Evaluating your corn stands allows you to assess the effectiveness of planting and make notes on how to improve in the future. If corn stands are evaluated soon after rows are visible, or about 5 to 7 days after the first plants emerge, you can determine whether replanting is needed before the feasibility of replanting is greatly reduced due to a late replanting date.
In this year in Minnesota and surrounding areas, pay special attention to fields planted prior to April 22 due to extended periods of cold soil temperatures after planting.
When assessing corn stands, evaluate several representative areas in each field while also making note of atypical areas in fields with stand establishment problems. Documenting emergence problems with GPS and photos tagged with geographical coordinates can be useful for future decisions.
Corn growth and growing degree days
Corn emergence occurs when about 115 growing degree days (GDDs) have accumulated since planting, although the number of GDDs required for emergence can range from 100 to 120. From emergence until the V10 (10 leaf collar) stage, corn advances by about one leaf stage with every 82 GDDs.
Daily GDDs are calculated based on daily high and low air temperatures, measured in degrees Fahrenheit, according to the following equation:
GDD = [(High + Low)/2] – 50°F
where High is the daily high (set to 86°F if the daily high is above 86°F) and Low is the daily low (set to 50°F if the daily low is less than 50°F). For example, 8 GDDs are accumulated on a day with a maximum air temperature of 66°F and a minimum air temperature of 50°F or lower.
Corn plant population is linked to yield and is a key factor when assessing stand establishment. Knowing the plant population relative to the planting rate, along with the soil and weather conditions from planting to emergence can inform future planting decisions.
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When measuring corn plant population, accuracy increases when more areas of a field are sampled and a greater number of plants are counted. Only count plants that are healthy and likely to contribute to yield. Counting plants in groups of three can speed the process.
A common approach for measuring plant population is to count the number of plants in 1/1000th of an acre and then multiply by 1000 to get plants/acre. The length of row equal to 1/1000th of an acre is 17 feet 5 inches for 30-inch rows, 23 feet 9 inches for 22-inch rows, and 26 feet 2 inches for 20-inch rows. When taking stand counts from a length of row, consider counting two adjacent rows to increase the sampled area.
An alternative method is to count plants while pushing a measuring wheel, record the length of row with 100 plants, and then calculate plant population based on this value. This avoids bias due to taking stand counts from short lengths of row. For example, if there are 100 plants in 52.5 feet of row in a field planted to 30-inch (2.5-feet) rows, plant population would be calculated as:
100 plants / (2.5 feet × 52.5 feet) × 43,560 square feet/acre = 33,189 plants/acre
Field trials by the University of Minnesota found that on average, corn grain yield was 96% of the maximum with 26,000 plants/acre, 92% of the maximum with 23,000 plants/acre, 87% of the maximum with 20,000 plants/acre, and 81% of the maximum with 17,000 plants/acre.
Additional yield losses can occur if there are frequent gaps of missing plants within rows. Expect yield losses of around 2% for gaps of 16 to 33 inches, and 5% for gaps of 4 to 6 feet.
Uniformity in emergence
A goal of corn stand establishment is for all plants to emerge at about the same time. This is far more important than uniform within-row spacing of plants. According to a previous study in southwestern Minnesota, a corn plant that is one leaf stage behind its neighboring plants early in the season will only yield about 80% of normal, while a corn plant that is two leaf stages behind its neighboring plants early in the season will only yield about 50% of normal.
Uneven emergence is most commonly caused by irregular soil moisture in the seed zone. In such cases, late-emerging plants typically had seed placed in soil that was dry at or after planting. Uneven emergence can also be due to factors such as poor seed-to-soil contact, cold soil temperature after planting, soil compaction, and soil crusting.
When evaluating corn stands, dig up plants that appear weak and exhibit poor growth. Try to determine whether the reduced vigor of these plants is due to delayed emergence or factors such as herbicide injury, diseases, insect feeding, salt injury from fertilizers, ammonia toxicity, or nutrient deficiencies. Understanding the reasons for poor seedling vigor can inform future management.
If it appears that replanting might be needed, consider whether additional plants are likely to emerge, the potential yield of the existing crop, replanting costs, and the potential yield of a replanted crop. Averaged over 26 planting date trials conducted across Minnesota from 2009 to 2016, corn grain yield was reduced by 4 to 6% when planting occurred on May 20 to 25.
In comparison, a planting date trial conducted from 1988 to 2003 in southwestern Minnesota found that on average, corn grain yield was reduced by 8 to 13% when planting occurred on May 20 to 25.
If replanting is needed, consider whether an entire field should be replanted or only a portion of it. When replanting very sparse to non-existent stands, planting directly into the existing seedbed may be a viable option, especially if it enables an earlier planting date and if the surviving plants are small.
In other cases, the existing plants should typically be removed with herbicide or tillage prior to replanting. Tillage may be needed before replanting fields with hardened soils due to intense rainfall or soil compaction from previous field operations.
If replanting, consider the length of the remaining growing season and select hybrids of appropriate maturity. If corn replanting is delayed until May 22 to 28, consider switching to hybrids that are 5 to 7 or more relative maturity units earlier than those considered full-season. If replanting occurs on May 29 to June 4, consider switching to hybrids that are 8 to 15 or more relative maturity units earlier than those considered full-season.
Assess early-season soil conditions, as these can influence corn growth and uptake of water and nutrients. Notes on early-season soil conditions can be useful for guiding in-season scouting and adjusting future management practices. Look for compacted soil along the seed furrow (sidewall compaction) or below the seed, as this can restrict root development.
Sidewall compaction can occur when planter disc openers cut through soil that was too wet at planting. Soil compaction just below the depth of preplant tillage can also occur if soils were too wet at the time of tillage.
Soil compaction is of greatest concern for the nodal (primary) root system of corn. Soon after corn emergence, nodal roots grow from the base of the coleoptile, located about 0.75 inches below the soil surface. These differ from the seminal (temporary) roots, which grow from the seed (Figure 1).
At the V3 (three leaf collar) stage, corn plants transition from dependence on seminal roots and kernel reserves to the nodal roots. The nodal roots are the major supplier of water and nutrients from the V6 (six leaf collar) stage to maturity. Since nodal roots develop near the soil surface, they are susceptible to near-surface soil compaction, especially if it is not alleviated by timely rainfall.
It is also useful to note fields with limited residue coverage on the soil surface, as corn seedlings in these fields can be injured or cut off by blowing sand-sized soil particles.