ISSUE 10 July 9, 1998
CORN GROWTH AT CRITICAL STAGE
The 1998 corn crop will soon be reaching a critical stage - yield determination. The tassel is initiated at 8 to 10 leaves (usually knee-high). The growing point has now moved above ground and suckers may be developing. At this time, corn moves into a rapid growth stage by elongating the area between leaves on the stalk, called internodes. A typical corn plant will have 16 leaves that unfurl at a rate of 2 per week, taking about 8 weeks. At this time, the nodal root system is taking over and nutrient deficiencies are less likely because of the distribution of the roots in the soil.
Over the next 7 to 10 days, the number of ear shoots will be determined. After this is complete, the number of rows per ear is set. The ear will continue to lengthen until 1 week before tasseling. The maximum number of kernels per row is also determined. So, over this 2 to 3 week period the corn has set the number of ear shoots, the number of rows per ear and finally the number of kernels per row. Its easy to understand why the next 2 to 3 weeks is considered critical in the life of a corn plant and why any deficiencies or stresses can dramatically reduce yield.
With the setback by frost in late May, corn maturity is a big concern on farmers minds. Corn is usually physiologically mature (has reached black layer) by 8 weeks after pollination. Its interesting to note that the difference between the length of the growing season required by an 80-day hybrid and that of a 100-day hybrid is more in the number of days from planting to silking, than from silking to black layer. Thats why we plant our latest hybrids first and earlier hybrids last.
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 1950s 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 1930s 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
ESTIMATING YIELD POTENTIAL OF WHEAT/DURUM
Estimating yield in wheat, or any crop, is done based on the components that compose yield and only determines potential yield; crop yield is determined after harvest. Small errors in counting the components that contribute to yield can result in large errors in the yield estimate.
Yield in small grains is the sum of three components: i) the number of heads in a unit area; ii) the number of kernels that are produced on a head; and iii) the weight of each of those kernels. Kernel weight can not be determined until harvest so a historical average must be used.
bu/A = (hds/3ft X
spikelets/hd X kernels/spikelet X 0.142)
row spacing (inches)
hd(s) = head(s)
Conversion factor that incorporates area, kernel weight and volume
Using the formula:
1. Determine the number of heads in three feet of row. Small heads with two or three kernels should not be counted; they will result in biased high yield estimate.
2. Determine the number of spikelets per head. This should be an average of six or more randomly selected heads. Top and bottom spikelets contribute little to overall yield and should not be counted. Undeveloped spikelets should not be counted.
3. Determine the average number of kernels per spikelet. Hand thrash the heads used to determine spikelet number and divide total kernels by total spikelets for an average kernel per spikelet number. Small errors in this number result unrealistically high yield estimates; consequently, using a fixed number that accurately reflects long term yield trends is often best. Usually 2.3 gives the most accurate results, or 2.1 when the crop has been stressed.
4. Finally determine the drill row width. When unknown simply measure the distance between several rows of plants and use the average. Most double disc drills are set at 6, 7, or 8 inch row spacings. Air seeders place seed in bands that can range from three to five inches wide; in this case the band width plus distance between bands is used. Measure several rows from the left side of the band to the left side to determine the width.
As illustrated yield estimates are subjective. The correlation between an estimate and the final yield is related to how close head and kernel counts are to the real numbers and crop development during the remainder of the growing season. Keep in mind that no field is uniform and the yield potential varies tremendously within a single field. To get accurate counts the process should be repeated several times, not less than eight per field.
Michael D. Peel
NDSU Extension Agronomist
THE BIRDS AND BEES REGARDING TREE FRUIT SET
There are reports that apples and crabapples flowered this year, but did not set any fruit. What is the problem? Simple answer - the lack of availability of pollen and its transport to the flowers!
First, a basic sex education in getting tree fruit to set. Pollen from a flower must be carried by insects to the stigma of another flower. The pollen germinates, grows down into the ovary where sperm in the pollen tube fertilize the ovules. If pollen is not transferred to the stigma of a flower, or if fertilization (pollination and fertilization are two separate steps!) does not take place, the flower yellows and falls, and no fruit will develop. If fertilization does take place, the ovary or fruit soon starts to swell, possibly developing into mature fruit. Any factor that influences the supply of pollen, the growth of the pollen tube, or the transport of pollen to the stigma, will prevent fruit set.
Since apples, pears, and plums are the major fruit bearing trees in our region, some knowledge of whether or not these trees are self-compatible or not is necessary. In checking this out, it is found that the degree of self-compatibility of these 3 species is very low - 4% or less. This means that the pollen from that variety or cultivar cannot serve as a source of pollen for fertilization. This is why if you purchase a Hazen apple for your yard, it is highly suggested that another variety such as Norland be planted nearby. The exchange of pollen between these two varieties will then be a major step toward good fruit set.
But the pollen must be carried between trees. Honey bees and bumble bees are the major means of pollen transport in their quest of foraging for nectar in the flowers. Bee activity is high on warm, sunny days, and sufficient pollen transfer can be accomplished in one day or less. If the weather is rainy, windy, or the temperature is low, bees do little work. If these conditions extend through the entire bloom period, insufficient pollen is exchanged and fruit set ends up being light, if it occurs at all.
With the spring and summer weather weve had thus far, it is a small wonder that any fruit set took took place at all! Those that have a crop of apples, pears, or plums developing on their trees will definitely be in the minority this year. If there is an over-abundance coming up, consider marketing the excess - those of us without local tree fruit may be willing to do some bargaining!
NDSU Extension Horticulturist & Turfgrass Specialist