Management considerations for corn and soybean crops

Courtesy photo/Andrew Ahlersmeyer: Potassium (K) deficiency symptoms where leaf yellowing started at the tip of older leaves and moved down the leaf margins where (A) 0 lbs. and (B) 90 lbs./ac. were applied at planting, and (c) a healthy plant where 150 lbs./ac. was applied.

Sow and Grow

Small grains harvest has begun or will start soon and current crop condition reports are not much short of 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 is still important to check fields and get a general understanding of what’s going on. If you’re in an area where corn is looking pretty rough and dry, you may be seeing some interesting drought symptoms. 

We’ve seen a few different things happening in corn fields, potassium (K) deficiency being one of them. Corn will use up to 85 lbs. of K per acre on a given year; this year, drought conditions have resulted in K deficiency showing up in fields where it may not typically occur. Symptoms usually appear on older leaves 4-6 weeks after planting as the plant moves K from older to younger leaf tissue. Deficiency will usually show up as yellow to brown leaf tissue that starts at the tip of the leaf and expands down the outside margins.

Although you may have adequate K 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 potassium uptake and the plant can become deficient. This can happen more with tilled soils, as soils haven’t settled sufficiently since being disturbed, resulting in less contact between soil particles, slowing movement of K through soil water to plant roots (hence why K deficiency is sometimes not observed in wheel tracks). Other factors that may affect K uptake include presence of smectite clays, highly compacted soils, pest root pruning, sidewall compaction, and plant diseases.

What can we do about it? In a dry year like this one, if your K soil tests are sufficient, the best practice is to wait for a rain, as this will help increase the K availability in the soil, making it available for plant uptake. 

Typically, K deficiency symptoms will decrease or disappear after rainfall begins again in these situations. 

There is not enough current data in South Dakota to prove or disprove the effectiveness of rescue K treatments to an in-season corn crop. Regardless of potential effectiveness in research done elsewhere, additional potassium applications will not be sufficient if drought conditions continue. 

The best way to prevent K deficiency is to keep soil test levels adequate.

While checking fields, now is the time to start scouting for soybean aphids. Soybean aphid populations have been observed this year in South Dakota and although populations appear to begin small, it’s good practice to start scouting before they get out of hand. We suggest frequently scouting fields (weekly this time of year), and walking in a ‘W’ or ‘Z’ pattern when scouting.

There are two effective ways to scout for aphids:

1) Traditional scouting- choose 20 plants from multiple locations across the field, count the aphids present on each plant and record the total number of plants affected and aphids per plant (be sure to check the plant base and underside of leaves). Management is necessary when at least 80% 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 through R5 (beginning seed). After this time (R6 and forward), the observed yield loss due to aphids is dramatically reduced. The 250 aphid/plant threshold has been extensively researched in South Dakota and indicates that insecticidal control prior to this level is not profitable.

2) Speed scouting- this method uses a decision population of 40 aphids per plant and is accessible through a (printable or electronic) worksheet or the phone app called “Aphid Speed Scout”. This method involves first checking the aphid population on 11 random plants. If each plant has less than 40 aphids, a “-“ is marked down. If more than 40 are observed, a “+” is written down. After examining 11 plants, the “+” signs are added up, and if there are six or less, you have reached a ‘do not treat’ decision for the field. If there are 11, you have reached a ‘treat’ decision. If 10 “+” signs are written down, 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 before insecticide application occurs. For a copy of the speed scouting worksheet, visit extension.sdstate.edu and search “speed scout.”

Upcoming events

• July Drought Hour Webinar Series: July 26  at 11 a.m.. This webinar will include several drought related talks covering corn pollination, crop conditions, current forecasts, and dry lotting decisions. Free to the public, sign up at extension.sdstate.edu/events.

• 2021 Virtual IPM Field School: This self-paced course runs online Aug. 1-31 and is broken into modules that will cover a variety of topics. Register at extension.sdstate.edu/events

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