These pictures recorded multispectral digital images of the two fields, each with multiple plots, within the test. The pictures were taken with a one meter resolution with the CCD sensor sensitive to the visible green, red and near infrared spectrums. Each image was linked back to the weed populations collected on ground.

    The weed seedlings were 5 to 10 centimeters tall with one to six true leaves and the crop, soybeans, was 10 to 20 centimeters tall with four to eight true leaves during the study so that the staging timing coincided with the latest possible time to expect maximum performance from postemergence herbicides. Threshold levels were established in multiples of five to evaluate the weed populations in both the ground counts and for the estimations from the aerial photography. Stepwise discriminant
analysis was used to determine the reflectance bands that best ascertained each weed species and threshold levels from the aerial photography.

    Generally, all three weed species could be estimated with at least 75% accuracy. Sicklepod and pitted morningglory patches of ten or more weeds could generally be classified with at least 85% accuracy. Even with highly variable soil conditions in the fields and their variable reflectance properties as well as that of the surrounding soil and vegetation, the accuracy was good on estimating weed populations but not as well defined in estimating weed-free areas. Still, one field correlated the discriminant analysis correctly 84% in the weed-free areas and 75% correct on pitted morningglory infestations with 15 plants per meter squared or higher. Remote sensing technology has potential for weed detection. Treatment of only weed-infested areas could dramatically reduce herbicide usage. Linking remote sensing with weed mapping which then can be fed into a yield loss prediction/herbicide recommendation model and could then pinpoint areas infested above a determined economic threshold.
Data maps linked by global positioning would then allow herbicide delivery to the weed infested areas. With further advancements such as rapidity of image analysis, this prescription farming could be the wave of the future.

IN THE AFTERMATH OF WATERLOGGED SOYBEANS, MAP YOUR PLANS

    Crop scouting all fields and throughout each portion of each field will allow you to determine what action, if any, should be taken. The purpose of field scouting is to obtain an objective summary of the field situation whether it be flood, drought or a pest situation. Observations now may help in knowing what to expect in areas of the field now as well as next year. First, select observation stops within a field on a representative basis. Try to develop random patterns each time you observe a field. Be
aware that fields recently under any type of stress are more prone to future problems. Be aware that many pests can invade from field borders. Walk to within 50 feet of every border. If a pest problem is identified, determine if it is isolated to the border area or is more widespread and what pattern any pest invasion is taking. Randomly select sampling sites if looking for pests or checking plant health. You might select each new site within a field by stopping in a spot that is 40 steps ahead (or to the left or right) of the previous site. Even while focusing on this task, scan the field when walking from one area to another in order to notice any topographic or unusual crop conditions that might herald more intensive checking.

    Troubleshoot fields by first looking for patterns of occurrence within the field. Identify areas where the problems occur and where they are absent. Problem areas with sharply defined boundaries often are related to field operations or equipment. Problems that correspond to the topography of a field or to soil type are often soil related or define environmental stresses such as flood, low areas, or erosion paths. Problems that are worse on one side or edge of a field could be related to spray
drift or to insect movement. Some problems such as root and stem rots can occur on isolated plants. When scouting, learn to identify plant symptoms. Crop plants often are very cause revealing by the symptoms exhibited. Disease, environmental stresses, nutrient deficiencies, insect damage, chemical damage and poor growth symptoms can often be seen. Multiple symptoms can often be diagnosed and the step-wise progress from one problem to another determined if the plant is observed at the correct time. Remember too that although the top portion of the plant reveals much about the plant health, the roots can also provide useful information. Plant samples including the roots from suspect problem areas of a field may cause the loss of a plant but may save you yield if the root symptoms provide a clue to the problem developing in the field and if action should be taken.

    After a problem develops in a field, that field should be checked more frequently. Often fields are checked every week, but a field that has been under stress should be checked every one to three days. The later an additional problem is noticed, the more difficult it is to diagnose or possibly correct the problem.

    Soybeans are a difficult crop in which to determine economic thresholds for control measures as they have a relatively great ability to compensate for problems. Small gaps in the rows may be filled in by adjacent soybean plants that branch and compensate. Plants that are hit by hail or chewed by deer often, if at least one of the many growing points remain, can once again develop one or more stems. Due to the determinant varieties planted in our area, the soybean plant modifies growth to
parallel development with the night length, hastening toward maturation with the advance into shorter days. This crop plant retains that survival skill of throwing redoubled effort into the attempt to produce mature seed before the growing season ends. Even with this compensation ability, field scouting this rotational crop is important.

Denise McWilliams
Extension Crop Production Specialist
dmcwilli@ndsuext.nodak.edu