NDSU Crop and Pest Report

Plant Science


ISSUE 14  July 31, 2003

PRE-HARVEST DRY DOWN IN SMALL GRAINS WITH GLYPHOSATE

The early planted small grain crops in the state are beginning to mature. There are a number of herbicides labeled for pre-harvest use in small grains (except oats) for weed control and as harvest aids (see the 2003 North Dakota Weed Control Guide for a complete listing). Glyphosate, however, is the only labeled herbicide that can be used to aid in the dry down of the crop itself (as opposed to controlling and drying down weeds in the crop). Glyphosate is a systemic herbicide and takes from 7 to 10 days to effectively kill the growing parts of the crop so the dry down process is not immediately visible. Some of the potential advantages of using a pre-harvest dry down application of glyphosate are: allows earlier combining, eliminates the need for swathing, enables faster and easier harvesting and promotes more uniform ripening. Whether or not you will realize any or all of these potential advantages will largely depend on the climate and the characteristics of your crop at the time of application.

Consider the following when using glyphosate for pre-harvest dry down in small grains:

Joel Ransom
Extension Agronomist - Cereal Crops
joel.ransom@ndsu.nodak.edu

 

SWATHING AND HARVESTING CANOLA

Canola fields are rapidly nearing the stages for swathing as the warmer temperatures during the past week has greatly hastened the ripening process. Swathing canola at the optimum stage of ripening reduces green seed problems and seed shatter losses, and ensures the quality required for top grades and prices. Field inspections should be made every other day when there is some color change in the first formed pods on the bottom of main stem. Canola seeds within the pod will change color an average of 10 percent every 2 to 3 days. Under hot conditions, seed color changes can be very rapid.

Examine only those pods on the main stem. Seeds in pods on the "bottom third" of the main stem were formed earlier and will turn color much sooner than seeds in the pods on the top third of the plant. When overall moisture content of seed from the total plant averages 30 to 35 percent, about 30 to 40 percent of the seeds in

pods on the main stem only will have changed color or have started to change color. Seeds with only small patches of color should be counted as color changed. Remember, the color of the seed is more important than the overall color of the field in determining the stage of maturity.

Most of the seeds that have changed color will be from the bottom third of the stem. When seeds in the bottom pods slightly turn color, seeds in the top, last-formed pods are filled or nearly filled. At this time, most of the seeds will be firm and roll, as opposed to break, when pressed between the forefinger and thumb.

Seeds in all pods on a plant complete filling (physiological maturity) at about 40 percent moisture and then slowly turn from green to light yellow, or reddish brown to brown depending of variety. Once filled the seeds rapidly lose moisture at about 2 to 3 percent or more per day, depending on the weather.

Green Seed Problems

Cutting too early with high temperatures and rapid drying can lead to excessive green seed count. Two percent or less green seed is currently the allowable limit. Any higher than the 2 percent and market discounts can occur. The key to curing the crop is moisture. The enzyme responsible for clearing the chlorophyll requires moisture. Therefore, seed moisture is critical. If the stems and seeds dry too rapidly after swathing, then chlorophyll can be fixed.

Leaving canola in the swath longer can help eliminate some green seed problems or potential. A rain will also help reduce green count in canola. Once the moisture content of seed is 20 percent, chlorophyll will begin to be moved out. In some cases however, when swathed too green in hot weather the chlorophyll will not be reduced to any great extent. Cool temperatures and light frosts in August and September slow the enzyme activity that breaks down chlorophyll. Frosts from 32 to 33 degrees F disrupts that system, more specifically it can reverse it and restart the synthesis process. This is very sensitive in the seed development stage, and the window is very narrow. This can cause differences between adjacent fields that are only days apart in maturity, or differ in uniformity of maturity. Even canola swathed four to six days before a frost will retain relatively high levels of chlorophyll. Thin stand counts can result in plant with more branching and more variability in seed maturity and are more likely to have immature seed at swathing. Late seeded canola may be impacted by all these situations. When looking at uneven stands, its suggested that one do a count early on the ratio of early emerged canola which is bolting or starting to flower and the late emerged flush of young more immature plants.

If one knows the ratio of early to late emerged canola plants, a better decision can be made as to how soon to swath or wait until the later crop catches up. If the stand is on 20-25% early and 75-80% late, then waiting to cut later may be the best strategy to reduce the amount of green seed.

Two years of NDSU research has shown that at 0_5 seed color at swathing time resulted in 3.5 percent green seed content which is higher than the 2% allowed in the market place before a discount will occur. Approximately 180 lbs/A of yield gain was noted when swathing was delayed to the 15-20 percent seed color change.

Another sign of canola being very near the swathing stage is the natural yellowing and senescence of leaves and leaf drop. When canola plants consist only of stems, stem branches and pods, it is probably very near the optimum time for swathing.

Canola should be allowed to cure and ripen from ten to 14 days in the swath before combining. If combined too early, the chance if increased green seed in the harvested crop is much greater.

"Be in a hurry to swath on time and prevent shattering, but take your time in moving the combine in the field to ensure maximum drying, maturation and quality of your harvested canola."

 

CORN POLLINATION- WHAT’S HAPPENING

Recent hot weather has helped the corn crop with heat units and the crop is now near or at the critical pollination time. Soil moisture to supply the corn plant for growth and development is critical at this time.

Unlike all other major grain crops, the corn plant has separate male and female flowering parts. The tassel and ear shoot are the male and female flowering structures, respectively, of the plant. The flowering stage in corn, which involves pollen shed and silking, is the most critical period in the development of a corn plant from the standpoint of grain yield determination. Drought, high temperature stress, as well as hail damage and insect feeding have the greatest impact on yield potential during the reproductive stage. The following is an overview of some of the key steps and phases of the corn pollination process. Information was taken from the text entitled "Modern Corn Production" by Aldrich, Scott and Hoeft.

Pollen shed usually begins two to three days prior to silk emergence and continues for five to eight days with peak shed on the third day. On a typical midsummer day, the shedding of pollen is in the morning between 9:00 and 11:00 a.m.

The tassel is usually fully emerged and stretched out before any pollen is shed. Pollen shed begins at the middle of the central spike of the tassel and spreads out later over the whole tassel with the lower branches last to shed pollen.

Pollen grains are borne in anthers, each of which contains a large number of pollen grains. The anthers open and the pollen grains pour out in early to mid morning after dew has dried off the tassels. Pollen is light and is often carried considerable distances by the wind. However, most of it settles within 20 to 50 feet.

Pollen shed is not a continuous process. It stops when the tassel is too wet or too dry and begins again when temperature conditions are favorable. Pollen stands little chance of being washed off the silks during a rain storm as little to none is shed when the tassel is wet. Also, silks are covered with fine, sticky hairs which serve to catch and anchor pollen grains.

Under favorable conditions, pollen grain remains viable for only 18 to 24 hours. However, the pollen grain starts growth of the pollen tube down the silk channel within minutes of coming in contact with a silk and the pollen tube grows the length of the silk and enters the female flower (ovule) in 12 to 28 hours.

A well developed ear shoot should have 750 to 1,000 ovules (potential kernels) each producing a silk. The silks from near the base of the ear emerge first and those from the tip appear last. Under good conditions, all silks will emerge and be ready for pollination within 3 to 5 days and this usually provides adequate time for all silks to be pollinated before pollen shed ceases.

Pollen of a given plant rarely fertilizes the silks of the same plant. Under field conditions 97% or more of the kernels produced by each plant are pollinated by other plants in the field.

The amount of pollen is rarely a cause of poor kernel set. Each tassel contains from 2 to 5 million pollen grains which translates to 2,000 to 5,000 pollen grains produced for each silk of the ear shoot. Shortages of pollen are usually only a problem under conditions of extreme heat and drought. Poor seed set is more often associated with poor timing of pollen shed with silk emergence (silks emerging after pollen shed).

As a general rule in our northern corn growing region, grain corn is usually physiologically mature approximately 50 to 55 days after the mid-silking date. That would mean if would be fairly safe from a killing frost. If corn is mid-silk on August 1, then it would be safe from damaging frost on Sept. 20 or by Sept. 25 depending on maturity. Cool August days and nights can further delay the maturing process.

Duane R. Berglund
Extension Agronomist  
dberglun@ndsuext.nodak.edu  


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