Harvesting of the small grains has started or will begin soon and current crop condition reports are not far from disappointing. However, there is always an opportunity for growth and improvement, especially if you do the right rain dance.
At this point in the growing season, corn management often starts to slow down, but it’s still important to check the fields and get a general understanding of what’s going on.
If you are in an area where the corn looks quite rough and dry, you may see some interesting drought symptoms. We have seen different things happening in the cornfields, potassium – K – deficiency being one of them.
Corn will use up to 85 lbs. of potassium per acre in a given year. This year, drought conditions have resulted in potassium deficiency in fields where it does not usually occur. Symptoms usually appear on the older leaves 4-6 weeks after planting, as the plant shifts potassium from the tissues of the older leaves to the younger ones.
The deficiency usually appears as yellow to brown leaf tissue that begins at the tip of the leaf and extends along the outer margins.
While you may have adequate potassium soil tests, this deficiency can still occur, as corn plants need actively growing roots and water uptake to move potassium to the plant.
Under drought conditions, root activity slows the absorption of potassium and the plant may become deficient. This can happen more with plowed soils, as the soils have not settled enough since they were disturbed, resulting in less contact between soil particles, slowing the movement of potassium through the water in the soil. soil to the roots of plants – hence the reason why potassium deficiency is sometimes not observed in the wheel of the tracks.
Other factors that can affect potassium uptake include the presence of smectite clays, highly compacted soils, pruning of pest roots, sidewall compaction, and plant diseases.
In a dry year like this, if your potassium soil tests are sufficient, the best practice is to wait for the rain. Rain will help increase the availability of potassium in the soil, making it available for plant uptake. As a rule, the symptoms of potassium deficiency decrease or disappear after the resumption of precipitation in these situations.
There is not enough current data in South Dakota to prove or disprove the effectiveness of potassium relief treatments for a seasonal corn crop. Regardless of the potential effectiveness of research done elsewhere, additional applications of potassium will not be sufficient if drought conditions persist.
The best way to prevent potassium deficiency is to maintain adequate soil test levels.
While checking the fields, now is the time to start looking for soybean aphids. Soybean aphid populations have been seen this year in South Dakota and although populations seem to start off small, it is recommended that scouting begin before they get out of hand.
We suggest that you explore the fields frequently – weekly at this time of year – and walk in a “W” or “Z” when scouting.
There are two effective ways to detect aphids.
Pick 20 plants from several places in the field, count the aphids present on each plant and record the total number of affected plants and aphids per plant – be sure to check the base of the plant and the undersides of the leaves.
Management is necessary when at least 80 percent of the plants are infested with 250 or more soybean aphids. We recommend insecticide control within five days of scouting while plants are in the vegetative growth stage until R5 – seed start. After this period – R6 and beyond – the observed yield loss due to aphids is considerably reduced. The threshold of 250 aphids per plant has been extensively researched in South Dakota and indicates that insecticidal control before this level is not cost effective.
This method uses a decision population of 40 aphids per plant and is accessible via a printable or electronic spreadsheet or the phone app called Aphid Speed Scout.
This method consists of first checking the aphid population on 11 random plants. If each plant has less than 40 aphids, a “-” – minus – is marked. If more than 40 are observed, a “+” – positive – is noted.
After examining 11 plants, the “+” signs are added, and if there are six or less, you have made a “do not treat” decision for the field. If there are 11, you’ve made a “treat” decision. If 10 “+” signs are written, additional plants should be checked.
When using this method, it is important to return to the field three to four days later to confirm that an aphid population is still present prior to the insecticide application. To get a copy of the speed scout spreadsheet, visit extension.sdstate.edu and search for “speed scout”.
Sara Bauder is an agronomist field specialist at the Mitchell Regional Center at South Dakota State University.